© Brooke Clarke 2000 - 2023


Earth's Magnetic Field
Compass Dip Needle
Helmholtz Coil(s)
Magnetic Shielding
Magnetic Stars In Outer Space
Superconducting Magnets
Sensor Types
Fluxgate  Fluxgate & Other Sensor Patents
Combined magnetostrictive and the piezoelectric
MIDIM (mirror image differential induction-amplitude magnetometer)
Magnetoresistive (MR)
Giant Magnetoresistive (GMR) & Spin Valve
Coil Antenna
Hall Effect
Magnetic Precission
Silicon Integrated Circuit
Medical Applications
Nuclear Magnetic Resonance (NMR)
Magnetic Reasonance Imaging (MRI)
Magnetoencephalograph (MEG) & MCG
UXO UneXploded Ordnance Detection
MICR (Check) Processing
Magnetic Ink in U.S. Paper Money
Credit and other cards with a Magnetic Stripe
Anti-Shoplifting Garmet Tags
Sensing a Magnet
Demagnetizer - Magnetizer
Geophone Based Intrusion - Seperate Web Page
Earth's Gravity
Acoustic - Infrasound 

Light      Photometers
IR        Detectors
Laser      Distance Measuring
Cloud     Turbidity




Building Wall Stud


Gyroscopic   North Finding Module

Human movement    REM Sleep 

Explosive Detection





Rotary Angle

Hobo Data Logger



Web Links


There are a number of technologies that are used to sense magnetic fields.  I have also included some places where magnetic fields are used like in MRI, NMR, particle accelerators that use super conducting magnets.

1 Oersted = 79.58 Ampere-Turns/Meter
1 nano tesla = 10 micro Oe = 1 Gamma
1 Tesla = 10,000 Gauss
Papers about Magnetism -
Journal of Magnetism and Magnetic Materials - For over 100 years, Elsevier Science has been dedicated to facilitating the exchange of information.
The Magnetic Measurements Handbook -

NASA has a series of web pages devoted to magnetic topics called: The Great Magnet, the Earth & A Millennium of Geomagnetism 

Earth's Magnetic Field

Magnetic Field Calculator - need Lat, Lon, elevation, can choose WWM2005 or IRGF 10 models. Outputs 7 components:

Nominal value of Earth's field = 0.5 Oe = 0.5 Gauss = 39.79 A-T/m = 5E-5 T = 50 uT = 50,000 nT.

Lat: 39° 11' 24''
Lon: - 123° 9' 50''
Elev: 280.00 m
+ East - West
+ Down - Up
+ North - South
+ East - West
+ Down - Up
5/26/2007 15° 6' 62° 20' 23,183.6 nT 22,382.9 nT 6040.4 nT 44,224.8 nT 49,933.1 nT
Change per year - 6' per year - 1' per year -24.7 nT/year -12.7 nT/year -48.0 nT/year -69.6 nT/year -73.2 nT/year
8 May 2015 - Magnetic Field Calculators
62.2494° 22,893.5 nT
22,195.2 nT
5,611.0 nT
49,167.4 nT
-0.1003°/yr -0.0056°/yr -41.7 nT/yr
-30.6 nT/yr
-49.1 nT/yr
-89.5 nT/yr
-98.6 nT/yr
0.34° 0.22° 133nT
138 nT
89 nT
165 nT
152 nT

The National Geophysical Data Center lists a number of models and is the key US web site for the Earth's magnetic field.  The World Magnetic Model   is updated every five years by the DoD.  This model will give you all three components of the Earth's magnetic filed if you know the date,  Longitude, Latitude and Elevation.  They also have a good links page.  The WMM95 model is built into the current versions of the Motorola VP Oncore GPS receivers.  A magnetic compass and this GPS receiver could be used to give true bearings, not just magnetic bearings.  Now add a long range laser range finder and you can locate anything within range in absolute coordinates  . . .  just what the military needs for targeting.

Compute Field Values for givenlocation  and date -
Experiment Relating to the Vertical Component of the Earth's Field -
National Geophysical Magnetic Data Center - On line mag field calculator 

The International Association of Geomagnetism and Aeronomy -
USGS - Real-time monitoring of the Earth's magnetic field - geomagtism page -
WWV - Official Announcements - includes geomagnetic field activity activity
Hermanus Magnetic Observatory - CSIR -
International Geomagnetic Reference Field - World Magnetic Model
British Geological Survey - Geomagnetism -
Bartington Instruments Ltd - Magnetic Instrumentation Products -combined with theodolite
Australian INTERMAGNET Observatory Information -
Schonstedt Instrument Company - Surveying, Unexploded Ordnance Detection, Construction, Police, Utility (Water and Electrical), and Undersea pipe and cable location.
Dunham & Morrow - surveying marker locator designed by the same people that designed the Schonstedt GA-32/52 series.
Institute for Defense Analyses - Sensors, Surveillance & Target Acquisition -   DARPA MEMS -
MicroStrain - 3DM  - Orthogonal arrays of magnetometers and accelerometers are used to compute the pitch, roll and yaw angles over a wide angular range. Dev Kit $695
NGDC  - Utility Programs for Geomagnetic Fields
Swedish Institute of Space Physics (IRF) - Real Time plot - Today's and Earlier Space Weather -
AAVSO Solar Division - McWilliams magnetometer - bar magnet suspended on "E" guitar string  photo sensors detect rotation
The Great Magnet, the Earth by  Daniel Stern of the NASA-  Commemorating the 400th anniversary of "De Magnete" by William Gilbert

3 Axis Magnetic Field Sensor - looking for information on this


The direction of the horizontal component of the Earth's magnetic field can be determined by a compass.  These range from the "Cracker Jack" type to very high quality models.

Note that the Earth's magnetic field can be thought of as having three components (Calif. values shown):

  • down in the direction of gravity  = 44,862 nt
  • to true North (not magnetic North) = 22,460 nt
  • and an East-West component at right angles to true North = 6,479 nt
The East - West component causes what is called declination, or the compass points to "magnetic north".
The true North component is in the horizontal plane, but
The down component is much stronger than the North component!  If the compass is not level then it will have a large error because it is starting to read the down component of the Earth's field.  This is why all conventional compass' need to be held in a level position. <>Chris M. Goulet's Frequently Asked Questions - Magnetic Declination
Suunto- World MapNorth America - Field Compass -  MC-1 G Global - unique needle balanced for worldwide use.  No longer on their web pages?
Suunto has many patents.
3813792 Device for Declination Correction of a Magnetic Compass, Suunto Oy, Jun 1974, 33/356 - just declination dial on bottom to manually set
7129835 Wrist-top computer, Suunto Oy, Oct 31, 2006, 340/539.11 ; 340/539.12; 455/100 - belt transmits to wrist display

Vector wrist top computer (thermometer, altimeter, barometer, compass, watch - I have one of these) - Although I have not experienced it, there are reports that all batteries have ferrous metal and typically are magnetized and might cause trouble with the compass.  The instructions say to re-calibrate the compass every time the battery is changed.  The person making the complaint may have been in a strong magnetic field that changed the batteries magnetic field.
Recta - at TugawarPhotos -
Silva -makes a number of quality compasses and other outdoor items like Head Lamps
Brunton - Pocket Transit = Army M2 comes calibrated in either degrees or mils (1 mil is 1 meter lateral @ 1,000 meters range)
Celestaire, Inc -Marine and Air Navigation Instruments - Compasses -
Stocker & Yale, Inc. - manufacturer of  Lensatic Compass  -
Cammenga - Current  Lensatic Compass manufacturer
Point Research Corporation - PointMan - drift-free dead reckoning navigation system - Engineering Evaluation Kit is $2,495 (2/13/99) 5583776 -
There are 3 magnetometers and 3 accelerometers on orthogonal axes, plus a barometric altimeter. The accelerometer triad senses the gravity vector to tilt compensate the compass, and is also used for stride detection.  Distance is measured by counting the user's steps (whose length is calibrated by GPS or other method) rather than by double integrating the accelerometer data. GPS and DR data are combined in an on board Kalman filter.
Jack Runyon
Microstrain, Inc. - Orientation sensor 3 magnetometers & 3 accelerometers
Photos: Surveyor's - M2 - Lensatic - German - Army Marching -
How Stuff Works - How a Compass Works -

Static vs. Dynamic Compass

Note that to determine the compass bearing when static requires both sensing the Earth's magnetic field and sensing it's gravitational field.  A simple way to do this to level the compass using a bubble vial.  That's what's done on the Suunto wrist compass. For a conventional magnetic needle compass the needle is weighted to counteract the dip caused by the Earth's magnetic field for a limited range of latitudes.  In order to get a reading the compass needs to be held level or the needle will not be free to turn.

For an electronic compass an accelerometer can be combined with a 3-axis magnetic field sensor, called an orientation sensor, to determine both North and down.

But if an orientation sensor is used in a dynamic environment, like a boat, car or plane, it will not work because the "down" direction will not be valid when the vehicle is not going in a precise straight line.  To fix this a gyroscope can be added to sense turning.  This is why "Gyro compass" is a term herd on vehicles.

Dip Needle

Magnetic Dip NeedleThe compass works in the horizontal plane and is compensated for it's lattitude of operation to balance out the effect of the vertical component.  The dip needle works in the vertical plane, which is often the stronger magnetic field.

Notice that the scale is only on half the circle and that a chain with a hook is provided to hang it so the needle axis is horizontal.  Not sure why two covers rather than the single cover used on a compass, but maybe so you can see whichever side is nearest you.

This one needs cleaning up and the needle is too weak to work properly so needs to be magnatized.

Helmholtz Coil(s)

Field Generation

A Helmholtz coil consists of two coils aligned on a common axis, wound in the same sense and connected in series.  The radius of the circular coils is R and the separation between them is also R.  Their fields add in such a way that there is a region at the geometric center where the magnetic field has constant magnitude and angle.  The magnitude of the magnetic field is linearly proportional to the applied current through the two coils.

The magnetic field is given by B = (4/5)^(3/2) * (Uo * N * I)/R
where the R units are:

Uo =
1.26E-6 Tm/A
1.26E-4 Tcm/A
4.95E-5 Tin/A
This is a good way to test Earth field type magnetic sensors.

A single Helmholtz coil could be positioned so that it's axis was in line with the Earth's local magnetic field and then the DC drive current adjusted to cancel the Earth's field.  An alternate procedure is to use 3 Helmholtz coils in an X-Y Z configuration.

Wood, plastic, fiberglass are suitable for either DC or AC driven Helmholtz coils.  For Earth's field work where only DC will be used aluminum, brass, and copper can also be used because there will be no AC eddy current problems.
UC Press - Hermann von Helmholtz & the Foundations of Nineteenth-Century Science -
Magnet-Physik - Moment measuring coils - for measuring permanent magnetics
Biot-Savart law - web plotting
Walker Scientific - Helmholtz coil - Gaussmeters - Fluxmeters - magnetometers -
Rapid Magnet Evaluation -
3m diameter Helmholtz Coil at the National Physical Laboratory -
Billingsley Magnetics - 1 meter, 2.2 meter - coils
5126666: Method and apparatus for substantially eliminating magnetic field interference to a magnetometer caused by DC current carrying conductors
Lake Shore Cryotronics - and magnetic field instruments

Field Measurement

If a permanent magnet is placed inside a Helmholtz coil and then rotated 108 degrees a Grassot Fluxmeter (Wiki) (aka: integrating Fluxmeter) can determine how strong the magnet is.  Instead of using a Helmholtz coil a "search coil" can be used in a similar manner.  For example by placing the search coil at the maximum field, zeroing the meter, then pulling the search coil out the strength of the magnet can be measured.

The W.W.II Signal Corps Fluxmeter TS-15 has two meters.  The probe is a meter movement without any magnet.  It's placed into the field to be measured (properly orientated).  Then the main box is used to adjust the current through the probe until the probe shows full scale.  At that setting the main meter has a deflection proportional to the current, but the meter is calibrated in magnetic field.

Maxwell Coils

The Maxwell coil arrangement is similar to the Helmholtz coil pair except there's three coils.  The center coil being slightly larger than the outside coils where the radius of the outside coils are (4/7)^1/2 times the radius of the center. The coils are spaced apart by (3/7)^1/2 times the radius of the center coil.  These are used mostly in MRI where they are trying to get the largest volume with a constant field.

Magnetic Shielding


The electron beam in a Cathode Ray Tube (CRT) used in computer monitors and TV sets is bent by the magnetic field of the Earth.  In some applications where precision beam positioning is important the CRT has a Mu-metal shield.  There are numerous permeabilities offered some alloys are called Netic (soft iron) and Co-Netic (mu-metal).  Metglas is a high initial permmability material.

Home Made Zero
              Guass CanI tried common metal objects like food cans, but the soft iron just translates and concentrates the Earth's magnetic field.  Using Co-Netic material (Netic has a much higher saturation level and is for strong fields) in a can shape both inside and outside a 10" length of 3" diameter ABS plastic pipe results in a reasonable shield.  Using the same length, but with 2.5" diameter PVC would be better (longer L/D), but the 2.5" PVC is not available at my local hardware stores.
The material in the Magnetic Shield Corp. LK-110 is in 10" x 15" sheets.

When working with magnetic sensors it's difficult to tell what the zero field output is.  There is a continuous bipolar change from the max North indication and the max South indication.  Because of offsets half way between these two is not the zero field output.  But with a zero field can you can tell what's going on.  A Helmholtz coil aligned with the Earth's magnetic field vector or a 3-axis coil system could also produce a zero field, but with much more complication and also with some uncertainty.

The 10 & 4 mil sheets are easy to cut using ordinary paper scissors.  Although not as good as spot welding an overlapped joint can be spot soldered.

When a FGM-3 sensor is pointed alone the Earth's field vector it reads 37 kHz or 135 kHz depending on polarity, but when pointed in the same directions inside the can it reads 58 or 66 kHz.
If the Earth's field is + or -50 uT and that corresponds to a span of 98 kHz then the sensitivity of the FGM-3 is about 1.02 uT per Hz.  The field remaining in the can is about 8 * 1.02 or 8.1 uT.  It seems that the can should be doing more, but maybe it's too short or needs spot welds, or thicker material. (it was too large in diameter)

17 Nov 2005 - I now have a third can that fits inside the above two cans and can be seperated from the inner can above by using bubble wrap or cardboard.

The attenuation for a cylinder is proportional to (material thickness) / (diameter) for a single can.
When two cans are nested and magnetically isolated from each other the final attenuation is the product of the two attenuations.  So if you double the thickness on a single can you get 2 times the attenuation, but if you use the same material to make two cans you get about 4 times the attenuation.  So you get more attenuation for the dollar by using a number of thin wall cans than a single thick wall can.

There appear to be different effects at work:
(1) as a probe is moved along the centerline of a can starting from the open face there is a 1/(e^(a*x)) type of decrease in field strength. where x is expressed in terms of L/D with L being the distance down into the can and D the ID of the can.  This suggests that the inner can should be shorter than the outer can.
(2) after some distance the field strength flattens out and this must be related to the material thickness and permeability as well as the can I.D.
(3) It may be that the overlap joint in the can has some leakage that prevents the field from becoming smaller and smaller at the deeper depths.
(4) There is no material saturation problem with these materials and only the Earth's magnetic field at work here.

