These are just quick and dirty gadgets to have a look at some electro optical idea. Presented in random order.
This is one of the TAOS Light to
Voltage ICs. It takes the current from a photo diode and drives
the inverting input (virtual ground) to an op amp which converts the
current into an output voltage. They typically offer different
model numbers that have different gains and bandwidths.
22 Jly 2007 - A few minutes using the
TSL267 with the Battery Top Power Supply and the Printer Encoder Strip
looped back on itself seems to confirm my idea that the slot opto
interrupter is not working because the light source is closer to a
point source than a collimated source. With a point source the
light falling on the strip not only passes at 90 degrees, it also is at
other angles, making it much harder to block the light. I ran
this test in the day time, but room IR background saturates the TSL267
so need to wait till dark to really try it out.
22 July 2007 9 pm - Used angle head Flashlight about 4 feet from TSL267 and it's working.
The output voltage goes between 0.67 and 2.63 for a Pk to Pk signal of
about 1.9 volts. The speed I was sliding the encoder strip on
itself resulted in a frequency of about 633 Hz.
So it's important that the light source is collimated and desirable that it has near IR output.
Blurry photo hand held auto time exposure.
Theoretically the wave form should be a triangle with points at the top
and bottom and the scope image is close but the top and bottom are
rounded so looks like a sine wave. That may be related to the
sampling scope's low one shot bandwidth.
So to use the strip ambient light needs to be blocked and a collimated near IR source used.
The Light to Voltage family of TAOS parts uses a photo diode as a solar
cell generating a current proportional to the light input. By
connecting the photo diode output to the virtual ground negative input
to an op amp the current gets transformed into an output voltage
The negative feedback circuit consisting of a resistor in parallel with
a
cap allows trading gain and bandwidth to some extent. The rise
time for these varies from a few to a few hundred microseconds.
Methods of using Photodiodes:
- As Solar Cell - no bias -In this mode the output current is
proportional to the light input. At a former residence I had a
small solar panel (1x3 inches) flat on the roof driving an analog
current meter as an indicator of the Sun's brightness. A voltage
meter connected to a solar cell does not give you the same
information. TAOS
uses an op amp connected as a transimpedance amplifier to convert the
current out of an unbiased photo diode (small solar cell) to a voltage
from a low
impedance in their Light to Voltage products. These have a
dynamic
range of many decades. There's a tradeoff between high gain for
high
sensitivity and lower gain for faster operation.
- Reverse
Biased - the leakage current is a measure of the light on
the diode. The back bias lowers the capacitance allowing the
diode to
be faster. There are a lot of complications related to the
presence of DC so this method is typically only used for things like
fiber optic receivers where speed is very important.
- Novel Method with high sensitivity - uses PICmicro
controller to first back bias the LED to charge up it's
capacitance (uses 2 I/O pins, cathode ground), then switches the pin
connected to the cathode from ground to a digital input then grounds
the other PIC pin. Now there's 5 volt input to the digital
input. The digital input pin has an extremely high
impedance. A
timer is started and stopped when the input switches low (good to make
this an interrupt pin). The time to discharge is a measure of the
light on the LED. Can take a second when light level is 0.0001
lux).
- The
output current from the photo diode can be used to control the
frequency of an oscillator. These devices have the effect of
integrating the light level and so are slower in responding than a
photo diode connected as a current source. This is what's done in
the TAOS Light to Frequency converters.
Idea for Much Higher Resoltion Incremental Encoder
When you look at two identical Printer
Encoder Strips as one is moved over the other the amount of light
coming through varies from completely blocked off (if they are well
aligned) to 50% of the light on the other side of the strip. So a
linear light intensity sensor set so it's full scale output goes from
black to 50% of the light source will have a sawtooth output as the
strip is moved. Two of these sensors with a separation that's
some integer of the pitch plus 1/4 pitch (i.e. a quadrature sensor
arrangement) would allow not only much finer resolution but also
direction of movement detection.
This method would not be good for fast turning motors but would be
great for things like telescopes that move slowly. Note that the
light intensity is directly proportional to the effective slit width so
the voltage output from the light to voltage converter is a straight
line function of the displacement of the two strips. Only an
offset and scale factor correction need to be applied.
Instead of taking the photo diode output as a binary signal, process is
through a transimpedance amplifier to get an analog signal. Use
an A/D converter to read how much light is there. There will be
some limit on how many bits can be added by I expect that 8 bits is not
out of the question.
