This lockable two-position servo control system was found in documentation from the 1990s. The error amplifier can be an operational amplifier and the sensors are phototransistors.
This circuit was found in Electronics Now. With it the dialing signals of a telephone are...
Found in American documentation, this simple circuit activates a buzzer with embedded oscillator. A...
I found this circuit in an old English magazine from 1979, but it can be easily assembled with...