Betabot Photopopper

Betabot was my first attempt at the Miller Photpopper circuit. Up until this point I had only build Fled photopoppers, but I was intrigued by the improved efficiency of this circuit, and I thought I would give it a go. Betabot is powered by a Solarbotics 37 x 33mm solar cell, connected to a 4700uF capacitor. This time around, I chose my components more carefully. Capacitors increase in size with voltage rating, and if I wanted to keep this robot small, I was going to chose a power capacitor with an appropriate voltage rating. This cap is rated at 6.3 volts. Since my miller engines would be firing at approximately 3.4v, I wouldn't have to worry about exceeding the capacitor's voltage rating.

I couldn't be happier with Betabot's performance. He quickly scoots across the desk under lamp light. The miller photpopper circuit is my favorite circuit to build, and Betabot would not be the last. Changing values for C2 and C3 will increase or decrease the step size of the bot. If you're building a bot that needs longer steps, increase C2 and C3 appropriately. Keep in mind that you may have to increase C1 as well. You can't pull more power per step if the main power source is empty!

This brings up another good topic that was originally written by Wilf Rigter regarding the capacitance of C1 in this circuit (or the Miller solar engine as well).

With a small storage cap that is nearly discharged at every trigger event, the avarage storage cap voltage and therefore average solar cell voltage is well below the peak power point voltage. By adding a much larger storage cap across solar cell that discharges only a small fraction of its stored energy at every trigger, the capacitor voltage will only fluctuate a few 100 mV allowing the solar cell to operate continuously very near the peak power point. The trigger of the SE must be set to correspond to this optimum operating voltage and the SE on time should be limited to avoid deep discharging the storage cap.

This larger cap must have low resistance to delive a high current pulse and is usually one 1F - 2.5V gold cap or two 1F - 2.5V gold caps connected in series running at 5V. The current pulses will cause a small drop in voltage across the internal resistance. This large cap can also can be a 1F - 5V high internal resistance "memory" cap in parallel with the 3000uF electrolytics.

The drawback is that the large cap requires an initial long charging time after which the SE will pop much faster. 
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