A plot of field strength for the 2.4" dia x 9.75" high can shows that the field increases slightly from the center value to the cap indicating that the cap has some leakage because of the folding done to make it.  From 2 to 6 inches up from the cap the field is constant  at about 2.8 uT compared to a theoretical value of 44.3 uT for the vertical field.

A much better shield could have been made by making the I.D. much smaller.

Note that an open cylinder that was twice as long as a closed can should give the same results without the need for making a cap, but that takes twice as much expensive material.

Some of the material supplied in the Magnetic Shield Kit is soft iron.  It's good for knocking down a very high field since it's saturation level is higher than mu-metal.  But the initial permeability of soft iron is many many orders of magnitude lower than mu-metal and so even a very thick shield does little to attenuate the earth's field.

AD-Vance Magnetics -
Amuneal -
Magnetic Shield Corporation  - FAQ -Netic and Co-Netic - Helmholtz Coil Assemblies  -Zero Gauss Chambers - Lab Kit 110 with 10" x 15" sheets of material.
Magnetic Shields Limited - UK mu metal supplier that has formed shapes, not just sheets or rolls.
National Electronic Alloys - mu metal and other shielding alloys in sheet or roll (they call it coil) forms.
Sunbo - Korea raw materials & shield cases for magnetic recording heads
Riken - Magnetic Materials Laboratory - Japan
MuMetal - Specifications - MµShield materials - Gauss meter rental - Equations
U. of Texas - Magnetic shielding - note equation (611) at the bottom of the page.
\begin{displaymath} {\bfm B} \simeq \frac{3}{2}
              \frac{\mu_0}{\mu} \frac{a}{d}\, {\bfm B}_0.
where B is the field inside a sphere and Bo is the field outside the sphere
a is the radius of the sphere
d is the thickness of the material
mu zero is the permeability of free space 1.257E-6 Henry/meter
mu is the relative permeability of the shield material.

So to improve shielding you can make the sphere smaller, use thicker material, use material with a higher mu.

Reference Data for Radio Engineers, Handbook of Chemistry and Physics, and other books have tables of relative permeability for various materials.  What Magnetic Shield corp. calls Netic (plain cold rolled iron) mu is about 200, cast iron is 90, machine steel is 450.  Mu-metal is 20,000 and Supermalloy is 100,000.  So you can see why using iron pipe, soup cans or cookie boxes will not give you much shielding. 

The high mu materials also are designed to have very low coercivity (they do not become magnets).

Also note that the same materials that work well for shielding would also work well as the core of a fluxgate sensor and for the same reason, they concentrate the filed lines.

Lakeshore 4060 Zero Gauss Chamber

Lakeshore 4060 Zero
              Gauss Chamber
The cylindrical space inside is 0.51" dia x 2.04" inside the plastic sleeve.  When the sleeve is removed the space is 0.605 dia x 2.114".
The field at the bottom of the hole is essentially zero.  Comes with the cloth bag.


If a magnetic sensor is used in a feedback loop to control the current in a Helmholtz coil and bring the sensed field to Zero those devies in the sweet spot  volume of the coil will not see any magnetic field.


National Geophysical Data Center - related servers -
Scripps Paleomagnetic Laboratory - analyze the magnetic properties of rocks - Equipment & Facilities - links -
Geomagnetism and Paleomagnetism Section - of the American Geophysical Union -
Gisco -geophysical instrument source - Magnetometers -
Coast Ranges of Oregon and Washington - Magnetics in the exploration of the ground -


The idea of a flux gate is that when the core is saturated it essentially disappears and when it's out of saturation it bends the earth's magnetic field lines which cross the coil.  The switching of the core in and out of saturation is gating the earth's magnetic filed through the coil thus generating a voltage according to Faraday's law.

Fluxgate sensors were called "Flux Valves" prior to WW2.  At that time the indicator was a 400 cps Syslen and the Flux Valve was constructed on a "spider" that had 3 legs each 120 degrees apart.  The coils were wound on bobbins and slipped onto the legs.  An outer ring was then attached to the central core and this ring had three gaps.  The whole assembly was mounted on a pendulum and part of the calibration was to be sure that the center of mass of the Flux Valve was centered so that it would hang plumb.  See my Fluxgate Patents page for more on them.  The MC-1 uses this "Y" type core.

See: O. V. Nielsen, B. Herhando, J. R. Tetersen, and R. Primdahl, "Miniaturization of low cost metallic glass fluxgate sensors", J. Mag. Mag. Matr., 83, 405, 1990, for a general background on fluxgate magnetic sensors.

There are a number of different ways to process the signal from a fluxgate sensor.  See the Fluxgate Patents web. 
The most common ones are:

Many fluxgate circuits make use of two coils, one for driving the core and another one or two coils for sensing.  But there are many fluxgate circuits where there's only a single coil that's used for both drive and sensing.  In all cases there needs to be a way for the sensing circuitry to ignore the drive signal and maximize sensitivity to the output signal.  I have tagged patents that have a Two Terminal Coil (TTC) on the Fluxgate Patents web since these would be less expensive both in terms of winding the coil and simpler circuitry.

Pacific Rim (Japan) A-Plus

A-Plus Photo  They make fluxgate magnetometer unique in that they use a sandwich of printed circuit boards instead of winding wire around a core.  There are 3 seperate coils, one with turns  through the center of the toriod, one ourside for X and another outside for Y.   The turns are done with traces and plated through holes.  They may be a big player in the automotive compass market. Their fax in Japan is 03-5281-2064 & tel 03-5281-2067
Paper on very similar idea -
Applied Physics Systems - Fluxgate Magnetometers, DC SQUID Magnetometers
ASC Scientific - Magnetometers -
Barington @ ASC Scientific -
Bartington Instruments - Instruments, not sensors
Billingsley Magnetics - Swales Aerospace - Atitude Control Magnetometers -
Don Lancaster's article muse104 - another muse 103 -
EMDS - (301) 840-5710
Fat Quarters Software - They sell units made in England by Speake & Co Limited.  They have an internal oscillator and an output whose frequency varies with magnetic field.  Also carried are some ASIC (microcontrollers) to do common functions.  A number of app notes are also available, now as HTML documents. 
SAM (Simple Auroa Monitor) - PIC uC kit for using the FGM-3
KVH Industries - C100 - They have a commercial monocular that has an optical readout of the magnetic bearing of the item sighted.  Typically used by mariners.  D0325041: Multifunction monocular  5020902: Rangefinder with heads-up display
Magnetic Research, Inc. - book with detailed circuit diagrams and fluxgate sensors - he died no further info
MC-1 Aircraft fluxgate compass field calibration kit -
Multilayer Air Core Inductor Calculator - good for fluxgate coil design with the core removed. (IE only, not Netscape)
Simple Fluxgate Sensor - FGM overview & App notes Single cal coil details start at pg 9
Speake & Co - make FGM- series of sensors carried by Fat Quarters
Stefan Mayer Instruments -
Watson Industries - combinations of heading sensors, accelerometers, angular rate gyros, and angle references as well as a line of magnatometers.

These are some fluxgate type magnetometers that use the Metglas material, yet are different from the PNI use of the material, yet also still have low power consumption.
5124648 Single winding saturable core magnetometer with field nulling 324/253; 33/361; 324/225; 331/65
5039945  Single winding magnetometer with automatic distortion compensation 324/253; 33/361
4859944 Single-winding magnetometer with oscillator duty cycle measurement 324/253; 33/361; 331/65
A History of Vector Magnetometry in Space -


The early cores were "Y" laminations like are used in transformers.  Later Tape Would Cores (torrids) were used.  Note that in both cases the core material comes in sheets.  The current highest initial permeability material is Metglas.
Magnetics - Strip Wound Cores -
Arnold Magnetics -
Magcore -
Vacuumschmelze GmbH - ?Tape Wound Cores?
MK Magnetics - only does large cores
Applied Physics Systems - makes magnatometers and cores

Magneto-Inductive (MI)

The book Magnetic Sensors & Magnetometers says that these are really Fluxgate sensors.  That may explain the reliance on the circuit rather than on the sensor itself.
Note: Anti shop lifting sensors, like my local Staples store uses, have a thicker strip of steel and a thinner strip of Metglas material inside the plastic package that's about 1 3/4" by 3/8" by 1/16" thick.  So these would be a source of Metglas samples.

American Microsystems, Inc -  magnetic sensing ASIC implements the 5239264 patent and is available for licensing from Precision Navigation Inc.  See the March 2000 issue of Sensors Magazine for an article.

Combined magnetostrictive and the piezoelectric

Spinix - US patent 6580271  class numbers 324/260; 324/244 June 17, 2003 describes this innovative technology

MIDIM (mirror image differential induction-amplitude magnetometer)

Magnetoresistive (MR)

This is a technology that makes use of semiconductor type processes.  The resistance of an element changes due to magnetic field strength.  It is now being used in the read heads of computer disk drives.  Note in this application the output signal does NOT depend on the speed of the disk like it does with inductive heads.  Giant MR disk heads are starting to be used for disk drives, but GMR technology is not as good as MR for Earth field sensing.  The science of magnetic materials is a practical application of quantum mechanics.  This makes the study of these materials a Phd project.  The magnetic properties relate to both the electron orbiting and the spin of the electron itself.  Electrons can spin in the "up" direction or the "down" direction.

Some uses for magnetometers are compassing where finding North is the goal, science where measuring the Earth's (or some other field is the goal) and signature analysis where measuring the change in magnetic field caused by some ferrous object is the goal.  For signature analysis it's helpful to get a dynamic signal with reasonable bandwidth from the sensor and for that the MR sensor seems to be the most desirable.


Even after the manufacturer has laser trimmed each part there still remains an offset voltage from the bridge when there's no magnetic field.  This offset is specified to be up to a few times higher than the output of the bridge when aligned with the earth's field.  Most products handle this by turning down the preamp gain and then letting the micro controller handle it.  But this has the effect of reducing the resoltion by a few bits.

Flipping Set-Reset

Many of these sensors have a Flip coil built into the package.  It turns out that left alone for some time the magnetic domains can change states.  The commonly published aspect of this is a total flip where the sensor output polarity changes.  But another thing that can happen is that the sensor has no output or low output until it receives a flip pulse.

The offerings form Honneywell and Philips Semiconductor appear to be about the same in capability.
Honeywell- Has extensive app notes on many aspects of these including the "setting" and "re-setting" circuits. Honeywell

3902167 Magnetic Thin File Switch Aug 26, 1975 365/139; 335/3; 365/136; 365/195

4533872 Magnetic field sensor element capable of measuring magnetic field components in two directions August 6, 1985 324/252; 33/319; 324/247; 338/32R

4847584 Magnetoresistive magnetic sensor July 11, 1989 338/32R; 257/E43.004; 324/252; 324/260

5247278 Magnetic field sensing device Sep 21, 1993 338/32R; 324/252  - Set & Reset i.e. Flipping

5820924 Method of fabricating a magnetoresistive sensor Oct 13, 1998 427/130; 427/131; 427/256; 427/372.2

5952825 Magnetic field sensing device having integral coils for producing magnetic fields Sep 14, 1999 324/252; 338/32R - Compensation Coil

6717403 Method and system for improving the efficiency of the set and offset straps on a magnetic sensor April 6, 2004 324/252; 338/32R -

Philips Semiconductor - Has very little information on their magnetic field sensorsKMZ51 units.  DigiKey p/n 9082CA-ND (9000 series Fig 2) is an 8-pin SOIC to 8-pin SIP surfboard for the KMZ51 package. Scanner Image - 9082 w/KMZ51 -

6100686 Magnetic field sensor with double wheatstone bridge having magneto-resistive elements August 8, 2000 324/252; 324/207.12; 338/32R - by placing two bridges on the same chip but with opposite output polarity the two bridge outputs can be added to find the zero point offset and eliminate it without using flipping.  Since the temperature effect is realted to mask alignment the two bridges need to be made on the same chip (i.e. using the same masks).  This means that you can not use two different ICs in this mode.  As far as I know there is not product yet available using this method.

Leica has a 3 axis unit that is very accurate because it includes an inclinometer. They do not say which technology is used in the top web page, maybe further reading will reveal the technology.

Pegasus - InSb Magnetoresistors -
San Diego Magnetics - MR, GMR sensors for tape and a new type of magnetic card

5260653  Thin film very high sensitivity magnetoresistive magnetometer having temperature compensation and simple domain stability
5119025 High-sensitivity magnetorresistive magnetometer having laminated flux collectors defining an open-loop flux-conducting path
Searching for ( (magnetoresistive) <in> AB) and ( (Eastman Kodak) <in> PA) at (now a fee service)- yeilds 24 patents that may now be assigned to SDM
Arconame robotics - Devantech - Magnetic Compass Dual axis board using the KMZ52 sensor intended for compass, PWM and serial data+clock outputs.  50 or 60 Hz jumper to reduce line noise.
Magnetometer with magnetoresistive sensor KMZ10 / KMZ51 - Practical info re use as Earth's field sensor

Hitachi - HM55B - It's a 2-axis MR type sensor combined with analog circuitry and a digital interface, ready for a micro controller.  Intended for rough compassing. about 1 uT per bit.  The earth's field is about 50 uT so it's not too sensitive.  16 pin SMT package with 0.65 mm pitch.  Easy to interface with uC or Basic Stamp.

Giant Magnetoresistive (GMR) & Spin Valve

These are technologies being looked into for the next generation computer hard disk drives.
GMR Physics -
NonVolatatile Electronics Inc. - makes a number of magnetic sensor based products

Coil Antenna

The May '96 issue of Scientific American Amateur Scientist article "Detecting low-frequency electromagnetic waves" by Carlson; 3 Page(s) on page 98. This is a follow up to the Fraser-Smith (Stanford) detection of low frequency waves prior to the Loma Prieta Quake.
Stanford University -Space, Telecommunications, and Radioscience Laboratory - ULF/ELF Magnetic Noise -ULF Group - VLF Group
A. C. Fraser-Smith: Research Interests - ULF Earthquake Measurment Sites in Calif. -
Electromagnetic Field Measurements - U.C. Berkerley
The Earth-ionosphere cavity resonances - Stanford STAR
ElectroMagnetic Instruments, Inc. - Magnetotelluric (MT) Earth impedance measurements

Hall Effect

Most Hall sensors that are commonly available have a on/off output and are used in things like keyboards or shaft rotation sensors, not for measuring analog field strength.  Most of the (if not all) of the analog output Hall sensors are aimed at applications where there's a local magnet providing the field to be sensed, rather than sensing the earth's mag field.

Hall effect sensors also have an offset similar to an MR sensor.

