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Feivel was my very first robot. He's a little phototropic (light-seeking) robot built into an old computer mouse case. I'm nowhere near clever enough to come up with the simple circuit that controls him -- it's the classic 'Herbie' circuit by Randy Sargent, using an LM386 op-amp (originally intended for use as an audio amplifier) to read the difference in light level between the two infrared phototransistor 'eyes' and use that to split the voltage between the two motors. When both eyes are seeing ...
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/robots/show/feivel