For most applications only the final position is important so the photo
diode output can be split into two channels. One channel is for
conventional digital counting and direction. During high slew
moves the analog channel may or may not be able to keep up. So a
speed based switch driven from the digital channel can turn off the
analog output or for lower cost just ignore the lower significant
digits during slews. This might be a problem when in a GOTO mode,
so there should be two speeds used, the first for getting near the
target and a slower speed used to creep up on the desired
setting. I think this is how they now work, but would be required
to use the higher resolution mode.
The reasonable priced angle encoders now have 500 slots per turn (0.72
deg) and the minimum step size you can get is 1/4 pitch (10.8 min
angle).
But this might be changed to 2.5 sec angle using the analog method.
When the number of bits in the D/A converter gets high enough
variations in the pitch of the strip or wheel will start to show
up. A way around that is to use the index mark to allow
calibration of a complete circle by using an external index head to set
the position and build a table in EEPROM.
Electronic Gold Mine
G15602.
A 13" long x 0.237" (330 mm x 6 mm) with alternating clear and dark
bars at 150 line pairs per inch (5.9 lp/mm). The dark lines are
0.157" long ( 4 mm). There is a slot at each end, both angles at
45 degrees and parallel to each other. I think this was used on a
printer to locate the print head. The obsolete
HP Deskjet 3810/3820 Printer series has manuals with product number C8952A and the encoder strip is marked "C8952A-80005".
At 150 lp/inch one line pair takes up 0.006666" (0.17 mm( which is
smaller than the 0.5 mm slit in the above Sharp slot type opto
interrupter, but that shouldn't matter since when two strips are in the
gap and perfectly aligned but out of phase such the bars one one strip
are aligned with the spaces on the other no light should get
through. And this is what happens. The data sheet for the
Sharp sensor mentiones that the sensor slot is vertical, i.e. aligned
with the long axis of the gap but I don't think that's much of an issue.
It's impossible to hold a folded encoder strip in your hands and adjust
it to be black for more than about 1/4". The problem comes when
the alignment of the two strips is off a little the you get a
Moiré pattern (
Wiki). The effect can be calculated (
Wiki).
For two strips with the same pitch the big distance on the pattern
between dark lines is D = p/a where a is in radians and D and p are in
the same units. So for p = 0.0066666" and a = 1 degree or
0.017453 radians, D= 0.38"
angle deg (radian)
|
radian
|
D =
|
0.029 (1.7 arc min)
|
0.000513
|
13"
|
0.25
|
0.004363
|
1.5"
|
0.5
|
0.008727 |
0.76" |
1
|
0.017453 |
0.38"
|
2
|
0.034907
|
0.19"
|
4
|
0.069813
|
0.095"
|
19.5 deg
|
0.34
|
0.5 mm
|
If the angle between the two strips was within 1.7 arc min the whole 13" would appear to be black.
The bottom line is that it's not trivial to make a linear encoder that works.
If the two strips are aligned to within 19 deg across the 0.5 mm wide
slit then the brightness will be a function of the linear offset
between the two strips which will vary as the phase of the pitch.
I've heard that the commercial linear encoders use two sensors for the
"A" phase and two for the "B" phase where the two sensors for the same
letter are out of phase so one is dark and the other is light.
Theoretically only one A and one B are needed, but by using
complementary sensors it works better. With the folded strip this
would be an "A" only incremental sensor, i.e. no direction information.
|
|
Black battery holder, photo diode, w LEDs green is the back
side of the solar panel.
This may be a sililar circuit dirgram
|
PCB
|
After reading an interesting analysis of two different
Solar Garden
Lights
I got one to see the high technology they contain. The
interesting ones have a single cell battery to store the power so they
include a Switch Mode Boost Power Supply to get enough voltage to drive
a LED.
This
one has a metal cylindrical frame about 5" dia x 6" high. A
square solar panel a little smaller than 2.5" on a side drives the
electronics. Three AAA Ni-MH cells store the power. A photo
diode senses if it's night. Two 5 mm LEDs shine down and some
light would fall in a circle around the fixture and what light hits the
bottom reflector is spread horizontally. The batteries were dead
(this is a used discount store unit) and are now on the
C401FS charger, then to the
C9000 for discharge analysis and cycling. Since there's a three cell battery no need for the SMPS.
The batteries showed fully charged in less than 10 minutes. They
are rated at 1.2 Volts and 600 mAh. Discharging them showed about
33 mAh capacity, so they now are on the Break-in mode of the
C9000. After the Break-In the cells now have 279, 229 and 308 mAh
capacity almost 10x what they were down to after sitting. But
it's still half the label capacity.