These typically are not sensitive enough for Earth's field applications.  There is a cleaver product (model 1490)  that uses Hall Effect sensors combined with a rotating magnet (compass) for coarse direction indication made by Dinsmore Sensors. 1814-T Remell St. Flint, MI 48503 4482 USA Tel: 810-744-1330 Fax: 810-744-1790  There are a couple of new sensors Model 1525 and 1655 (uses Allegro 3507) [ the 3507 is obsolete, but they have better sensors now]  that use a moving magnet (like in a compass) and a couple of analog Hall sensors to allow getting a bearing reading. This will require reading 2 analog signals and computing a vector.  
5157841: Portable electronic compass
4402142: Electronic compass for automotive vehicles 1983 compass needle & 4 Hall sensors
4250626: Electronic compass - Lazar; Harold - Feb. 17, 1981 / Aug. 20, 1979

Not by Dinsmore an interesting combined sphercial LED tilt and compass sensor  based on 3 flux gates.
5526022: Sourceless orientation sensor - 1993

Allegro MicroSystems Inc. - Integrated Circuits - AN 27702A for strong mag fields apps, not Earth's field
Ratiometric Linear Hall-Effect Sensors - A1321 - 5 mv/Gauss is a 3 terminal 5 Volt device.  The noise level is 40 mv, i.e. 16 times greater than the Earth's field, so a lot of averaging/filtering is needed.  This sensor uses rotation of the driving terminals to get better stability.  The zero field offset is <= Vcc/2 +/- 75 mV.
US 5621319 Chopped hall sensor with synchronously chopped sample-and-hold circuit  Apr 15, 1997 324/251; 324/225; 327/384; 327/511; 327/551

Analog Devices - AD22151G - This is a Hall Effect unit with built in temperature compensation, it is for strong magnetic fields like from a nearby permanent magnet, not for Earth's field application.
Honneywell - Hall Effect sensors -
Micronas - Hall Sensors - HAL401 -linear non-programable (no spec sheet Mar 2007)
F W Bell (Sypris)- Hall catalog.pdf - has theory section (Many of the FW Bell gaussmeters use synchronous demodulation of the Hall plate output in order to see small fields combined with a temperature sensor near the Hall plate)
Panasonic -digital @Digikey -
Melexis - MLX90215 Precision programmable linear Hall effect sensor
Melexis MLX90393 3-axis wide dynamic range good for compass and strong fields!
8552721 Hall Sensor Array
6278271 Three dimensional magnetic field sensor
Advanced Hall Sensors (AHS) - using III-V compounds to achieve very good specifications - no data sheets yet
Lake Shore Cryotronics - Magnetic Measurement Instrumentation & Temperature Sensors and Instrumentation
Xensor - has a "Spinning-Current" Hall sensor with a claimed sensivity of 1uT.  This is done by rotating the current and sensing contacts around a circular Hall plate in the time domain.  White paper They make 1, 2 and 3 axis versions.
Earthquake Seismometers, Seismographs and their Various Unique Sensors and Related Topics -
Linear Magnetic Sensors for Seismometers -
$2 Diamagnetic Suspension Seismometer - uses magnetic legation of carbon rod


These look like a zero center panel meter with an analog pointer.  Most of these you see on eBay have a scale of -10 to 0 to +10 gauss, but the M25 can be purchased with a -0.5 to - to +0.5 gauss scale.  Typically used to test metal for residual magnetism.
R.B. Annis - Magnatometers, and demagnitizers - Calibration using a single Helmholtz coil that positioned using a compass and dip needle so that there's an axis with no earth's mag field.

 Magnetic Precission

This is a high end (read expensive) technique used to measure the total magnetic field.  To get total field with any of the sensors that only read a single axis, three sensor outputs need to be combined. This is tough to do since all three need to be very closely matched and be perfectly orthogonal.

Build a  Proton Precession Magnetometer -
Quantum Magnetometry Laboratory - Processor Overhauser Sensor POS-1 - The Quantum Magnetometry Laboratory (QMLab) was established in 1981 by the USSR Government attached to the Ural State Technical University (USTU), for a purpose of the development of sensors on the base of magnetic resonance for geology and defense.
Geometrics - Magnetometer -  Cesium, Proton
Scintrex's - Magnetics -   Cesium, Proton
STL Systemtechnik Ludwig - sub and ship signature - Digital Magnetometer Systems -
 Naval Technology - Magnetometers - Marine Magnetometers - Digtial Magnetometers Fullversion.pdf<>.pdf
ASC Scientific - Magnetometers -
Quantro - underwater Proton magnetometer - $2,750+
J. W. Fishers Proton 4 Mag - $9,495
Dan's Homegrown Proton Precession Magnetometer -
Proton Precession Gradiometer by Jim Koehler - 1 1MB file * spreadsheet - these are very sensitive metal detectors
The Proton Magnetometer Mailing List  -  is a gathering place for persons interested in homebuilding proton mags
Phil's Magnetometer & Prospecting Pages -How to build a low cost Differential Proton Precession Magnetometer. CD-ROM
Gem Systems magnetometer (K-Mag) and - quantum magnetometer technologies, including the Overhauser magnetometer, optically pumped Potassium Proton Precession magnetometer and Proton Precession magnetometer

SQUID - Superconducting QUantum Interference Device

Jena Superconductive Electronics Foundry - Department of Cryolectronics - LTS SQUID group - no English
ASC Scientific - Magnetic & Paleomagnetic Instrumentation -
F.I.T. Messtechnik GmbH - HS20 SQUID Chip -
Applied Physics SystemsSuperconducting -
Images from IBM Scanning SQUID Microscope Group - Scanning SQUID Microscope & the Half-Integer Flux Quantum Effect -
Institute of Applied Physics Justus-Liebig-University Giessen- Squid Sensors -
Stanford -Einstein - Superconducting Quantum Interference Devices
Johns Hopkins University - Applied Physics Lab
NRL - Code 6177 Surface Nanoscience and Sensor Technology - Force Amplified Biological Sensor (FABS) -

Silicon Integrated Circuit

Integrated Resonant Magnetic Field Sensor - a current carrying coil formed in Silicon mechanically vibrates and it's motion is effected by the Earth's magnetic field.  The effect is sensed by means of capactive sensors.
5731703: Micromechanical d'arsonval magnetometer Google patents

Magnetic Stars In Outer Space

On 27 Aug. '98 high energy gamma rays from outer space passed through our solar system.  These came from a Magnetar.  This had effects of HF radio propagation.
Press release.  This is very interesting since it came from outside our Solar system.

Nuclear Magnetic Resonance (NMR)

Stanford - Stanford Magnetic Resonance Laboratory (NMR) -
Varian Inc - NMR page -

Magnetic Reasonance Imaging (MRI)

Magnetic Resonance Imaging FAQs - for people about to get an MRI scan
Magnetic Resonance Imaging on WWW  - good set of links and they are organized
News Groups: sci.physics.electromag & sci.technologies.mag-reasonance -

Magnetoencephalograph (MEG)

Shimadzu - SBI-100 -
Siemens - Schematic presentation of the SQUID system -
Tokyo Denki University - Applied Superconductivity Research Lab -

Superconducting Magnets - mostly used for particle accelerators

Tutorial on Superconducting Accelerator Magnets -
AMI - Superconducting Magnet Systems -
National High Magnetic FIeld Labratory - User Projects - Magnet Technology -
Fermi National Accelerator Laboratory -


A long time ago I had a wire recorder.  Like a tape recorder except the medium was a spool of fine wire.  When it broke, the wire could easily get tangled.  We now have tape, floppy disk and hard disk recording devices.  The early devices used sensors that were just a coil of wire on a magnetic core with a gap next to the magnetic media.  Now the high tech devices like the hard drive in your computer are using MR and GMR read heads.

Santa Clara University -The Institute for Information Storage Technology - Inventor Poulsen and History of Analog Magnetic Recording -
IEEE Magnetics Society -
International Disk Drive Equipment Materials Association - IDEMAJ -
Magnetism and Magnetic Recording Devices - on line book based on tape recorder, but good general information
Magnetic Recording Books  -
Alison Chaiken's Magnetic Recording Web Sites - Excellent extensive, sorted list of all kinds of magnetic recording & other magnetic web sites
NCITS Announces the Public Comment Period for ISO/IEC 13923:1996 - 3.81 mm tape cartridge
National Storage Industry Consortium (NSIC) -
Data Storage - magazine has links: Companies, Advertizers, Organizations

UXO UneXploded Ordnance

The world has an extremely large number of land mines that continue to maim and kill people after the military force that installed them leaves the area.
Unexploded Ordnance Detection Using Imaging Giant Magnetoresistive (GMR) Sensor Arrays - by Alison Chaiken
See my Demining page for more information

MICR (Magnetic Ink Character Recognition) Check Processing

The numbers at the bottom of your checks are read by a magnetic reader, not an optical character reader.  There are 16 characters 10 numbers and 6 special characters.  The early systems used a single gap read head following a write head to magnetize the characters.  The magnitude of the read waveform vs. time was used to recognize the characters at about 1,000 documents per minute.  The character pitch is 1/8".  Some newer higher speed machines use multiple gaps and actually form an image of the character providing a lower false recognition rate at a higher system price.

4107653 Print Specifications for Magnetic Ink Character Recognition, 382/320 ; 235/449
X9 - U.S. MICR standards and others - requires $250/yr fee to access documents
American National Standards Institute (ANSI online) -search on MICR -> Banking, Information and Imaging Standards from a User's Perspective - has a history and lists a lot of standards -
Canadian Payments Association - Standards And Specifications For MICR-Encoded Documents - the actul "Standard 006"
World Magnetics - makes the sensor to read the check numbers
Stanford Research Institute - in 1960's developed ERMA & MICR technology

Magnetic Ink in U.S. Paper Money - NOT - well OK there is something here

The magnetic stripe idea is a myth, in the new paper money.  I received an email for the Treasury and the stripe is a plastic, it glows in UV light and it's position in the bill indicates the denomination.
Coin Currency & Document Systems Inc. - Currency Processing Equipment - MD-3 Magnetic Ink Detector - I don't know what this is
4/25/99 - I have been told that the ink used to print US currency has magnetic properties and a magnetic sensor is used in some bill changers. The Nov 2001 issue of Sensors has an article about reading the magnetic signature of a new $20 bill.

Credit and other cards with a Magnetic Stripe

ANSI on line - standards for magnetic cards - search magnetic cards for a large number of hits
Anatomy of The M.I.T. Card - M.I.T. engineering students analyzing their school ID card
Dorado Systems - Magnetic Stripe Encoders & Readers
Standards for Smartcards -

Anti-Shoplifting Garmet Tags - Electronic Alarm Systems (EAS)

See How Stuff Works for a good explanation.
Electronic Shoplifting Deterrent Systems - Magnetic tag notes - Knogo -

5288980: Library check out/check in system  - references 40 patents going back to 1939 - label reader, not anti-theft

Resonant loop tags

2774060 Detecting means for stolen goods, Thomas F Thompson, 1956-12-11, - a resonant circuit in the tag causes the Tx oscillator to be loaded.  3493955 teaches that this method works very poorly because the decrease in signal is very small. Cited by 76 patents.
3500373 Method and apparatus for article theft detection, Arthur J Minasy, National Bank, 1970-03-10, - Tag is flat coil and cap to make resonant circuit.  - this is a poor design since the effect is very small. Cited by 98 patents.
3521280 Coded labels, Maurice Janco, Arnold T Nordsieck, Charles W Perkins, General Research, 1970-07-21, - microwave resonators, swept source.
3624631 Pilferage control system, Marc Chomet, Donald E Ellison, Robert F Watterson, Sanders Associates, 1971-11-30, - Portal Tx swept frequency, tag: resonant circuit with fuse that's blown at checkout.
4429302: Electronic security system with noise rejection - sweeping through a resonance and counting the pulses out put frequency - Checkpoint?

Mixing loop tags with a diode

3895368 Surveillance system and method utilizing both electrostatic and electromagnetic fields, Lloyd L Gordon, Robert D Williamson, Sensormatic, 1975-07-15, - Portal 915 MHz
4063229 Article surveillance, John Welsh, Richard N. Vaughan, Sensormatic, 1977-12-13, - the whole system
4642640: Signal receptor-reradiator and surveillance tag using the same - mixer type- good overview of mixer tags
4139844: Surveillance method and system with electromagnetic carrier and plural range limiting signals, Willes W. Reeder, Sensormatic,  - mixer type
5349332: EAS system with frequency hopping - calls out 10 prior art patents including the two above
6011474 Multiple-use deactivation device for electronic article surveillance markers Jan 4, 2000 340/572.3; 340/572.6; 340/572.7; 340/572.8 - Sensormatic

Magnetic materials tags

3747086: Deactivatable Ferromagnetic Marker for Detection of Objects Having Marker Secured Thereto and Method and System of Using Same - this patent is called out by 47 other patents so it is a good place to start - (Harmonic tag)
3990065: Theft detection system - magnetic method
4510489 Surveillance system having magnetomechanical marker 340/572.1; 148/304; 148/307; 148/310; 148/311; 148/403; 340/572.8; 428/928 - Electromagnetic (EM tag) mechanical vibration of one of the strips in the sensor causes electrical reasonance. Sensormatic makes these labels.
4553136 Amorphous antipilferage marker, Philip M. Anderson, III, Ryusuke Hasegawa, Robert M. VonHoene, Allied Corp, 1985-11-12, -
5534836  Deactivator for theft-deterrent markers 335/284; 340/572.3 - Sensormatic CD/DVD (EM type)  When the EM tag is magnetized, it is deactivated.
5821859 Concealed magnetic ID code and antitheft tag
5963173 Antenna and transmitter arrangement for EAS system 343/742; 340/575; 343/867 - acousto-magnetic (AM tag,aka magneto-mechanical When the AM tag is demagnetized, it is deactivated.

Transponder tags

These are different from RFID tags in that they have no battery.

3493955 Method and apparatus for detecting the unauthorized movement of articles, Arthur J Minasy, Monere Corp, 1970-02-03, - Portal Tx at 27.2 MHz, 20 Watts (CB rules then).  Portal Rx at 5.1 to 5.9 MHz.  The tag consists of a Rx resonant circuit/antenna, rectifying diode, oscillating transistor and a Tx resonant circuit/antenna.
3859652 Method and apparatus for detecting the theft of articles, Roger L Hall, Edward S Wainwright, Kenneth Clarkson, North American Systems, 1975-01-07, -

MRAM Magnetic Random Access Memory

Conventional DRAM chips use the electric field on an electron to store a binary state.  These need to be refreshed frequently or they loose their data.  By using the spin on the electron to store data magnetically a nonvolatile memory can be made that promises to be faster than DRAMS.  I think the first patent was by Honeywell 4455626: Thin film memory with magnetoresistive read-out
Filed March 21, 1983 for a military version.  Then Nonvolatile Electronics, Inc started development of commercial versions.  They have a good online white paper.  HP, IBM, Motorola and others have dozens of patents in the area.  This technology uses standard CMOS chips with the magnetic element sitting over a single CMOS FET.  It has the potential to replace FLASH chips and hard disk drives.
A computer that used MRAM would be like the early computers that had magnetic core memory.  You would have protection from power outages and have "instant on" operation.  Whatever program you were running when you powered down would come up running when you power on. The military is very interested in this technology and Honeywell is offering products.  In Dec 2000 NVE stopped sub licensing this technology in exchange for a million dollars.

Sensing a Magnet

The idea is to have a device that will detect a magnet while not responding to ferrous metal.
5134370 Apparatus for the detection of magnetic tags (fish) 324/247; 324/253; 340/573.2
4527123 Improved method and apparatus for detecting and locating residually magnetized items (tools) 324/254; 324/67; 324/260; 324/326

Demagnetizer - Magnetizer

The common demagnetizes work by AC line power and have a momentary push button for the power switch.  They generate an alternating magnetic field.  To use one you hole the object far from the unit and press the button then bring the subject to the high field area and away from the field and when far away release the switch.

