Photovore
Photovore is a BEAM robot that moves following the light.
By analogy you can think of insects in nature that fly at night around a light source. There are many designs of photovore bots on the internet; some of them are using solar cells, some photoresistors, some photodiodes or phototransistors.
Schematic
I simulated the schematics in Proteus software and I obtained realistic results. Here are some screen shots!
We have a voltage divider which polarize the base of the transistor. When the light does not fall on the photoresistor, it’s electric resistance is at the maximum value so basically all the voltage source drops on the photoresistor. The current flow is insignificant.
When the light hits the photoresistor, the electrical resistance drops so according to Ohm Law the voltage on the resistor drops t0o.
We also have some current flow through the circuit. The difference of the voltage source is taken by the resistor between BASE and EMITOR. So we have enough voltage applied on the base of the transistor. The obvious gain is the current throw the collector(20 times bigger that the on the base). In this situation, there is enough power to drive the motor. In the video below, the bot is somewhat clumsy. That’s because I didn’t tune enough the two potentiometers in the base of the transistors and the beam of light is diffused so it touches the both photoresistors.
The motors had to be positioned in an “X” shape, otherwise the bot would have run away from the light.