Onset G-Pendant

Brooke Clarke 2007

G-Pendant major
G-Pendant, Coupler, USB Interface
PIC side Bat side
PIC side
Battery side

    G-Pendant data logger
    Base Station & Coupler
Solution to hidden Status LED
Event Triggering
Related Products


As part of an effort to measure Rapid Eye Movement while sleeping there's two things needed, one is the eye movement and the other is body movement.  REM sleep is very similar to being awake except during REM sleep there are no large muscle movements.  The G-Pendant is a simple way of logging head position while sleeping.

I have used the Hobo H-8 logger and like it so got the G-Pendant from the same company.


G-Pendant data logger

The G-Pendant made by Onset Computers consists of a small PCB ( 1" x 1.5") housed in a small water proof clear plastic can whose lid is sealed with an O-Ring.  On the PCB is a PIC 16F690 micro controller, an Analog Devices XL330 3-axis accelerometer, an 24LC512 EEPROM, a reed switch, 2 ea. LEDs, 2032 buttton cell battery and glue parts.  But to use it you also need a computer coupler and software which is included in the G-Pendant Kit.

When the battery is changed in the pendant be sure to install the PCB into the groove in the can.  The battery should be visable on the groove side of the can.  The label can be read without the distortion from the groove. It's possible to install the PCB upside down then the LED in the can and the base station will not talk to each other.


The G-Pendant uses a 3 Volt CR2032 coin cell.  It's voltage is recorded by default along with the X, Y and Z acceleration values.  A good scale is 2.5 V min to 3.2 V max when plotting.  If you autoscale a plot the voltages will be set to display the maximum and minimum recorded values which is not as clear as the above fixed values.

After replacing the battery be sure that the PCB goes back into the can in the slot .  Also be careful to get the O-ring in is groove and not pulled out.

The XL330 3-axis accelerometer IC is designed to run on less than 200 uA and expect that it's only turned on long enough to make a measurement so the battery drain is very small.

25 Apr 2007 - have used for 40 nights about 10 hours/night or about 4000 hours and the battery is still at 90%!  I don't sleep for 10 hours each night but often do not stop the logging until 10 to 12 hours after starting.  This is a good example of both the improved Analog Devices 3 axis accelerometer and the Microchip Technology "Nano Watt" feature set on the newer micro controllers.

31 May 2007 - have continued to use every night and the origional battery is still strong. 40 + 36 = 76 nights * 10 hrs = about 760 hours of operation.
13 Jun 2007 - origional battery is still working fine 89 nights or about 890 hours.
25 Jun 2007 - still working about 1,000 hours

Optic USB Base Station for Pendant & Coupler

The base station is USB powered so doesn't require a battery.  The active part of the base station is packaged in the same type clear plastic can as the pendant and has an LED in about the same place.  It's a USB to optical interface.  The base station plugs into the coupler plain end and the pendant plugs into the end that has the permanent magnet.  The magnet activates a reed switch in the G-Pendant causing it to look for digital data at the interface LED.  The groove in the coupler and the ridge on the can is to make sure the two interface LEDs are looking at each other.

The red blinking LED that shows through the hole in the label is different than the interface LED.

The reed switch in the G-Pendant also serves a couple of other purposes.  When you start the newer Onset data loggers (they call it launching) there are options relating to when the logger starts such as right now, on the next even minute, at some specified time, or later in the field when the event button is pressed.  In the case of the G-Pendant the button is replaced by bringing a magnet near it.

In order to save pendant battery power the PIC controller powers down when there's nothing to do.  When the time between measurements gets too short to allow sleeping between tasks the current drain goes up.  This is also why you will drain the pendant battery if you store the pendant in the coupler even when the base station is disconnected from the computer.  What happens is that the PIC in the pendant sees the magnetic field and wakes up and starts looking for the base station LED data which is not there, but it keeps looking anyway.

6826664 Interleaving synchronous data and asynchronous data in a single data storage file, 711/157 ; 707/102 - In a standard data logging file the header would contain the start data - time and the interval between data fields.  That way no starage is wasted lodding a date and time for each data point.  An event logger only logs the date + time when the event occurs and otherwise does nothing, for example a rain gauge may be implemented as an event logger but the temperature and barometric pressure would be data loggers.  This patent is a way to combine the two different types of data in a single file type.


The Hoboware Lite softeware that comes in the kit supports setting the time interval, starting time/mode and what things will be logged and does the launching.  It also has a status mode that allows you to see real time data from the G-Pendant.  But the more interesting part is reading to a file and plotting the results.  I'm now in the process of learning how to use the plotting function.  Not only does it let you plot each individual axis, but in addition the total vector or tilt in any of 3 planes.  Also things like battery voltage and various events can be plotted.

Onset has a patent on how they record events in a data logger.  Note that when data logging you do not need to remember a time or date for each measurement.  All that's required is to remember when you started and the time interval between measurements.  But if an event is to be stamped with a date & time a different format is needed and that's what patent 6826664 Interleaving synchronous data and asynchronous data in a single data storage file covers.

Although the G-Pendant has a reed switch that's used to start logging in the field by using a magnet or empty coupler, that switch does NOT support event recording (that's too bad, it would be a nice feature when long intervals are used between measurements).