To use as a magnetizer you need to cycle the pushbutton with the subject in the strong field.  By luck you will power off the field when the AC voltage is near a peak and that will magnetize to subject to some extent.  Commercial magnetizers use a DC pulse or a gated 1/2 wave of the AC line.

The field strength needs to be greater than some value that depends on the material.  For example older magnetic recording tapes might need 400 Oe and modern hard drives might need 4,000 Oe to demagnetize.  Since the Earth's field is about 0.5 Oe it will not erase a tape or disk.  But to get the field high enough to demagnetize it takes a lot of amp turns.

The coil measured 17.5 milli Henrys, but it's hard to say if that's accurate.  It would be about 6.6 Ohms inductive reactance which would limit 110 VAC to 16.6 Amps.  But there's also the DC resistance of 7 Ohms for a total reactance of 9.8 Ohms for a current of 11 Amps.  The line cord is very heavy duty and marked 16/3.

L-W Chuck Co 600
              Watt DemagnatizerL-W Chuck Co 600 Watt Demagnetizer

The central hole is 3 7/8" I.D.
The label reads:
To Demagnetize
Hold work in opening.  Push button and pull work out quickly. (B.C.: maybe should say slowly?)

To Magnetize
Hold work stationary in opening.  Push button, then release button. (B.C.: maybe should say a number of tries may be needed)

Do not hold button down over five seconds.  Do not connect to D.C. current or other A.C. voltage than indicated below.

Mfd. by L-W Chuck Company, Whitehouse, Ohio
Volts: 110   cy.   60, Watts: 600
Form R4   Serial 3415

D34015 Demagnetizer, Otto M. Knoblock, Jan 29, 1901,

Plastic Block type

YouTube: How do these Magnetizer/Demagnetizers work?, 9:10 - use of the steps for demagnetization - uses Eisco Labs Magnet Probe.


There are two types of magnetic measuring technologies. 

Gauss meter - An instrument used to measure the instantaneous value of magnetic induction, B.
                      Typically an AC instrument used for 50/60 Hz power line magnetic fields and ghost hunting.

Magnetometer - An instrument for measuring the magnitude and direction of a magnetic field.
                         Typically a DC instrument where the earth's field is full scale.

Flux meter - An instrument used to measure magnetic flux by the current it generates in a coil.
                   Can be either AC or DC and used to measure magnets.


U.S. Surface Gravity Calculator - Need Lat, Lon and Elevation as inputs.
See the U.C. Berkley on line tutorial  Types of Accelerometers  for some information.
Tutorial on Inertial Navigation Systems - with sensor errors in an appendix, 84.46 minute Schuler oscillation derivation

Force Balance

Jewell - Inertial Sensors, Accelerometers , Inclinometers -
Systron Donner 4310 series - web page with patent links (maybe the same as Jewel)

Micromachined (aka MEMS)

These have a beam with many fingers that are interleaved with other fingers that are at ground.  The capacitance between the fingers on the beam and the grounded fingers is measured.  In this was acceleration can be measured.
Analog Devices - Made for automotive air bag, New ADXL150 & ADXL250 lower noise versions of the popular older ones.  They have a large number of app notes on their web site covering all kinds of practical areas of concern like temperature dependency in DC coupled systems and noise reduction.

ADXL202 - Low-G, 2- axis - Crossbow Evel Kits - has PWM output for direct microcontroller interface.  Note that this is the first acceleration sensor designed for static gravity work.  The other sensore have been aimed at the automotive air bag crash detection market.

Rocket Altimeter Project (RAP) - PIC micontroller based with ADXL190

Brooke's ADXL202 web page

KMI - sisemic sensor - used to shut down equipment when strong motion quake detected
Button-free Tilt and Gesture Input for Itsy - new imput method for PDA
Adding a tilt sensor to the PalmPilot - PalmPilot Programs and Hardware Hacks -
SmartQuill - Handwriting recognition is achieved through accelerometers that monitor hand movements
Microsoft Corporation's Hardware Division for its new SideWinder® Freestyle Pro game controller

5621157: Method and circuitry for calibrating a micromachined sensor
5540095: Monolithic accelerometer
5465604: Method for adjusting sensitivity of a sensor
5345824: Monolithic accelerometer

Applied MEMS - 3 axis accelerometer withlow noise.  double integration for 60 seconds gives less than 0.1 inch movement on a fixed sensor.
Motorola - Like the Analog Devices model it is a micromachined Silicon sensor aimed primarily at automotive air bag applications.
     05814727 09/29/1998 Semiconductor accelerometer having reduced sensor plate flexure
     05590241 12/31/1996 Speech processing system and method for enhancing a speech signal in a noisy environment
     05545912 08/13/1996 Electronic device enclosure including a conductive cap and substrate
     05487305 01/30/1996 Three axes accelerometer
     05313835 05/24/1994 Integrated monolithic gyroscopes/accelerometers with logic circuits
     05249465 10/05/1993 Accelerometer utilizing an annular mass
     05044201 09/03/1991 Double-integrating silicon acceleration sensing device
     04677913 07/07/1987 Safe and arming device
     04378510 03/29/1983 Miniaturized accelerometer with piezoelectric FET
     05818093 10/06/1998 Semiconductor device having a movable gate
     05400644 03/28/1995 Knock detection system
Silicon Designs, Inc. - Combines the micromachined chip with conditioning electronics.
           4736629: Micro-miniature accelerometer
Kistler Instrument Corporation - Piezoelectric, Piezoresistive and Variable Capacitance Based Sensors.  These tend to have lower noise than the MEMs type sensors.
05347841 09/20/1994 Self-testing transducer curcuit
04771637 09/20/1988 Accelerometer
Draper Labs -
5507911: Monolithic micromechanical vibrating string accelerometer with trimmable resonant frequency
5817942: Capacitive in-plane accelerometer
Entran - Accelerometers - small & Automotive including racing and R&D

Freescale Semiconductor (formerly part of Motorola) - 3 axis sensor on one chip MMA7260Q.  In addition to the 3-axis capability this chip has dynamically programmable acceleration ranges.  It appears to be the replacement for the ADXL202.
Kit 3112 G Select - contains the MMA7260Q XYZ sensor, and a uC along with software, etc.
The 3112 refers to App Note 3112.  There is another kit that just has the SMT sensor on a board with a few parts to make it easier to work with.
6936492 Single proof mass, 3 axis MEMS transducer August 30, 2005 438/50-Fig 4,5 & 6 is the invertion FIg 1,2,3,7 & 8 are prior art.
6845670 Single proof mass, 3 axis MEMS transducer

14 Feb 2006 - I tried to order the Eval Kit a month or so ago after receiving the Freescale email that I subscribe to, but it was not on the linked web page.  Today I tried to use the Eval kit and when reconciling the packing list with what I received the discrepancy is with what the packing list calls a "Quick Card".  I would hope this would be a how to use it guide and not the 3x5 card with sales info.

There is nothing in the first or second level folders on the CD other than sales hype.

After calling the support number three times no one could help in answering simple questions about how to get it to work. 
Call 1 - I was given the phone number of a Field Applications Eng, but he no longer worked there.
Call 2 - after explaining what I wanted and righting down the part number of the eval kit the support lady hit the disconnect button.
Call 3 - The support person tried to find anything in the eval kit to answer my question "Hot to make it work" and after giving up started a Service Request.

The email in response to my Service Request pointed me to app notes that are mostly sales hype but there were a couple of paragraphs describing how to get one of the stand alone functions to work (free fall detection), but the other (battery saver) did not work.

I did find the PC software, but it's buggy and when you click on help the message is . . .  that there is no help, come back later.

It seems that Freescale either has an incompetent person in charge of marketing, the product was rushed to market before there the support structure was in place or there's some other problem.

This is the second and probably the last eval kit I get from Freescale.  The RD1979MPXM21042A Barometer was also a disappointment.

The eval kit has a free fall detection function that uses the on board battery power and an LED.

Spark Fun - has a number of products supporting the MMA7200Q -

Kerfott -
5656778: Micromachined acceleration and coriolis sensor
4945765: Silicon micromachined accelerometer
Crossbow - Silicon Accelerometer based products & Analog Devices eval kits
Breed Electronics - Lakeland, FL
(Sumitomo Precision Products) Silicon Sensing Systems Japan Ltd - Capacitive Type 3-axis Accelerometer C3AP - Links -
Integrated MEMS Inertial Measurement Unit - accelration & gyro sensors on a chip
Manufacturing Low-Cost MEMS Inertial Sensors - Draper Laboratory
Flexible Manufacturing of Dissolved-Wafer Silicon Capacitive Sensors (CAPS) - Integrated Sensing Systems, Inc.
VTI Hamlin - SCA600 -
Measurement Specialties - MEMS Acceleration - ICS3052

Silicon Piezoresistive Accelerometer

Fujikura Ltd. - FACS Accelerometers -Single Chip 2­ and 3­ axis signal conditioned, 1 and 2g
Oceana Sensor Technologies - Low cost, Piezoelectric Accelerometers - Eval Kits -
Morgan Matroc -  PZT piezoelectric ceramics manufactured for specific application
Mitsubishi Electric Corporation - Semiconductor Acceleration Sensor - Because of their ability to detect accelerations of static characteristics, low-acceleration sensors can be applied to Inclination meter, Vibration detection, game-playing equipment, etc. High-acceleration sensors have a high frequency response and can be applied to airbag systems based on analysis of shock waveforms and other signal waveforms.
Summit Instruments - ? Technology
Measurement Specialties Incorporated (MSI) - piezo thin films, analog test printed circuit board - with schematic
EG&G (now called PerkinElmer, Inc) -Data Sheets -3031, 3038, 3022, 3028.    3140- 2G, 3145-2G 0.5 - 5.0 V out.   3255- 50, 500 G, 3265 - 5G - 3052 & 3058 -2G have internal temp comp capability good for quality DC apps
GS Sensors Inc. -FA1131 -  Ephrata, PA


Electromagnetic & Capactive
Laser Vibrometers
Oncque - Vibration switch - Ball in Cage switches.
spring over Rod or in tube
spring supported at both ends
sensitive at right angles to long axis
Pedometer - 2 balls held by spring
Sensitive along long axis
2 balls trapped between hemispherical ends
sensitive in all directions
1 ball 4 terminals stock application cross wired to form 2 terminals
Ball normally no contact, under jerk contacts 2 of 4 terminals

2369296 Electrical switch, Emmett M Irwin, George E Lewis, Richard H Duval, Bruce H Rule, Ray M Chenoweth, Cooperative Development Co, Priority: 1938-10-04, W.W.II, Pub: 1945-02-13, - is a relay where the moving part is a ball bearing.  Sort of a ball in cage.
Part of patent 2336101 Well Survey System.

Metrix model 5160DV vibration meter.  Uses a piezo accelerometer as the sensor and then the electronics processes the signal to get either Displacement or Velocity hence the DV suffix on the 5160DV model.

To change the 9V battery remove 2 screws at probe end and slip off sleeve.
The whole meter movement acts as the knob for the switch which has a "D" shaped brass shaft.
Too much torque on the "knob" will break the switch shaft (like if you try to open this instrument by unscrewing the meter).
                          model 5160DV vibration meter.

Variable capactance
Servo (Force Balance)
5808198: RF balanced capacitive vibration sensor system


A device to determine how far a person (horse?) has gone.   Used in modern dead reckoning systems.
5724265: System and method for measuring movement of objects - this patent references many others
4991126: Electronic-automatic orientation device for walkers and the blind - distance & direction
SignalQuest - Ball in Tube sensors
see Oncque VBS02 just above
Also see Ship's Log \ Pedometer


Remec - Humphrey - thermal accelerometer (click on New Products)
The IC is carried by Fat Quarters.  This is a patented low bandwidth acceleration sensor that depends on the inertia of heat. They have an app note on making an Earthquake sensor based on this unit.
Memsic - Dual axis, analog 7 digital outputs, absolute or  ratiometric - a # of Evaluation Boards - June 01 Sensors mag article - Comparison of Accelerometer Technologies by Memsic Inc.

Earth's Gravity

Links to Gravity Groups that have FG5 absolute gravimeters (and Web Sites)
The Earth's gravity has been very thoroughly mapped in areas where rockets are launched.
A cleaver way to measure gravity is to drop, in a vacuum, a retro-reflector whose position is being measured by a laser in real time.  See the Micro-g Solutions web page for information on this method.  Note that the acceleration due to the Earth's gravity is NOT 1.0 every where.  You should not use 1.0 as a value for calibration of accelerometers unless you have measured the acceleration at your test location first. A-10 Outdoor Absolute Gravimeter -
Intelligent Detection Systems - Scintrex CG-3 - Seagrav -
LaCoste & Romberg - Dr. LaCoste - a pioneer in the development of these instruments - has a large line of meters - Portable Earth Tide -
Scintrex -
Carson Services Inc./Aerogravity Division - US5661649: Airborne gravity and magnetic field surveying method
NSF Absolute Gravity Facility - uses the FG5
24 April 2001 LaCoste & Romberg, Scintrex, and Micro-g Solutions Merge
Possible new theory of everything by David Allan (Mr. TIME) can be tested by measuring gravity

Seismometer -

 Seperate Web Page for Seismometer & Geophones
Redwood City Public Seismic Network - Home construction information & links
USGS Earthquake page - automated reports
Don Lancaster's info -
Geometrics - R and RX Series Seismographs - SmartSeisTM Exploration Seismograph  -
News Groups:  sci.geo.earthquakes & seismic.general -
Seismo-Watch Earthquake Reports -
Kinemetrics - Products - App Notes - Links -
Pinpoint Earthquake News - Interesting links to a lot of diverse areas
UC Berkeley Seismological Laboratory - Hayward Fault - Seismometers- Make your own seismogram! -
Simple seismograph -
Welcome to the Caltech/USGS SeismoCam -
University of Wisconsin - Milwaukee's Seismic Station -
How to Build an Inexpensive Seismometer -
SEG Virtual Museum - Seismic Items
Outdoor Intrusion Detectors - many of these use geophones as seismic sensors
Hugo Benioff Seismometer - uses a changing air gap (variable reluctance) and a 100 kg weight suspended by bicycle spokes.  Damping by oil dash pot.  Complete unit is about 250 pounds.
Silicon Designs - Model 1221 -
GangZhen - JC-V104 Short Period - MBS-1 Broadband -
Güralp Systems - CMG-3T Broadband - Realtime (updated every 10 minutes) from Wolverton, UK.
Colibrys Inc. (formerly Applied MEMS) - replacing geophones in oil & gas exploration - Si-FLEX (TM) Series - Strong Motion - Unmanned Ground Sensor (a classical geophone app)-

Electrolytic Acceleration Sensors

These are different from electrolytic tilt sensors.
eentec - Electrolyte movement sensing has low current consumption and low noise
6576103 Electrochemical transducer and a method for fabricating the same June 10, 2003 class: 204/412; 29/592.1; 204/252 - assignee PMD Scientific -

2735949 Electrolytic Acceleration Sensing Device Feb 21, 1956 class: 310/300; 73/514.09; 361/500 - Mercury drops in sulpheric acid where the mercury is held by bands, but the inventor did not understand how it worked.

