Here is a small robot that will run from sunlight. Just powered by a solar cell. Here are the circuit diagrams, assembly instructions, and the parts list.
On a table next to a window the robot runs continuously during the daytime, even in the shade. But when it has to work itself from the middle of the room to the window, the solar panel will sometimes not even supply that one milli-amp. It makes sense to save up some energy, then run a short distance, repeating the process until enough light is found to run continuously. A large capacitor can be used for storage. The circuit waits until the solar panel raises the voltage over the capacitor (4700uF) above some 2.7 V before switching on the oscillator. If the voltage drops below about 2.2 V, the transistors turn off and the capacitor is recharged. The run/charge ratio depends on the amount of light. It is close to sunset when the run time drops to zero.
The prototype seaches vigorously for better sunlight, while neatly sidestepping most obstacles. Apart from two clocks with wheels attached, it has two photodiodes (its eyes) and two feelers. The clocks turn clockwise. The robot can only switch them on and off. Yet it moves towards the light and it recoils from obstacles.
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