Launching is the term Onset uses for starting a data logger.  The Launch window has a number of fields.  At the top there's an identification of the device, it's serial number, a battery gauge, the number of times it's been deployed, a text input box for a Description of the log.

In the Channels area there's a list of the possible channels with a check box to select the ones you want to log.

In the logging interval area there are radio buttons for Fast or Normal mode.  In normal mode there are text input boxes for Hours, Minutes and Seconds.  And if your logger supports Fast Logging (intervals shorter than 1 sec) there's a drop down list of 8 frequencies from 2 Hz to 100 Hz or a selection for Custom where the icrement/decrement window allows you to specify an interval in 0.01 second steps.

Once you have set the logging interval and selected the channels to be logged these two factors and the programs knowledge of the memory size for your logger are used to compute a logging duration.  This duration does not account for Events (button presses for those loggers that surrpot that feature.  The G-Pendent does NOT support events, too bad).

The Launch Options are:

Solution to hidden Status LED

Hole in CouplerDrill a hole in the Coupler so that the status LED can be seen when the G-Pendant is installed.  This makes apparent some new things.
1) When the G-Pendant is in the Coupler the status LED blinks once a second!  That adds to the battery drain.
2) If it's been launched and left in the coupler it continues to blink once per second.
3) When removed from the coupler the LED is blinking at an 8 second interval.
4) If installed in a coupler (with or without the base station in the coupler) the blink rate goes to once per second.
5) Now if the coupler is removed it will launch, i.e. the LED gives a burst of blinks within maybe 1/3 second of being removed.  If  you remove it too slowly you will miss the burst.  But after the burst the LED is at a 4 second interval, i.e. logging.
6) If the G-Pendant is put back into the coupler the LED blinks once per second and it's still logging.  So a once per second blink rate means the logger is in the coupler and may or may not be logging.

10 Apr 07 - After stopping logging and downloading the data I shut down the software while the G-Pendant was in the coupler.  The status LED was NOT blinking.  Does this mean the G-Pendant is smart enough to know to not look for data and so can be left in the coupler without the battery drain that it must have when the LED is blinking once per second?  I think not because if the G-Pendant is now installed into the coupler (base station connected to computer) the computer senses a new USB device and the status LED is NOT blinking.  So in the above cases where the status LED was blinking once per second the software was running.

12 May 2007 - Drilling a hole in each side of the coupler allows seeing the LED when you launch a G-Pendant go to the 1 blink per second mode (confirming that it sees the magnet).  This is good since if you pull it out slowly you will mis the rapid blink indicating that it has started logging.  Not a big deal since you can then determine the blink interval and hence the mode.

Hint About Trigger Launching

G-Pendant Event
          Triggering If  you choose the Trigger launch option and put the G-Pendant into your pocket with a loose coupler, there's a good chance that the G-Pendant will get launched accidentally.  One way to avoid that is to carry them separately, but that's not as conveinient as it might be.  So just plug the G-Pendant into the base station side of the coupler.  This will keep it separated from the magnet.

By drilling a hole on the base station side in the right spot you can see the LED on the G-Pendant to confirm that it's in the pre trigger state.

Event Triggering

25 June 2007 - It turns out that you can do event recording with the Pendant, and also probably with all other Pendants.  Whenever the Pendant is inserted into the coupler an event is date-time stamped.  So if you plug the Pendant into the coupler (the base station is not part of this) the Pendant senses the magnet in the coupler and records that event.  So a magnet near the Pendant acts the same as the push button on other modern loggers.

G-Pendant Event
          TriggeringIn the photo at left the G-Pendant is inserted into the Base Station side of the coupler.  This is how I carry it after launch in the start logging on trigger mode. To record an event just unplug from this side and plug into the other side where the G-Pendant will see the magnet.

Now the question is what would be required in the way of an electromagnet to trigger the Pendant? 


There are two classes of applications, slow logging, where the time between measurements is 1 second or longer and fast logging where it's less than 1 second.  The latter are things like dynamic vibration or acceleration.

In my case I have the G-Pendant on a headband while I sleep.  This is related to studying REM sleep.  With a 30 second interval there are about 700 measurements in a 6 hour sleep which is well under the max storage capacity.  Also after 4 nights the battery is still showing full.

Related Products

Other Pendants

 - all the Pendants are packaged in the same can as the G-Pendant and they all can use the same couplers and Shuttles.
Temperature w/Alarm -   8 k memory
Temperature w/Alarm - 64 k memory
Temperature & Light -   8 k memory
Temperature & Light - 64 k memory
Temperature & Events - Instead of the simple lid with hole for mounting screw that the above Pendants use, this one has a sealed coupling with a wire that's the same as used on the Optic USB Base Station.

Base Stations

Universal Optic USB Base Station 


A Shuttle is a small device that can download data from a logger and relaunch it.  It's a convenient way to collect data from a number of loggers in the field without the hassle of taking a laptop into the field.

Hobo Waterproof Shuttle
Hobo U-Shuttle - can also run some sensor tests.

Brooke's Home, Hobo H8, Sensors,  Page
[an error occurred while processing this directive] Page created 21 March 2007.