Seismometer Patents  367/1 about 10,000 patents,  367/182 about 84 patents

5996416 Seismometer 73/655 ; 73/1.85
4466084 Seismometer 367/182 ; 367/184; 73/652
4188612 Piezoelectric seismometer 367/160 ; 310/329
4152692 Low distortion, high sensitivity seismometer 367/182
4107643 Unidirectional horizontal seismometer 367/178 ; 181/122; 73/653; 73/665
2348225 Magnetic Seismotomer Feb 13, 1940 367/183 ; 267/161
1784415 Electrical Recording Seismograph, Dec 9, 1930, 340/870.16 ; 310/15; 340/870.31; 340/870.44; 346/107.1; 346/65; 367/179; 367/182; 73/654


Two popular outputs from pressure sensors measuring the atmosphere are the barometric (like in barometer) pressure and displaying the answer in units of altitude.  These same sensors come in various pressure ranges and are also used for measuring, for example, the pressure in a pipe or other pressure sensing application.

I know of a security system where they had copper tube that looked exactly like bare #10 ground wire like used in electrical systems.  It was run in the electrical conduits and ended up in some fixture in a room that was being protected.  At the other end in the security center there was a very sensitive pressure sensor.  If anyone opened a door or window or cut a hole in a wall in the protected room they could sense the pressure change and it set off the alarm.
 Exploratorium  - Sound Spectrum Chart -

Simulator, Barometric Presssure, SM-384/URM-125, Sperry Utah Company, Division of Sperry Rand Corp., Order No. 682-PM-58, U.S.
This is an instrument (9" high, 4.75" wide and 14" long) that contains a manual pump, valves and a calibrated barometric altimeter.  The range is - 2,000 to + 18,000 feet.  It does not have a provision to set the local pressure, i.e. it is an absolute reading altimeter.  Available from Herback & Rademan as TM98GGE3132.
Name Plate,    Front Panel,   Diagram -


Advanced Custom Sensors Inc.-
DH Instruments, Inc - pressure & flow calibration
Entran - Sub-Miniature Silicon Chip Diaphragm - Silicon bridge
Exar -Click on Silicon Microstructures
FISO Technologies Inc. - Fiber optic 0.55mm OD x 0.5 mm size -50 to 300 mmHg
Fuji - pressure sensitive film changes color to record max pressure
Honneywell - High end - Pressure and Force Sensors - industrial
Integrated Micromachines Inc. - Silicon Low Pressure Sensors -
Integrated Sensing Systems Inc. - ISSYS - Pressure Sensing -
Intersema - absolute pressure - 1 meter resolution altimeter - MS5534B sensor 10 - 1100 mb 15 bit A/D
Kavlico - various technologies available 1/2" h2o to 20,000 psi
Lucas - NovaSensor - Better linearity than Motorola - Many Versions of the NPC-1210 are available from 1800 962 7364 x6118.
Melexis - tried to use change in ring oscilation frequency as measure of pressure in their MLX90211 Altimeter pressure sensor - not recomended for new designs
Mitsubishi - piezoelectric-resistance Pressure Sensors -
Motorola - Extensive app notes: Click on Sensors then Pressure Sensors
The Motorola sensors business has been transferred to Freescale Semiconductor.
I tried their RD1979MPXM2102A Evaluation board.  It has the sensor output conditioned by a 3 ap amp instrumentation amp circuit, but then it feeds a Morotola uC with only 8 bits A/D.  This makes it all but useless.  And there's no test point to hookup a DVM.  While a TQM-34 barometer in the same room was showing readings between 986 and 976 millibar the eval kit showed a constant reading and the history for a few days showed constant, with no forecast as the weather went from sunny to heavy rain.  The App Note associated with the eval kit shows that you need about 16 bits of A/D to get resolution in the 1 foot area, 8 bits gives tens of feet.  Also the eval kit elevation input sub menu did not work as it should.  I.e. when I put in my 900 foot elevation and then go to the barometer altimeter function the altitude displayed was -3070 feet.  By inputting that number plus 900 feet I got it to read 900 feet, but that's not how it's supposed to work.  There's a procedure to set two calibration points that probably was not done at the factory and it's not something that I can easily do.  NOT recommended.
Pressure Systems - quartz very high accuracy sensors
SensoNor - SP10 tire  Patent 5591679: Sealed cavity arrangement method - method for making sealed cavities on silicon wafer surfaces, they also make accelerometers for crash sensing
Sensortechnics - high pressure piezoresistive with fittings
Setra - packaged with fittings, long term stability
Servoflo - Fujijura pressure & Oxygen sensors -
Siemens - 100 pressure sensors in a mat to go under passenger seats in vehicles.  Can recognie baby seat and size of occupant.
Smartec - Pressure - low pressure range units for blood pressure and others for barometric (0 - 5 psig  $15 online at SensorGuys)
Standard MEMS, Inc. - Silicon On Sapphire Pressure Transducers - virtually hysteresis-free sensing
TI has acquired Integrated Sensor Solutions - Industrial & automotive pressure sensors - steel & brass housings
Visteon - Automotive Products - MAP sensor.pdf -
For a low cost sensor the MAP (Manifold Absolute Pressure) sensor form a late model GM car should provide a rugged temperature compensated unit for almost nothing from a wreacking yard, although I have not tried it.  If you do let me know.
VTI - SCP1000 Series - MEMs technology 17 bit SPI digital interface 30 - 120 kPa 17 bit A/D 1.5 Pa res 10 uA current SPI or I2C


Altimeters - Pretel - Thommen - Avocet - Peet - complete products rather than just a sensor
Altimeters - Thommen Pocket - Suunto/Pretel ALTIplus - AIR, Inc. -
Aviation Safety Reporting System - International Altimetry -
Revue ThommenAirspeed Altimeter - at Abnet - at Gioielleria Fabbrini -
Avocet - Vertech - Barometer - Altimeter - very accurate
US Patents:
5058427: Accumulating altimeter with ascent/descent accumulation thresholds
5295085: Pressure measurement device with selective pressure threshold crossings accumulator
4694694: Solid state accumulating altimeter
Ciclosport - HAC4 - bicycle data system with 1 meter altutide resolution (katalog-catalog)
Peet Bros. - Pocket Altimeters - Book: "The Sportsman's Altimeter/Barometer"
4969358: Electronic altimeter/barometer
5001929: Electronic altimeter
Casio - Timepieces - Altimeter-Barometer - Forester ATC1200-1V - G-Shock Alti-Meter Watch - Twinsensor Alti-Thermo - Pathfinder -
Dalvey - Barometer Altimeter - conventional aironoid type
Suunto USA - Vector Altimeter compass watch -
Swift Instruments -Weather478 Altimeter / Barometer -
Geneq (many types of sensor)- Barometric Pressure Sensor II - Micro Series Surveying Altimeters - Air Inc. Models -
Using an Aircraft Altimeter as a Barometer  -
Back Yard Nuclear Weapons - The Mechanism of The Bomb - altimeter used in fuse
John Coker's Rocketry - Altimeter Comparison with web links - Aerotech hybrid rocket motor


Microphones are the common sensors for picking up sound.  One application is locating gunshots in cities.  It seems that a way better than the patents below would be to use a wireless internet packet based system.  This would also allow Network Time Protocol synchronization of each sensors clock.  The trick is in coming up with a good microphone and signal processing package that minimizes false alarms for each pole unit.

Note that soldiers can tell what type of gun by just hearing the sound.  An M16 makes a sound that's different than that from an AK-47 which in turn is different from an M60, etc.  Some of the below systems say they can do this but do not tell how.  Many patents disclose just enough information to be granted while keeping some details as trade secrets.
The military GRA-114 system uses the "boom" to locate enemy artillery and mortars.

6965541 Gun shot digital imaging system November 15, 2005 Johns Hopkins University 367/118 - aimes TV camera to source of shot
6847587 System and method for identifying and locating an acoustic event January 25, 2005 367/127; 367/906 - Time of Arrival system with signal processing at the microphone.
6185153 System for detecting gunshots February 6, 2001 US Navy 367/124; 367/118; 367/906 - adds min and max pulse width
6178141 Acoustic counter-sniper system January 23, 2001 GTE 367/127; 367/124; 367/906 - military bullet type
5973998 Automatic real-time gunshot locator and display system October 26, 1999 Trilon Technology 367/124; 367/129; 367/906; 367/907 - Menlo Park, SRI system based on Earthquake location methods.  While the Showen system radically reduces the sensor density compared to prior art systems, as high as 80 sensors per square mile, each sensor, nonetheless, still requires a dedicated phone line. As one can see, to effectively monitor a large metropolitan area, an outrageous number of telephone lines would be required, resulting in a substantial investment in infrastructure, not to mention large on-going costs. Such systems can often take months to install.
5703835 System for effective control of urban environment security December 30, 1997 367/124; 367/129; 367/906 - see 5504717
5586086 Method and a system for locating a firearm on the basis of acoustic detection December 17, 1996 367/127; 367/906 - aimed at supersonic bullet direction, not urban area coverage. The system comprises an antenna having at least three microphones that are spaced apart from each other and connected via a microphone conditioning stage to a processing and display unit. The processing and display unit includes means for processing the signals and for determining the location and possible type of projectile being fired.
5504717 System for effective control of urban environment security April 2, 1996 367/124; 367/129; 367/906 - City Gunshot detection 50-14,000 Hz input bandwidth on pole units.  peak detection of 3 ms events and averaging of longer time events.  The system detects potential gunshot signals using a dense grid of "pole units", i.e. a collection of microphones placed on utility poles at every street corner (i.e. about every 500 ft.) in an urban area to be covered. Since this high density grid requires more than 80 sensors per square mile covered, it is very expensive to deploy and maintain. Sharkey does not, however, disclose or suggest any method for reducing the number of pole units required by his system, and overcoming this significant disadvantage.   The acoustic signals at each pole unit are analyzed to identify potential gunshot events. In particular, if a signal at a pole unit is 5 times louder than the average noise level, then an event is reported. A central processor uses the relative time data from reported events to perform triangulation and locate the origin of the gunshot. Sharkey mentions in general terms some signal processing techniques which may be useful in distinguishing a gunshot from other sounds. The mentioned techniques, however, apply only to the analysis of signals from single sensors. That is, these techniques are limited to discriminating gunshots from other types of sounds at each pole unit independently of the other pole units. Notably, Sharkey does not disclose any technique for detecting gunshots through the inter-dependent analysis of multiple signals from multiple pole units. Nor is it at all obvious from Sharkey what specific techniques one might use to perform concurrent analysis of several pole unit signals to improve the reliability of gunshot detection. Sharkey also does not disclose any technique for determining which of several potential triplets of sensors should be used in the triangulation to obtain the best location estimation.

The below patents are related, but not really aimed at city gunshot detection
5046053 Acoustic signal detection circuit September 3, 1991 Cyber Scientific 367/98; 367/127; 367/900; 367/907 - stylus location based on acoustic travel time
4279027 Acoustic sensor July 14, 1981 Honeywell Inc 367/125; 340/943; 367/117 - time when bullet is closest to microphones
4091366 Sonic monitoring method and apparatus May 23, 1978 J.H. McDaniel Tele-Communications 340/524; 340/539.1; 340/539.14; 340/540 - sound level triggered alarm (this may be the Sonotrol burglar alarm method?)
4083031 Acoustic presence detection method and apparatus April 4, 1978 US Navy 367/135; 367/129; 367/901 - water borne vessel detection
4001771 Intruder detecting security system January 4, 1977 IBM 367/129; 340/517; 340/566; 367/127; 367/136 - only alarms if the noise comes from a guarded area.
3854117 - discloses a passive system employing two or more microphones in a direction finding scheme. A phase difference detector is provided to receive first and second versions of two different sources of a common input signal to give a non-linear output indicating the phase differences. The reference clearly does not comprehend measurement of the closest point of approach since it is strictly designed to locate the direction of a sound source.
3341810 - ultra-sonic helicopter based
3147384 Flash detection and location system, Fenton Stuart J, Stoliar Arthur P, Bulova Watch Co Inc, Sep 1, 1964, 250/231.14, 250/347, 356/141.5, 89/41.6, 356/141.4, 356/622 - Motor rotates glass plate.
2966657 - wherein the particular application of the apparatus is employed for locating the origin of sound waves due to gun fire and other similar sound-wave generating sources. Microphones or "cosine law" microphones are disclosed in an arrangement where signals of opposite polarity are produced when the signals are received from opposite directions in combination with an omnidirectional third microphone. Circuitry is provided to permit the determination of the arrival direction of sound waves through the processing of these three microphones.
2459162 - acoustic apparatus is disclosed for determination of the direction of a sound wave source. A receiver is mounted on a rotatable shaft which then is employed in combination with a reflector such that there is a visual means controllable by the receiver and the reflector for indicating the direction of compressional or sound waves.

Infrasound (Wiki)

Sound in the .02 to 10 Hz range. The seismic outdoor intrusion sensors, like the PSR-1 use geophones to sense the infrasound and up convert it to something that a human can hear.  You can instinctively tell the difference between a man, woman or child walking, a cat, a dog, etc.
Infrasonic Symphony The greatest sounds never heard - overview
The Elephant's Secret Sense: The Hidden Life of the Wild Herds of Africa by O'Connell, Caitlin (Wiki), 2007
The Secret Lives of Elephants: Caitlin O'Connell-Rodwell at TEDxStanford

Air Seismology -
InfraSonic Introduction - InfraSonic Links by VK2ZTO now in pdf file that can be dlownloaded from here
Infrasonics Program - Univ of Alaska
Parapsychology - A Sound Theory? - may explain some haunted buildings
EarthSky Communications - Infrasound-from-volcanos-might-warn-airplanes, Researchers learn elephants' secret language, The extensive vocabulary of African elephants
Elephant Infrasound  -
Geotech Instruments, LLC - Latest Projects - Korea Seismic-Acoustic array.pdf -
Honeywell - Data Instruments Products - DCAL401DN
Listening to the Heartbeat of the World -
Infrasound Monitoring with a Microbarograph -
Infrasound monitoring of the Comprehensive Test Ban Treaty -
MKS Instruments - Type 223B Baratron Differential Capacitance Manometer -
Nuclear Explosion Monitoring Research & Engineering Program - Infrasound Monitoring Research - United States Infrasound Prototype -
NOAA - Infrasonics Program - weather and natural phenomenon related
Sonic Weapon of Vladimir Gavreau -
Swedish Institute of Space Physics, Umeå -
Comprehensive Test Ban Treaty Organisation (CTBTO) - Infrasound monitoring -
Bringing together seismology and atmospheric sciences -
The Inexpensive Infrasound Monitor Project - unit for sale
7269537 Infrasound sensor with disturbance filtering - many references
20110098950 Infrasound Sensor, Symphony Acoustics, Inc. - optical based sensors 8797548 - New Projects (Feb 2015): Infrasound Microphone - Infrasound -
Hyperion Technology Group - Sensors rejecting seismic energy
Chaparral Physics - Sensors -
Operations Manual for the National Center for Physical Acoustics Infrasound Microphone - National Center for Physical Acoustics - detect and monitor severe storms
Citizen Scientists League - Infrasonic Microphone Part I
Infiltec Model INFRA20 Infrasound Monitor & USB Adapter & Data Logging Software - eBay seller: infiltec_factory - The Inexpensive Infrasound Monitor Project
NASA Langley Researchers Nab Invention of the Year for Infrasound Detection System
US20090022341A1 Extreme Low Frequency Acoustic Measurement System, Qamar A. Shams, Allan J. Zuckerwar, (NASA), 2007-07-20 - 24 prior art citations,


Sound above 20 kHz
SensComp - was Polaroid, Electrostatic transducers & Piezo transducers - as well as ranging modules
Listening to Bats

Light =40 nm to 1,000,000 nm

VUV (Vacuum UV) 40 to 300 nm
UV is 320 to 400 nm
Visable 400 to 750 nm in wavelength
Near IR 750 to 3,000 nm  not body heat, viewer requires light source M18 binoculars
Mid IR 3,000 to 30,000 nm
Far IR 30,000 to 1,000,000 nm
The atmosphere has a window that passes visable light, IR around 10,000 nm and radio in the 5 cm (6 GHz) to 10 meter (30 MHz) range,
all other wavelenghts are blocked by the atmoshpere.
Exploratorium - Electromagnetic Spectrum Chart - a verry well done chart
Hamamatsu app notes: Optical Detector Selection: A Delicate Balancing Act - Silicon Photodiodes Principle of Operation -


EIT, Inc. - radiometers for UVA (320nm-390nm), UVB (280-320nm) , UVC  (250-260nm), and UVV (395-445nm) mainly for UV cure process characterization
International Light, Inc. - 250-1100 nm spectroradiometer
NSF - Polar UV Monitoring Network - SUV-100 high resolution, scanning spectroradiometer - ozone work
Multi-channel Solar Ultraviolet Radiometer - for ozone work
Optix - UVA&UVB Radiometer 300 ÷ 370 nm
OL-754: UV/VIS/NIR Spectroradiometer 200 to 1600 nm
Wedgewood Technology - AF44 UV Sensor -
Uvitec - uses filter over Si detector
Grunder - Schottky-type UV-photodiodes 230 - 370 nm in TO-18 & -39 transistor packages with no visible sensivity
Apogee - hand held UV meters 250 - 400 nm
Spectrex - SharpEye UV Detectors - flame detection 185 - 260 nm  1 sq.ft. gasoline fire at 50ft
Solar Light - UV light sources, solar simulators, and radiometers. sunscreen product development and SPF certification worldwide.


