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Fred is my first
'photopopper' style bot. I had designed and built a similar bot before,
but that one had three motors arranged radialy around a central axis. It
didn't work nearly as well as Fred does.
The secret to Fred's success is the modified FLED solar engine circuit. I
tried a FLED circuit, but the low-light performance was abysmal. I needed
to put my 60W reading lamp 10 cm away from the solar panel before it would
fire! This was clearly inadequate, so something had to be done. I tried
putting in extra diodes, extra capacitors, extra resistors, and eventually
settled on the design shown here.
Now Fred will pop away quite happily 40 cm from my desk lamp, which is the
height that it normally sits when I'm reading. He will also pop all day
when indoors, so long as a window is nearby to let in some diffuse
daylight.
Specifications:
Voltages:
Switch-on: 2.4 Volts
Switch-off: 1.4, or 0.7 Volts, depending on what mood Fred is in.
Dimensions:
43 (length) x 27 (height) x 38 (width) mm.
Solar Cells:
One 33 x 24 mm Panasonic 5 Cell amorphous array (Part # BP-243318).
Storage Capacitors:
One NEC 0.033 Farad capacitor, with a 47uF cap in parallel to lower the
internal resistance at switch-on.
Motors:
Two namiki pager motors, with extra padding on the shaft, directly resting
on the ground.
Electronic components:
2 x BC337 NPN transistors
2 x BC327-25 PNP transistors
2 x Red Flashing LED (FLED)
2 x 3.3 K resistors
2 x 33 K resistors
2 x 4.7 uF capacitors
Performance:
Fred is quite an active bot compared to my other mobile bots. At 11:00 in
the morning sun, he takes two seconds to recharge after a big step, and
one second to recover from a small step. Sometimes he takes big steps,
sometimes he takes small steps. When he takes a big step, the capacitor
discharges all the way to around 0.8 Volts, but when taking a small step
the cap discharges to only 1.5 or so volts.
This results in interesting behavior. It seems that he takes small steps
when directly facing the sun, and big steps when facing at 90 degrees or
more. If Fred starts out facing into the sun then he will take lots of
quick, little steps of maybe 45 degrees and follow the sun around the sky.
But if you start with the bot facing away from the sun, or at 90 degrees,
then he will take giant 180 degree strides, alternating sometimes with
little steps to aim more towards the center of the light pool.
Construction and development:
I originally breadboarded a single FLED solar engine, and tested the
circuit using this setup. This resulted in the poor performance that many
FLED SE builders have experienced and cursed. I'm the sort of person who
doesn't give up very easily (My friends just call me stubborn!) so I toyed
with the circuit until it reached the stage it is now.
I started by experimenting with the 3.3k resistor. I found that the reason
that many FLED SE's lock up in lower light levels is that the FLED is
being kept high by this resistor. Current is going through the motor,
through this resistor, feeding the FLED, but every time the FLED fires, it
discharges a little bit from the storage cap. If the light level isn't
sufficient to replace the charge lost each time the FLED fires, then you
reach an equilibrium point where the SE appears to 'lock up'. I tried
adding a diode in series with this resistor, but that didn't solve any
problems - in fact it created some more! The SE would lock up at 0.6 volts
instead of the more usual 2.2 V!
By this stage I was thinking about making the popper phototropic as well.
I added a phototransistor to the anode of each FLED. This raised the
switch-on level, but made the bot nicely follow the light.
I then thought about putting a capacitor in series with the FLED. My
reasoning being that since the FLED only has to trigger the two
transistors, not actually hold the base down, then you only need a little
spike, not a long on-time. This proved to be the key to good performance.
Of course, I also needed another resistor to discharge the 4.7 uF cap in
between FLED flashes. I found that the best place to put this was on the
motor terminal, instead of the base of the PNP transistor. The reason for
this is that the motor terminal swings a lot more than the base of the PNP
(2.2 V as opposed to 0.7 V), so it helped to 'latch' the circuit more
easily.
My problem now was that the damn phototransistors were affecting the
switch-on level too much. I wanted the bot to have a lower switch-on level
in low light (because the solar panel won't produce as much voltage) but
when I wired the PTs in that configuration, the switch on level would
shoot up to 2.9 volts! This was way too high for me, so I decided that
since no-one was looking, I would dispense with the PTs altogether and see
how much the FLEDs were affected by changes in ambient light. To my
delight, Fred saw the light and turned towards it! The changes in ambient
light were enough to change the characteristics of the FLEDs, and make
Fred choose the brighter side over the duller side when ready to fire a
motor.
Tuning:
If you have a look at the schematic for Fred, you
will notice that there is no way to 'tune' the circuit if one side of your
photopopper is more likely to turn on than the other in equal light.
Bad luck.
The way I tuned Fred is that I selected two FLEDs that were as equal as I
could find, in the selection that I had. The FLEDs are the most critical
part of tuning. Get them right, and the rest just takes care of itself. So
how do you choose two FLEDs that are about equal? Simple.
All you need is a solar panel (or a power supply with a 2.2k resistor in
series), and a reasonably large storage capacitor. Put the solar panel,
the cap, and two FLEDs in parallel, + to +, - to -. Short out the cap, and
wait for it to charge up. The FLEDs should come on at around the same
time, and should flash with equal brightness as the storage cap charges
up. Go through your selection of FLEDs to find two that are as similar as
you can find.
A better way to do it is with a multimeter. Put the meter on resistance
measurement, put the FLED in a dark environment, and measure the
resistance. Do this for all your FLEDs. I was amazed at how much variation
there is in a batch of the same FLEDs: I had measurements from 0.5
MegaOhms to 8 MegaOhms. FLEDs within 95% of each other should work fine.
Putting a 0.5M and a 8M FLED into the one popper will give a very lopsided
outlook on life.
Measure the resistance twice to see how much your test setup has changed
between measurements. This will give you an idea of how sensitive these
components are.
Of course be careful not to let your fingers touch both leads at once!
This will distort your measurement a lot.
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