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.
Found in Popular Electronics from the 90's, this circuit uses 555 integrated and common...
This circuit activates an incandescent lamp when the light stops falling on the sensor. The...
In the figure we have the configuration of a variable voltage regulator with two transistors and a...