T.I. - Eval Module - TSL-230 Series Light to frequency converters with very wide dynamic range
TAOS - has taken over the TI line of optical sensors - TSL230 Evaluation Module for $100
Gigahertz-Optik GmbH - both sources and measuring instruments
Visible Haze Sensor -make using LED (used as a narrow band optical detector) and op amp. 
Huygen - makes the Weston 594 (and a more modern hermetically sealed unit).  These have a spectral response that's matched to the human eye very closely.  Based on a Selenium diode, not Silicon.

Lateral Photo Diodes

 X-Y output of light spot position.  This is NOT a 4 quadrant detector but rather a very unique way to locate a spot of light very precisely.
UDT Sensors Inc. -
Hamamatsu  - Position Sensitive Detectors -
Steinbichler - COMET 100 Digitizing Sensor - 5289264: Method and apparatus for ascertaining the absolute coordinates of an object
GOM - The Mobile Optical 3D-Coordinate Measuring Machine -

Dual Diode

Osram (formerly Philips, Siemens) BPX 48 & SFH 221 - two photo diodes (0.7 x 2.2 mm) with a very narrow gap (90 um) between them.  400 to 1150 nm range.  So if for example a star is imaged on the part the two diodes will only have the same output when the image is split between the two diodes.  The LT1057 may be a good choice for an amplifier.

Avalanche Photo Diodes

Advanced Photonix, Inc. - 581 series

Silicon CCD

The quantum efficiency of Silicon for capturing light is way better than the human eye or photographic film.  This is why the astronomers are using CCD arrays for viewing objects in the night sky.
Mitsubishi - artificial retina - CCD array & image processon in one package 32x32 or 128x128 pixels - aimed at low cost applications like games and surveillance, hand gesture recognition, etc.  but may have many other uses.

GameBoy has a camera add on that uses the aritficial retina

Search for "game boy camera"
Inside Game Boy Camera - the start of some reverse engineering
GameBoy Developers -
AVR + GameBoy(tm) Camera = Fun -
M64282FP Artificial Retina Technicial Information -


Tektronix -
Sencore - CP288 - Auto "ColorPro II" Color Analyzer -
International Light -



Servo - IR detectors. - Thermistor [2,000 - 20,000 nm]( traditional bolometers both hand made & semiconductor type) & Pyroelectric [2,000 - 15,000 nm](Lithium Tantalate &  P(VDF-TrFE))  hi-end stuff.  This is long wavelength IR.
Photonics IR Detector manufacturers list -
EG&G - IR Detector page listing many chemistries and their wavelength range -
Opto Diode Corp. - IR emitters & detectors
I.S.I. Group, Inc. - VideoTherm® 2000 : 8 - 14 micron uncooled  focal plane array camera
Fermionics - HgCdTe Infrared Photodetectors - room temp to 2.8 um, thermoelectric cooled to 6 um, LN to 16 um

Hamamatsu - IR detectors - response curves for different types -
Litton's Infrared Product Group - IR line and area arrays -
Linear Laboratories - Non contact IR temperature measurement

Vishay Telefunken - automotive winshield Rain Detector TSRF900 -
CMC - Cincinnati Electronics Corporation - IR detectors - IR Imagers -
Thermometrics - MEMS & CMOS IC in same package thermopile sensor (measure temperature of objects)-
    filters: standard 6 - 13 um for tire temp, ear thermoters, human body detection
             F1          8 - 14 um for occupancy detectors 7 intrusion alarms
             F2          2 -   5 um for flame detectors
             F3          1 - 10 um flame detectors & high temp thermometry
Judson - 800 to 26,000 nm & 4 quadrent position sensors

Radio Shack 276-145 is an IR Phototransistor in a two lead Clear T 1-3/4 LED package.  The clear package allows visible light to bias the transistor in addition to IR light.  For REM sensing a black plastic that blocks visible light is much better.

IR temperature

Melexis Microelectronic Systems  - MLX90614 - good to -70 deg,  SMB (I2C) interface
Mikron -
Raytek -
Smartec -IR sensors -  SMT-IR-9902SIL has a 7 degree view angle ($12.60 from SensorGuys) just the sensor and reference 1k thermister

IR Cameras

Electrophysics -
FLIR Infrared Camera Systems -
Indigo Systems -


The Oxenhope automated observatory has it's own Weather Station that includes a cloud sensor.  These are for use at night.  A Peltier junction is sandwiched between two plates.  On a clear night the sky is colder than the Earth and a voltage is generated.  When it's cloudy the output is lower.
Cloud Sensor revised (12/06/97) - description of the above sensor -Herbach and Rademan - Part Number TM92HVC2178 -
The Hanna City Robotic Observatory - Description of the cloud sensor electronics & circuit diagram -
Qualimetrics - 8329-A/B Laser Ceilometer -
Vaisala  - CT25K Laser Ceilometer -
PMS/CSIRO Hot Wire Liquid Water Probe -
ARCS Data Management Facility - Tropical Western Pacific Instrumentation - SKYRAD Upward Pointing IRT - Infrared Thermometer -
Synergy of Active Instruments in the Earth Radiation Mission - collection of papers
YES - YES is currently developing an automated cloud observation sensor based on a solid state, digital charge-coupled device (CCD) camera.
Penn State Dept of Meteorology - Measurement Systems and Field Sites -
Scinetific American - Detecting "Hot" Clouds -
Astronomical Society of Australia -Introduction - Thermopile Cloud Detector - Sensitivity to Cloud - Conclusions - References - 8 to 14 micrometers
Advanced Visible and Near-Infrared Radiometer (AVNIR) - satellite
My Cloud detector using the H&R TM92HVC2178 & a year 2005 10 nm IR thermoter
The idea of using the Sun's shadow will only work during the day time.


This would be the sensor to use in a swimming pool to determine the optimum time to clean the filter.
Honeywell -

IrDA (InfarRed Data Access?)

This is a protocol for transferring data by means of IR.  It is used in many lap and table top computers and is available as an add on product for any computer.
Aiglent (HP) - HSDL-1100-017

CCD - my web page about astronomical CCD sensors

Intelligent Vision

There are some light based sensors that have intellengence added.
Agilent - optical mouse sensor
Corrsys-Datron - "pupil splitting" technology HS-CE 3-Axis Measurements


Distance Measuring

By using various techniques the distance to an object can be measured with light.  Two big applications for this are military targeting systems and land surveyor "total stations" and "Distance Measuring Equipment (DME)".  The surveying models typically work with retro-reflectors.  A more consumer oriented application is the BushnellYardage Pro range finder(FAQ) for golfers that reads out in yards or meters.  Leica makes a line of OEM range finders with millimeter read out, binoculars  that have RS-232 output and a built in compass. Leica makes the Disto,  a hand held unit for the construction industry that is also carried by Edmond Scientific  It has mm resolution.  $895 to $1700 with web ordering.  Some of the Disto models have RS-232 interfaces and there is free Leica computer software and third party software for mapping.

Riegl has a number of laser distance measuring instruments as well as a 3-D image scanner.
Swarovski Optik - Laser Range Finder RF-1 - to 1,000 meters @ +/- 1 meter
Acuity Research - 0.1" to 2000'
Measurement Devices Ltd. (MDL) - LaserAce® 300 range finder & others - WIN CE PDA
Laser Technology - maybe makes the Yardage Pro for Bushnell? - WIN CE PDA
Laser Atlanta - Advantage - 5 to 2,000 feet accuracy +/- 1/2 ft., 0 to 359.9 deg compass & MiNiLASER - 0.6 to 100 ft
Cyra - 3D field measurements to 50m with 6mm accuracy

Leica Range Measuring Instruments: (the tilt is limited to +/- 45 deg, so no good for astronomy)
Model Range Compass Tilt Data Price
Geovid 7x42BDA  Binoculars 1 km Yes No No
Geovid 7x42BD  Binoculars 1 km No No No
Vector GIS  Binoculars 1 km Yes Yes Yes
Vector 1500  Binoculars 2+ km Yes Yes Yes
Vector IV  Binoculars 4+ km Yes Yes Yes
MRF2000 monocular 3.8 km No No Yes
SG12 Digital Compass na Yes ? Yes
Disto Basic 100 m No No No US$ 895
Disto Memo 100 m No No Yes US$ 1095
Disto Pro 100 m No No Yes US$ 1695
Power Disto >140 m No No ?
Data Disto GSI use with T460D >100 m No No Yes
Data Disto RS232 >100 m No No Yes
NIVEL20 Dual axis inclination sensor na No Yes Yes - carries the Disto
Yahoo Group for the disto =

Stanley (CST/Berger) now (June 2006 ) haslaser measuring instruments that are very attractivley priced.
Product brocure for all 3 models.  Much better than ultrasonic.  But no Computer port.  Maybe hackable?

100 ft
77-910 - TLM 100 FatMax™ Tru-Laser™ 1/4"
meas + 3
77-920 - TLM 200 Professional FatMax™ Tru-Laser™ 1/8"
meas + 6
77-930 - TLM 300 Surveyor FatMax™ Tru-Laser™ 1/16"
meas + 14

Shop: Google and eBay your friends.

SDC ( Symmetric Differential Capacitive) sensor

Has some properties allowing low cost very sensitive displacement sensing - 0.01 to 10 Hz seismometer, Cavendish force balance, etc.  Covered by US Patent 5461319.


Thermistors (Wiki)

Thermistors have a well known resistance -vs.- temperature characteristic.  It is called the Steinhart-Hart equation.
that is experssed as
1/T= A + B*Ln(R) + C*(Ln(R))^3
or a simplified form that's less acurate

T= 1/(Ln(R) -A)) - C

In the above equations, T is in deg K and R is in Ohms.

The following equation appears on the Agilent web site, T = 1 / (A + W * (B+C * W*W)) - 273.15
where T is in deg C, A, B and C are the thermistor coeficients, W= Ln(R) and R is the measured resistance (Ohms) of the thermistor.  For the Agilent 34308 (and probably most other) 5k Ohm at 25 deg C thermistors the coefficients are: A = 1.28463e-3 B = 0.23625e-3 C = 9.2697e-8

Note that although the Steinhart-Hart equation is of the third order with respect to R, it is linear with respect to the coefficients, thus you can fit temperature vs. resistance data for a given thermistor easily.  This can be done by using three points on the R vs T curve, but is better done by using a statistical fit on a much larger number of points.

You can buy thermistors  specified so that upon interchanging sensors you will have 0.1 deg. C accuracy.  This is much better than the semiconductor IC type or a thermocouple.  The readout resolution can be much higher for example 0.01 or 0.001 deg C for applications where you're looking for a change rather than absolute accuracy.

applet that solves 3 by 3 systems of linear equations - also AFAICR  a solution based on statistics can be done on the HP 41 or HP 48 calculator.
Yellow Springs Instruments - Thermistors
Radio Shack Precision 10 K Ohm Thermistor 271-110 - on the back of the blister pack is a table of R vs T (103AT)
Agilent 34308 5 k - 40653B 10 k - Practical Temperature Measurements (AN 290  5965-7822E.pdf ) 32 pages, highly recomended for all types of temperature sensors. Includesthe Steinhart-Hart equations.

Self heating is mentioned as a problem with thermistors, but it is also the basis of fluid flow measurements using thermistors. (see Hot Thermocouple)
If the thermisor is biased so that there is some self heating and a fluid is moving past it, the fluid will carry away heat in proportion to the speed of the fluid.  For normal applications you need to be careful to bias the thermistor so that is does not self heat.

If you make a circuit driven from a constant voltage power supply that outputs a voltage that is some function of temperature you can compensate for the reduced sensitivity at higher temperatures where there's little change in thermistor resistance.  The simplest circuit is a fixed resistor equal to the 25 deg C resistance of the thermistor in series connected to the positive rail and the thermistor connected to ground.  The voltage out at 25 deg C is half the rail voltage.  But there are more complex circuits.

Silicon Semiconductors

These are analog integrated circuits that have an output that is a linear function of temperature.  They are not as accurate as thermistors but are easier to use.
Dallas Semiconductor - Thermal Management
Analog Devices - Temperature Sensors  - ADT7320 has 16 bit resolution and 0.2 C accuracy. can be used for cold junction measurement for thermocouples.
Eval Kit on order 28 March 2013 -
National Semiconductor - Analog Temperature Sensors
Honeywell -
Airpax - temp sensors -
On Semiconductor (formerly part of Motorola) - LM75DM - 2-wire serial w/trip point


The Resistance Temperature Detector is a metal element whose restence changes linearly with temperature.  They typically are wired using a 3 or 4 wire Kelvin connection so that the resistance in the wires to the device are not part of the measurement.
Burns Engineering - ITS-90 & DIN 43760 and IEC 751 software downloads
Minco - small RTD on Polyimide tape - good for PCB temp

Fiber Optic

Nortec - non metalic sensor using light reflected from a semiconductive crystal.
FISO Technologies, inc. -

Ultrasonic Anemometer Thermometer

If two ultrasonic transducers are in fixed positions a known distance apart then by measuring the propagation time from A to B and then from B to A both the wind speed along the axis and the air temperature can be determined.  Three pairs of these sensors will give air Velocity (a 3 dimensional vector).  The air temperature is a byproduct of the wind speed computation and not all ultrasonic wind speed devices have this output.