After installing the batteries the light did not work. The
voltage at the PCB was low. Rubbing the battery terminals on my
pants and after installing rotating the cell in the battery holder
fixed that. Now when the photo diode is put in the dark the two
LEDs turn on. Used Radio Shack Lube Gel (Silicon Grease) on both
ends of each battery. You might think that's an insulator and
would stop good contact, but that's not the case. There's enough
spring pressure for the metal to push the weak grease aside and make
contact. But Lube Gel is made without any entrapped oxygen so no
air can now get to the joint to allow it to corrode.
|
|
Working Solar Garden Light
|
|
Put outside in a spot that gets some Sun, but only an hour or two per
day around Xmas. Light came one at dusk (noticed it was on at 5
pm) and at 7 am the next morning it's still on.
If each LED was running at 3.3 Volts and 10 ma that would be 20 ma
total and if the battery capacity was 600 mah the max run time would be
30 hours. Off at about 7:13 at dawn probably not because the
batteries have run down.
The job of the circuit would be to turn the LEDs on and off based on
how dark it is outside. It's hard on Connect the solar panel to
the battery when there's charge to be had in the most efficient manner
possible an. Three Ni-MH cells and the white LEDs are a good
match. For example it's desirable to stop the discharge of Ni-MH
cells at about 1 volt per cell, or in this case at about 3 volts.
The LED forward voltage is that of a diode so as the battery voltage
goes down as it becomes discharged the LED current also decreases.
My guess is that the number of hours of light depends on how much solar
energy gets put into the battery. During the winter it's only
going to get a couple of hours of sun and I've not put in into the
ground plumb, but rather with the top pointing kind of toward the
sun. If this solar panel is like the one below for the solar
fountain pump (35 mw/sq in) then it's good for about [35 * 2.5 *
2.5 =] 220 mw for some number of hours. The voltage at the end of
charging three Ni-MH cells is about 4.2 Volts so in the ideal case 220
mw could provide 50 ma of charge current and so it would take 12 hours
to fill the battery to 600 mah or considering the battery efficiency
more like 18 hours. Some type of power point charge controller
that matches the panel to the battery would help get more charge into
the battery.
It's clear that in the winter time the battery is going to be operated
most of the time at empty with only a small amount of charge and back
to empty. Being on all of last night was because I charged the
battery.
It may be that under these conditions the newer Ni-MH cells that have
extended shelf life would improve the performance. This is just a
guess based on the idea that self discharge may be more important when
the battery is not fully charged.
A low resistance super capacitor probably would be a better energy
storage method. The idea is that they should have better
efficiency that a battery.
27 Dec 2007 light was on again last night about 28 hours total. It's cloudy today.
1 Jan 2008 - the light has been on every time that it could be that
I've checked. I.e. it's on a little past 5 at night and till 7
something am. There's no way it's getting enough direct Sun light
to charge the batteries and my initial charage would have worn off by
now, so the solar panels must generate a small current just from the
sky light.
Second Solar Garden Light
This light also uses 3 AAA Ni-MH cells and after charging them the
capacity was very poor (around 30 mAh instead of the label 750
mAh). After using the break-in function on the Maha C9000 the
capacity was 420, 451 and 706 mAh which is too dissimilar to use in a
series pack. The problem being that when the lowest capacity cell
is discharged the other cells will power it in the reverse direction
which rapidly ruins that cell and can cause it to vent. That's
probably the cause of venting in the
Solar House number.
This appeared to be a new unit. It had a red flag, like is used
on aircraft "Remove Before Flight", only this one says "Remove This
Shipping Tab Before Use" and has one end inside the battery compartment
seperating a battery terminal from the battery holder contact thus
opencircuiting the battery pack. Also there are clear plastic
protectors on the four solar panels with the legend "Note, Please peel
off this protecting film before use."
Photo taken after removing the top from the rest of the light.
You can see that there are solar panels each about 2 1/8" x
3/4". There's also a photo diode on the top. Two 5 mm
plastic LEDs for the light and three AAA batteries for power
storage. I'm guessing the time and temperature profiles of
storage or poor initial quality of the batteries or both resulted in
bad batteries when in as new condition.
Solar Garden Light or Marker Light Patents
4486820 Lighting equipment with a solar cell, Y. Baba (Kyoto Ceramic Co), Dec 4, 1984, 362/183;
362/157;
362/190;
362/276;
362/394;
362/395;
362/431;
362/802;
361/171;
136/244 -
a 4 W fluorescent lamp can be lit for 6 hours with the use of a solar cell of 16
V, 12 W and a battery of 12 V, 90 AH.
