This very simple circuit uses a phototransistor and a very sensitive relay (reed type) being used over short distances with a lens system. The circuit is from a 1967 Japanese publication.
This is the traditional configuration of an AC amplifier using the 741 integrated circuit. I found...
This article is old, from 1994. But, for those who have an old PC and want to use a joystick from...
This simple circuit indicates when the two channels of stereo equipment are balanced. The diodes can...