Kaijo Sonic - FA-600, SAT  series  3-D Ultrasonic Anemometer Thermometer 

Nova Lynx - Weather Monitoring Instruments & Systems - 200-81000 Ultrasonic Wind Sensor - Synthesis of the scientific results obtained through the AutoFlux project - college paper


Buck Research Instruments - humidity instrumentation
There are a number of ways to measure the humidity.  The popular Honneywell and Philips units use a capacitor whose value changes with humidity.  Dew point can be measured very precisely using light bouncing off of a mirror and looking for the dew.
Automata, Inc. - AQUA-TEL-TDR Soil Mositure probe uses Time Domain Reflectometry Delphi Auto -under hood capactive humidity sensor
General Eastern - sensors,  including mirror type & generators
Yankee Environmental Systems (YES) - Total Sky Imager (TSI-440A) - Ultraviolet Instrumentation - Humidity -Michell Instruments - cooled mirror and other technologies
Ohmic Instruments Co. - Humidity Sensors - resistive eval kits
OKI Semiconductor MSCM64162 - This microcontroler is designed for temperature and humidity detection.  It is used in the Radio Shack 63-1015/Oregon Scientific Weather Station Temperature & Humidity sensor.    It runs on 1.5 volts and in this application the returned digital data stream is very feeble and will not directly trigger TTL level circuits.  This may be the cause of the 30 foot wire limit. It's LCD drive capability is NOT used in the WX-200 application but I suspect that it is used for stand alone temp & humidity displays that are battery powered.

Honneywell - HyCal - Humidity & Moisture - the latter recommended by Vernier Software
JLC - Humidity Sensor Elements   - capactance type
Micorconversion Technologies Co. - SAW based dew point
Panametrics - Sensors
Philips - Distrubitor Listing 691-90001 - Factory .pdf documents -
Precon - HS-2000 Combined calibrated Temp & Humidity sensor can be socketed and replaced. Analog or Digital out models
Rotronic - HygroClip Humidity & Temperature
Sensirion - Humidity - Sensmitter AH31 Digital RH & temp with SPI digital output - SHT21 - Digital (I2C) Humidity Sensor (RH&T)


American Electronic Components (AEC) - Tilt -
AOSI - Polymer based, non glass rugged shatter proof tilt sensors survive during crash testing, systems, Inclinometers and Compasses
Applied Geomechanics Inc. -  tiltmeter and clinometer technology
Columbia Research Laboratories, Inc. - Force Balance Inclinometers -
Fredericks Company - Liquid based
Kavlico Corporation - TS904 Tilt Sensor - for rugged applications
LEICA NIVEL20 with compact opto-electronic sensor unit - light reflected from liquid surface as used in theodolites
Maxim + PIC - Eng Journal vol 29 Adjustment free Inclinometer operates on 2.7V-
schematic and sensor Spectron L-211 or Fredericks 0725-5006 
Nanotron, Inc. - electrolitic liquid based smaller than 1 inch up to +/- 70 deg
OncQue - Rolling Ball Switch (RBS model numbers) switch patents both direct metalic contact and opto types.
ball in tube, 2 terminals, level sw
one version may have spring triggered off by vibration along axis

4 contacts all in one end horizontal plastic box - N.C.
4 contacts all in one end of vertical plastic box - N.C.
45 deg in 360  2 pin metal ball in tube

ball in tube, 2 terminals, level sw
+10 closed, -10 open

45 degree in 360
Up - ball on 4 pins plastic box
< 45deg tilt ball on 2 pins North, South, East, West different
>45 deg all pins open
on: 0 to 5 & 180 to 185 otherwise off

Single Ball in metal two part cone - N.C.

    6800841 Tilt switch 250/231.1; 33/366.11; 250/239 - this is an opto isolator with a moveable ball in-between source and detector.
Osram - LED-photo sensor type SFH 7710 - 4 terminal SMT package
SignalQuest - Ball in Tube sensors
Spectron - Tilt & Inclination - Liquid based
UST Research Inc. (Wagner Lipnharski)- Ball in Cage - 4 pins: Left Tilt:   Contact A + B, Center:    B + C, Right Tilt: C + D
Zircon International, Inc., Campbell, CA - 4827624 05/09/1989 Electronic level

Building Wall Stud

5619128: Stud sensor with over-stud miscalibration via circuit which stores an initial calibration density, compares that to a current test density and outputs result via indicator - Zircon International, Inc., Campbell, CA  capactive based
5352974: Stud sensor with digital averager and dual sensitivity Zircon International, Inc., Campbell, CA  capactive based

6886269 Locating device, Robert Bosch Gmbh, May 3, 2005, 33/773, 33/775 - Model D-tect 150
7453253 Locating device and corresponding method, Robert Bosch Gmbh, Nov 18, 2008, 324/67, - Model D-tect 150 - larger and heavier than the Zircon type units, but much greater capability, works to find pipes, rebar, PVC tubing, pretty much anything in a wall or concrete or any object.  There are less expensive models (the 100 and 120).
7414235 Handheld locating device with a sensor for detecting motion parameters, Robert Bosch Gmbh, Aug 19, 2008, 250/221, 250/222.1, 33/775, 33/124, 33/779, 33/782, 33/773 - wheels
The Milwaukee M12 scanner may be equivalent and much lower in cost.


Polaroid - ranging transductor etc. not only from cameras but built for other applications like cars
Mobile Robotics - Ultrasonic Positioning - based on the measurement of ultrasonic pseudorandom noise and its cross-correlation
Cheap Ultrasonics: Software - PIC microcontroller based
Fascinating Electronics, Inc. - Ultrasonic Radar - ploar plot
Massa Products - Underwater TR-208A/SQS-23, TR-237, TR-238, and the TR-330/SQS-56 & Air - Articles -
Airmar - Ultrasonic Air -
Honeywell -
Senscomp - Eval kits and sensors


A note on gyros.  When located in a dynamic platform, like an airplane, the gyro needs to be located at the center of mass of the vehicle.  This way when the plane banks, pitches or yaws the gyro will only respond to the angles.  But if it's located anywhere else it will be in error.  This applies to models as well as to full size vehicles.  When Harrison first tested his chronometer at sea the movement of the ship caused errors in time.  If he had located the clock at the center of mass of the ship I think it would have worked much better.

Gyro Technology and Long Term Drift Rate

In the March/April issue of Inside GNSS, "The Promise of MEMS" by Naser El-Sheimy & Xiaoui Niu, Fig1 "Bias Stability vs. Nominal Size of Mature Gyro Technology" from a Dr. Robert J Smith of Honeywell (using Ring Laser, electrostatic, hemispherical resonator, fiber optic, 2 degrees of freedom rotor, quartz plate.) Y-axis is the log physical size in mm the X-axis is log Long term bias stability deg/hr).  The interesting thing is that the data falls on a straight line.  At 0.001 deg/hr the line is at 100 mm and at 10 deg/hr (4 decades away) the size is 10 mm (one decade).  So as the sensor gets smaller the LTDR goes up as 10 ^ 4.

But if a MEMs sensor has predictable long term drift over some short time period then it can be backed out.  In that case the short term noise (100 second bias instability) becomes important.  In the above article there's an example system using MEMS gyros and accelerometers.  They are using forward filtering and backward smoothing to get a reasonable result for short GPS outages.

Drift deg/hr
Ring Laser
7E-4 - .5
Hemispherical Reasonator
Fiber Optic
Two Degrees of Fredom rotor
Quartz Rate Sensor
7 - 1000
Micro Electrical Mechanical System

Classical spinning mass (Rotor)

These are classicial spinning mass type gyros like are sold as toys except with more mass and electrical motor powered.
Arcmax - model helicopter - ?technology
Harold Crabtree, AnElementary Treatment of the theory of Spinning Tops and Gyroscopic Motion. This is the definitive work on tops and gyroscopes. It even discusses the mysterious rattlebacks we will see in the demo on tops.
Charles Stark Draper developed gyroscope systems that stabilized and balanced gunsights and bombsights and which were later expanded to an inertial guidance system for launching long-range missiles at supersonic jet targets. Patent No. 2752790
Charles Stark Draper Laboratory, Cambridge, Mass., was awarded a $35,310,026 contract for product improvement/producibility and evaluation studies on the urrent Trident II (D5) Pendulous Integrating Gyroscopic Accelerometers (PIGA's).  Work will be performed in Cambridge, Mass., and is expected to be ompleted by December 1999.  The Navy's Strategic Systems Program, Washington, D.C., is the contracting activity.
Elmer Ambrose Sperry invented gyroscope-guided automatic pilots for ships and airplanes that have also been applied to spacecraft. Patent No. 1242065
Litton Marine Systems (formerly Sperry Gyroscope) - Sperry Marine - Gyrocompass
Watson Industries - combinations of heading sensors, accelerometers, angular rate gyros, and angle references as well as a line of magnatometers.

Fiber Optic

KVH Industries - Fiber Optic Gyro - 5768462: Grooved optical fiber for use with an electrode and a method for making same ; 5739944: Support system for resonating element of phase modulator - 20 deg/hour = 1/3 deg per minute
5610815: Integrated vehicle positioning and navigation system, apparatus and method
4634282: Multimode fiber optic rotation sensor
4135822: Laser gyroscope
5365338: Wavelength sensor for fiber optic gyroscope - Secretary of the Navy

Crossbow - FOG


ATA Sensors, Albuquerque, NM  ARS-20 with no moving parts.
The Magneto hydrodynamic effect depends on the combination of a conducting liquid, a magnetic field and a force.  It was featured in the movie "Red October" as the propulsion system for a submarine.

Vibrating Tuning Forks & Wheels , Piezo

Analog DevicesADXRS Gyroscope - +/1 150 and +/- 300 deg/sec versions -
Note:  The Eval board is a small PCB with the surface mount gyro chip mounted along with some caps.  The two rows of pins are on 0.3" centers so it can plug into a standard breadboard socket just like an IC.
BEI Systron Donner - vibrating Quartz tuning forks
                                Aug '99 Sensors V 16 # 8 Cover Photo & Article on Coriolis-Based Micromachined Quartz Angular Rate Sensor
British Aerospace (Systems & Equipment), Inc.
CFX Technologies - gyros, accelerometers, Inclinometers, Ultra Low Noise Seismic Force Balance Accelerometers
Delphi Automotive Systems - MEMS angular rate sensor

MEMS Clearinghouse - Micromachined Gyroscope - Two-Dimensional Micromachined Gyroscope-
Gyration, Inc. - Gyration has designed a low-cost, micro-consumer gyroscope sensor for automotive applications; computer cursor management; and camcorder stabilization products

5825350: Electronic pointing apparatus and method
5698784 : Vibratory rate gyroscope and methods of assembly and operation
5594169 :Optically sensed wire gyroscope apparatus and system, and methods for manufacture and cursor control
5440326 : Gyroscopic pointer
5138154 : Shaft angle encoder with rotating off-axis interference pattern
D0378751: Graphic display controller
Airtronics - SG10 Piezo Gyro System for models
Bosch - Acceleration combined with Coriolis Yaw for automative handling applications
Draper Labs -
5481914  Electronics for Coriolis Force and Other Sensors -
5505084: Micromechanical tuning fork angular rate sensor - >100 other patents This patent for Silicon (prior art was quartz)
5496436: Comb drive micromechanical tuning fork gyro fabrication method - >110 referenced patents
5515724: Micromechanical gyroscopic transducer with improved drive and sense capabilities
5535902: Gimballed vibrating wheel gyroscope
5555765: Gimballed vibrating wheel gyroscope
5650568: Gimballed vibrating wheel gyroscope having strain relief features
5251483: Piezoelectric sensor element intended for a gyro
5600064 Electronics for coriolis force and other sensors, 73/504.04 ; 73/504.03 - calls 19 patents
Temic - DRZ-75X - automotive application?
Imego - Butterfly Gyro - SensoNor sells the SAR10
Inertial Science, Inc.- RRS75 Resonator Rate Sensor -
(Sumitomo Precision Products) Silicon Sensing Systems Japan Ltd - Vibrating Structure Gyroscope SGN1-100 - Links-
Fujitsu - GyroSensors -
Micro Piezo Gyro - on the Acroname robitics page
Murata -  ENC-03J GYROSTAR -
Melexis - MLX90609 - long term drift is 5% of FS, most sensitive is 75 deg/sec * 5% is 3.75 deg/sec = 225 deg/hr
Irvine Sensors Corporation - MicroSensors, Inc. - Micro Ring Gyro - 1 deg/min
Silicon Sensing Systems - MEMS rate sensors and gyroscopes  and 3-axis accelerometers
Home Made Piezo gyro by Scott Stephens (see the PIC archive)
"Getting Around The Coriolis Force" by David J. Van Domelen Ohio State university

North Finding Module

In addition there is a "North Finding Module" that can determine North very accurately.  The military is interested for targeting and surveyors use them inside caves where they work well while a magnetic compass can have large errors if there is ferrous metal in the ground nearby (which may be unknown).  These work by measuring the Earth's rotation (360 deg/day, 15 deg/hr, 0.004167 deg/sec) in 2 planes 90 deg appart and use an accurate plumb measuring device like an accelerometer.

A gyrocompass works using the same method and typically has a long settling time and different axis of rotation.
The ADXRS has a noise level too high for this application.  If you use some number "n" of sensors to average out the noise the improvement goes as SQRT(n) so it would take a very large number of sensors to get the noise down low enough.  Analog devices says that maybe in 10 years MEMS  technology will be good enough for this app.
AN/PAQ-3 Modular Universal Laser Equipment (MULE) - Target locator and guide for laser-guided projectiles - FAS - DDN - BH -
Azimuth Technologies Ltd. - NORS62 - not gyro but astronomic & GPS based
Defense North Finding Products -
Federation of American Scientists- SE.09.02.A North Finding Module -
KVH Industries - North Finding Gyro -
Smiths Industries Aerospace -
4694728: Fiber optic guided missile aimer - NFM North Finding Module; Sperry Gyroscope; May 1977. - Gyros, Platforms, Accelerometers; Kearfott Systems Division, General Precon Systems, Inc. 7th Ed. Nov. 1967, (pp. 26-28).
Wild T16 Theodolite with GAKII Gyro - by Jim Arnold P.E.

