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The BEAM Circuits Collection is a BEAM Reference Library site.

The SmartCap solar engine
Starting from a Bob Shannon design

Bob Shannon originally designed the SmartCap solar engine for a specific purpose -- to provide a "burst" of energy to a 'bot right after its source of light goes away. The idea was to introduce a specific behavior -- when things got dark for the 'bot, it would have the energy to notice and scurry toward the next spot of bright light.

Here's a circuit diagram (originally drawn by Wilf Rigter, I corrected it based on Bob's on-list feedback) for Bob's "Vore-N-More" circuit containing the SmartCap solar engine:


In this diagram, the "SmartCap" is just the 1.5 F storage capacitor, phototransistor, 2N3906 transistor, and 5.1K bias resistor. The idea is that after charging, a loss of light causes the 1.5 F capacitor to be discharged through the 75 Ohm resistor -- this gives the photopopper a sudden "recharge," which it can use to get to a brighter spot.

I prefer to butcher Bob's design and just use the SmartCap circuitry to drive nocturnal circuits. Here's the circuit diagram for this:


This design can be used for BEAMbots that you only want to be active at night -- "crickets", night-lights, and such "pummers." Note that you could just as easily replace the phototransistor here with a photodiode and get the same effect.

Parts list for basic circuit
Radio Shack

Storage capacitor


Solar cell


Germanium diode (1N34A)

$0.36, #900-6232

5.1 KOhm Resistor








2N3906 transistor

$0.15, #TR3906
$0.26, #2N3906-ND
$0.07, #900-5457

For more information...

Bob has some nicely-done pages describing his 'bots that use the "Smart Cap" solar engine at his Robot Roll Call.

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Page author: Eric Seale
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