4816970 Solar powered light Mar 28, 1989
5065291 Marking Light, J.S. Frost et al (Atlantic Richfield Co), Nov
12, 1991, 362/183; 362/431; 362/31; 362/800; 362/145; 136/291 - small
solar panel, minimal one transistor circuit and LED. - The battery
voltage must exceed the LED operating voltage and the solar panel
voltage must exceed the battery voltage. simple cleaver circuit.
5221891 Control circuit for a solar-powered rechargeable power source
and load, R.W. Janda, Jun 22, 1993, 323/350; 323/906; 320/21; 320/61;
362/183 - 2.7V, 180 ma solar panel, 2 x SubC Ni-Cad cells, 3 transistor
control circuit, #1767 2.3 volt incandescent lamp patents 5086267
5041952 use a very similar circuit
5984570 Self energized automatic
surface marker Nov 16, 1999
6013985 Sealed solar-powered light assembly, David R. Green (Carmanah Tech), Jan 11, 2000, Jan 11, 2000, 315/149;
315/159;
362/183;
362/800 - two timers, voltage reg, cur lim resistors on multiple LEDs - not too efficient
6406163 Solar cell lighting fixture integrated with heat sink, Tai-Her Yang, Jun 18, 2002, 362/183;
362/374;
362/276 - diminishing effect of solar heat on batteries and electronics
6573659 Solar-powered light assembly with automatic light control, Ion Toma (Carmanah Tech), Jun 3, 2003, 315/149;
362/372 - uses micro controller to dim light to allow it to stay on all night based on charge obtained the prior day.
6729742 Solar lamp for outdoor use, W. Wismeth, May 4, 2004, 362/183;
362/153.1;
362/431;
136/206 - seperate light sensor diode & solar panels facing different directions
My wife wanted to try out the Harbor Freight
91962
Floating Solar Fountain Pump but it came DOA. After going thought
the RMA procedure they said there was no need to return the dead one so
I opened it up in the hope of recovering the solar panels, which
worked. The probable reason for the DOA is very poor soldering at
the joint between the panel wires, the pump wires and a 1N4739A 9.1
Volt Zener diode connected cathode to red wires (positive), i.e.
normally back biased by would limit spikes from motor that might exceed
the solar panel breakdown voltage.
The two 6" x 4¼" panels are connected in parallel and put out
about 11.7 Volts in direct sun. Short circuit current of 165 ma,
but those are not at the same time. So the power out is going to
be less than 2 Watts. The Harbor Freight 41144 5 Watt Solar
Battery Charger is 18" x 12.5" or about 3.3 Watts / square foot.
At that rate these two panels would be about 1.2 Watts.
Pump Testing
The pump while pumping water draws:
# AA Batt
|
H2O Ht"1
|
mA
|
V
|
Ohms
|
W
|
4
|
1
|
220
|
3.95
|
18
|
0.9
|
5
|
2
|
240 |
6.5 |
27
|
1.6
|
6
|
4
|
250 |
7.6 |
30
|
1.9
|
7
|
8
|
213 |
8?
|
38
|
1.7?
|
8
|
16
|
240
|
10
|
42
|
2.4
|
Note 1 estimated, not measured
Load Testing Solar Panels
By measuring the voltage across a load resistor the power can be computed as( V * V) / R.
Load
Ohms
|
50 W
Desk Lamp
Volts
|
50 W
D.L.
mW
|
Oct1
noon
Volts
|
Oct
noon
mW
|
10
|
0.099
|
0.98
|
1.5
|
225
|
20
|
0.042
|
0.088
|
ng
|
ng
|
47
|
0.83
|
14.7
|
6.4
|
871
|
57
|
x
|
x
|
7
|
860
|
67
|
x
|
x
|
7.3
|
798
|
Note 1: Not really direct Sun, some tree filtering. October has less Sun energy than July.
Looks like 35 mw/sq in. [900 mw / (6 * 4.25)]
5040726 Solar energy powered water fountain , A.T. Dimitri, Aug 20, 1991, 239/17;
239/18;
239/20;
239/22 - seperate solar panel
6435422 Floating Fountain, Mark Wutschik, Aug 20, 2002, 239/23;
239/18 -this floating fountain
I'm using the word Glow to describe
light sources that generate light without flame and that get their
energy from being excited by photons. The photons may be visible
light, UV light or radiation.
Fluoresce relates to changing the
wavelength from one value to another. For example a fluorescent
light changes UV to visible light.
Phosphorescence relates to a light generating process that stores the external energy and releases it over some time.
08797 hits since July 09 2007
page created 9 July 2007.
