The Bare Bones photovore is a beautifully simple circuit that makes use of the fact that the switching voltage of the inverter inputs on the 74AC240 is roughly half the supply voltage. The circuit uses a pair of photodiodes as a voltage divider. The mid point of the divider is converted to a “high” or “low” signal by the inverter, which is used to determine which of two motors to run.
I’ve been toying with the idea for a while of using two solar panels in place of the photodiodes, and simply taking a mid-point voltage reference to figure out which panel is getting more light. Then, given the appropriate configuration of solar panels and motor wiring, it should be easy to make a bot that heads towards the light with no photodiodes. This is the result (with schematic this time!).
Its body is the 74AC240 chip, with a capacitor abdomen. The head is the Miller solar engine, which uses a couple of surface mount components; an old SMT diode, and a tiny capacitor, both salvaged from old circuit boards:
This guy works well. It’s a little slow to pop if the light is only hitting one panel, but takes very few pops to orient itself towards the light. Once both panels are getting equal illumination, both motors pop and it goes forwards.