1503436 Vertical Gyroscopic Compass, Sperry, 33/321 ; 33/324
3938256 Quick settling gyrocompass having interaxis decoupling compensation for improved response time, GE, 33/324 ; 33/318; 74/5.4
4283960 Gyroscope indexing drive mechanism, Sperry, 74/5.41 ; 33/326; 33/327; 74/5.47; 74/5.9
4383452 Transfer mechanism for a gyroscopic device, Bendix, 74/5.47 ; 33/324; 74/5.41 - can switch between North Finding and direction finding modes.
4503718 Azimuth determining gyroscope, Sperry, 74/5.46 ; 74/5.5; 74/5.6E; 74/5.8

North Finding
4033045 Portable surveying gyrocompass apparatus, Sperry
4302885 North finder with optical transfer provision, Sperry
4443952 Gyroscopic apparatus, Bendix
4461089 Instrument for the automatic determination of North direction, Bodenseewerk, 33/324
4603483 Thermal gradient compensator for fluid rotor gyroscopic apparatus, Sperry
4694728 Fiber optic guided missile aimer, Army
4741245 Method and apparatus for aiming artillery with GPS NAVSTAR, DKM Ent.
4945647 North Finding System, Allied Signal
5003171 Optical encoding arrangement for absolute angle measurements, Allied Signal
5060392 North finding system,Allied Signal , 33/324 ; 33/318; 33/321 -
5369889  Single gyro northfinder, Honeywell
5481957 Aiming and pointing system for ground based weapons equipment, Allied Signal
7107179 Passive target data acquisition method and system, Malchi & Malchi


Some camera stablization systems utilize accelerometers rather than gyros.  An acceleration over a time translates to movement, sort of a very short time frame inertial nav system.
Bicycle 9.35 Gyroscopic Forces -
Peleng 1240 GyroStabilized Binoculars -
Gyroscopic stabalized cameras -
Gyroscopic Stabilizers - for model helicopters - Airtronics - Futaba - Hitec/RCD
20 mm Auto Shotgun TL A Military Specs - stablized gun
Ship Gyroscopic stabilization -
Gyroscopic Beverage Holder -
ES 300 Naval Weapons Systems - Chapter 5 Automatic Tracking Systems -

Gyroscopic Links

Exploritorium - Gyroscope - a number of topics are covered, good to learn about them
BMI Surplus - Gyroscope Test Equipment -
Gyroscopic Ball - for golf training
Air Force Institute of Technology - MECH 712 - Nonlinear Oscillations -
Wright-Patterson AFB - Gyroscopic Controls on Sperry "Messenger -
World Scientific - Stability Of Gyroscopic Systems - book in chaos series
Comptons Encyclopedia - Gyroscope -

HEV Flywheel Safety- gyroscopic effects of the flywheel can cause a vehicle to roll over during a turn
Pendulum to Gyroscope - 1852 Gyroscope Jean Bernard Léon Foucault
TB 11-6625-247-35 Gyro Magnetic Compass AN/ASM-61 Test Set @ETM

Human movement

Simon Fraser University - School of Kinesiology - Human movement tracking technology: resources -British Columbia Association of Kinesiologists (BCAK) -
AN/GSQ-160 - seperate web page uses VHF radio field
SignalQuest - Fall Detection 
Freescale semi - Fall Detection using a 3-axis accelerometer

REM Sleep

Rapid Eye Movement (REM) sleep was discovered by Kleitman and Aserinsky around 1953 and since there's been a lot of research related to sleep.  Maslow's hierarchy of needs (A Theory of Human Motivation 1943) has as it's foundation: Breathing, Food, Water, Sex, Sleep, Homeostasis, Excretion.  So even before the work of  Kleitman and Aserinsky the importance of sleep was recognized. 

The stages of sleep start with awake (stage 0).  Stage 1 sleep is "half asleep" where if someone asks you a question you can hear them.  Stages 2, 3 and 4 can be differentiated using electrodes that probe brain activity where stage 4 is the deepest level of sleep.  Most people after reaching stage 4 transition to Rapid Eye Movement sleep which is very different from the other stages.  During REM sleep you dream and to keep from acting out the dream your body shuts down the motor neurons so you don't try to run when you dream about running, but the brain activity is the same as when awake.

If the stages of sleep are plotted where the Y-axis has 0 at the top, stage 1 just below and stage 4 at the bottom then REM is typically shown at the same level as awake.  The horizontal axis is time.  The plot looks like a negative going saw tooth, i.e.:
0 1 2 3 4 REM 1 2 3 4 REM 1 2 3 4 REM 1 2 3 4 REM 1 2 3 4 REM 1 2
These are called sleep cycles and if it took 90 minutes for a cycle then in an 8 hour sleep there would be a little more than 5 cycles.  But the cycle time varies a lot and you don't always have REM sleep so this is not something that can be timed using a clock.

There are a number of ways that REM sleep can be sensed. 


The classical (gold standard) involves attaching electrodes for EEG (for brain activity) EOG (for eye movement) and EMG for muscle movement (needed to differentiate awake from REM).  Needless to say that that's a lot of electrodes that need to be hooked to someone who's expected to sleep and it takes sophisticated (read expensive) equipment to record all the data then someone to read the data to get to the simple sleep cycle plot.  Sleep centers use these methods and add many more since they are looking not only for REM but other things related to sleep disorders.

Peripheral Arterial Tone

This is the only other FDA approved method of detecting REM sleep.  It's measured using a sensor on a finger tip similar to the pulse and oxygen sensors only this one is in a rigid housing and uses compressed air sort of like a blood pressure test.  REM detection is a byproduct of more advanced testing the PAT was made for.

6319205 Method and apparatus for the non-invasive detection of medical conditions by monitoring peripheral arterial tone, Goor, Schnall, Schnall, Sheffy, & Lavie, Nov 20, 2001, 600/485 ; 600/481; 600/483; 600/500; 600/504; 600/507
6322515 Method and apparatus for the non-invasive detection of medical conditions by monitoring peripheral arterial tone
6939304 Method and apparatus for non-invasively evaluating endothelial activity in a patient
Watch-PAT 100 System - monitoring system - Flyer.pdf 

Piezo Film on Eye Lid

This was one of the earliest methods.  The eye's lens sticks up above the radius of the eyeball so when there's eye movement there will be up and down movement of the eyelid caused by the passage of the lens.  This movement will cause a piezo film sensor to generate electricity if the sensor is stuck to the eyelid with double sided tape.
4836219 Electronic Sleep Monitor Headgear, Hobson, Mamelak, Helfand & Lavie, Jun 6, 1989,  600/595 ; 340/575; 340/576 - a product called the Night Watch was based on this method and was made by a number of companies.
NASA - Night Headband Monitoring System -
Sleep Sense - 1620-Kit  - the kit has a cable and 10 disposable sensors

IR LED and IR Sensor

The skin is transparent to IR around 1,000 nm so 940 nm LEDs and sensors can be used to look through the eyelid and see the pupil movement as a change in the reflected IR.
5507716 Equipment and methods used to induce lucid dreams in sleeping persons, LaBerge, Stephen, Rich, Wright & Kottke, April 16, 1996, 600/27 - the product was called the Nova Dreamer and consisted of two components, a sleep mask with IR sensors for eye movement and for head movement and a seperate box with a micro processor (not a micro controller).  These were expensive and are no longer being made.
Kvasar dreammask - is a more modern version of theNova Dreamer that uses a micro controller and so all the parts are in the mask.  Note that both of the above units are designed to flash an LED in the eye of the sleeper to get their attention but not wake them so that Lucid Dreaming can be stimulated.  The DreamMaker is another versio as is the REM Dreamer.
5551879 Dream state teaching machine, Raynie, Rodriguez, Forister & Crawford, Sep 3, 1996, 434/236 ; 600/26; 600/27 - Dream Weaver

4863259 Rapid eye movement sleep state detector, Schneider & Lloyd, Sep 5, 1989,  351/210 ; 351/209; 600/558 - uses an IR beam over the top of the eyelid similar to the beam breaker type of shop doorbells.

Rapid Breathing (Nightmare)

4420001 Respiratory measuring devices, Hearne, Dec 13, 1983, 600/537

Baby Rattle & Microphone to sense strength of Body Movements
4735199 Dream detection method and system, DiLullo, Apr 5, 1988, 600/28 ; 600/545; 600/595

Pulse Rate

4228806 Sleep state inhibited wake-up alarm, Lidow, Oct 21, 1980, 600/544 ; 368/12; 368/244
Detection of REM Sleep by Heart Rate.pdf -
5101831 System for discriminating sleep state, Koyama, Michimori & Hagiwara, April 7, 1992, 600/500 ; 600/26; 600/529

Gross body movements

During REM gross muscle movements are shut down.  Some twitching or fluttering movements may occur on the eyelids, toe, inner ear, etc.
4832050 Motion sensor, DiLullo, May 23, 1989, 600/595 ; 340/573.1; 446/419 - looking for gross movement on either side of REM
6669627 REM sleep augmentation with extra-ocular light, Campbell & Murphy, Dec 30, 2003, 600/27 - Toe twitching during REM
5479939 Sleep detecting apparatus, Ogino, Jan 2, 1996, 600/595 ; 340/575; 5/658 - piezo sensor on bed
G-Pendant (my web page) (Onset Computer web page for the G-Pendant Kit) - I'm trying this 3-axis acceleration data logger.  It's a small light weight data logger and reasonably low cost.

Galvanic Skin Resistance (GSR) & Spontaneous Skin Vasoconstriction

Skin sympathetic nerve function during sleep--a study with effector responses.  -

 The frequency of GSR and sweat rate on the dorsal side of the hand were significantly lower during REM sleep than during NREM sleep.
The frequency of spontaneous skin vasoconstriction (same as PAT above) was higher and blood flow was lower during REM sleep than during NREM sleep. These results indicate that sweating and blood flow in the skin are differentially regulated depending on the sleep stage.

Explosive Detection

-Smiths -
Also see my UneXploded Ordinance (UXO) page


Figaro - Semiconductor Gas Sensors Combustible, Toxic, Organic Solvents, CFCs (HCFCs and HFCs), Indoor Pollutants, Cooking Vapors, Oxygen, Humidity - These are intended for use in alarms with a set point rather than to measure the level of a gas.  The Sep 2004 issue of Nuts & Volts article "An Electronic Nose" aka "The Electronic Sniffer" shows how to use one of these to detect fumes from drug labs.  The project is made on a circular PCB so that a flashlight can be used to hold the batteries.  There is a newer sensor (TGS2620)  that draws much less power that could be used for this project.  Figaro has a number of products that only show up on the price list.
Chemical Sensors -
Electrovac - Amperometric Oxygen Sensor -
Hong Tak Electronic Co.,Ltd. - Oxygen Sensors
City Tech & SensoriC - electrochemical and catalytic bead gas sensors
IntelligentGas -


Another way of detecting common gasses is to use a pair thermopile temperature sensors each with a different wavelength filter illuminated with an IR source.  The air temperature will have peaks and valleys depending on what gasses are there and that can be optically sensed.
PerkinElmer -  TPS2534


Sensory Inc. - $50 eval kit - 15 words, user trained


Cyrano - wroking with Agilent (was part of HP)  to investigate the commercial viability of their electronic nose technology -
6085576 Handheld sensing apparatus, Cyrano Sciences, Inc., Filed: Mar 20, 1998 - Dec 2016: the list of patents that reference this one is >400!
HP (now Agilent, BC) and Cyrano Sciences, Inc. Sign Research Agreement -
Agilent 4440A chemical odor sensor -
Electronic Sensor Technology - zNose™
ACS: Diagnosis and Classification of 17 Diseases from 1404 Subjects via Pattern Analysis of Exhaled Molecules -


Pressure Profile Systems - Capactive arrays & processing/visualization

Rotary Angle

HP had a very high precision shaft angle encoder.  Maybe this is the patent:
4266125 Optical shaft angle encoder, Howard C. Epstein, John J. Uebbing, HP, 1981-05-05, -
4259570  Optical comparator, Mark G. Leonard, HP, 1981-03-31, -
4451731 Apparatus and method for modulating light to generate an index pulse,
4691101 Optical positional encoder comprising immediately adjacent detectors, Mark G. Leonard, HP, 1987-09-01, -

HP also made a small optical shaft encoder (HEDS-5000) for use as an input to test equipment.

Netzer Percison - FAQ - 360 deg absolute to 10 arc seconds after calibration, Sin Cos output

Hobo Data Logger

Onset Computer Corporation - HOBO® 4-Channel External Logger - Radio Shack can order as RSU12045522 - My HOBO web page.
G-Pendant acceleratin data logger.


Fujitsu Microelectronics America - Biometric fingerprint - metal contacts forming capacitors at 500 dpi + software to recognize duplicate prints.  Two types, one is a static 1.5 cm square the other, MBF300 is a 1.28 x 0.2 cm strip that the finger is swept over.  Designed for use on things like cell phones, PDAs, laptops, access systems, etc.  How hard would it be to make a latex copy and would it fool the sensor?


For an example and some patents that describe a method see SkyScan Severe Thunderstorm Detector which is a Lightning detector.  It uses two LF coil antennas tuned to different frequencies.
The AS3935 Franklin Lightning Sensor works using a single RFID antenna in a circuit that the IC tunes to 500 kHz and that has a resistor to set the Q to 15.  This is a single LF antenna.  NOte this company makes many other sensors.

Web Links

Motorola Sensors - Inirtial, pressure & smoke
Sentech - Sensor distributor
Texas Instruments -Calculator Based Laboratory (CBL) - has many low cost sensors used for education.
The interface between the calculator, although RS-232 based, has a T.I. proprietary format so that this very handy data logger can not be used with a lap top or anything other than a T.I. calculator.  The CBL is also sold by EduCALC.
Veriner Software - many sensors for the CBL & general use aimed at education
Sensors Magazine - As the title says they are devoted to Sensors.  There is also an annual trade show. There are sensors for almost anything that can be sensed.  I have only included the ones that I find interesting.  The Dec. 98 issue has a overview article on magnetic sensors.  Dec 2001 "List Servers" article by Brooke Clarke
In theFeb. '99 Issue Jack Johnson proposes the following end use based definitions:
Transducer - is an I/O device that transformes energy in one form to energy in the same or another form for the purpose of transmitting energy or power.
Sensor -        is an I/O device that transformes energy in one form to energy in the same or another form for the purpose of transmitting (e.g. displaying, indicating, generating) information (e.g. data, status).
Instrumentation & Control Systems - Industrial oriented but also has a lot of sensor information
Instrumentation of amateur rockets -
International  Frequency  Sensor  Association  (IFSA) - products and papers
Remote Measurement Systems - Sensor theory & Products -
Photonics Spectra - a magazine with lots of good information and a Directory + Dictionary + buyer's guide +
EG&G - Optoelectronics Product Guide -
Hamamatsu Photonics - USA page -
MIT Lincoln Laboratory's Programs with the DARPA Sensor Technology Office -
Temic (Dalimer - Chrysler) Automotive Sensors -
Sensors and Actuators A: Physical -is an ELSEVIER pub
Handy Board Robotics Links - a Motorola mocrocontroller based generic PC board for robotics applications
LEGO - Mindstorms - touch & light sensors - TECHNIC CyberMaster - includes 2 way wireless communications to/from PC.
Onset Computer Corporation - HOBO® 4-Channel External Logger - Also can be ordered from Radio Shack as RSU 12045522
Honeywell Sensors - main index to all kinds
PC Interfaces by Tomi Engdahl
Linear Technology - LTC2400 24 bit ADC on a chip
NASA - Instrument and Sensing Technology - Spin-off Sensor Technology -
JPL - Chapter 12. Typical Science Instruments -
Sensor Technology at the Lawrence Livermore National Laboratory -
Allen Bradley (Rockwell) Sensors - Presence Sensing
Sensors and Transducers - by Bruce Carter
Cornell Autonomous Helicopter Project - using Earth's Magnetic field & other sensors
Russell Bik Design - electric field sensor, 12' range to detect people in a room
Remote Measurement Systems - Remote Measurement Systems -
Tekscan - tactile sensors
Quantum Research Group - Charge Transfer sensors - many interesting applications for this patented technology -E97S Eval Board
Sensor History by All sensors -
Handheld Design - CF+ format sensors for PDAs -
Brooke's Home Page
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But my guess is that for it to work for us, or any rural area, virtually ALL aircraft would need to carry the hardware in order for it to provide something resembling reliable