1984 – Moth , a light-seeking robot – Gene Oldfield (American)

Extract from the book "Everyone Can Build a Robot Book " by Gene Oldfield and Kendra Bonnet, 1984.

"The concept behind the Moth is very simple. When you turn on the robot in a  dark room, the photocells have a high resistance rate that blocks the flow of electricity. The Moth does  not move. When exposed to light, however, the resistance is diminished. Current flows through the photocells to the transistors. The transistors act like amplifiers and increase the amount of current. The collectors send this boosted signal directly to the motors, which drive the Moth.

Each motor is connected to the opposite transistor. The reason for this is that when you shine a flashlight at the right cell, you instinctively expect the Moth to move to the right (or toward the light). But it is actually the left wheel that pushes the Moth to the right, and vice versa. The brighter the light source, the faster the Moth moves."

pdf of article.