This automotive grade circuit was found in a 1991 Electronics Hobbyist. The transistors are general purpose types. P1 sets the monitoring voltage.
The interruption of the light incident on the phototransistor causes the trip of this circuit that can...
Found on Popular Electronics from 1969, this circuit can be implemented with components that are...
This simple timer, which activates an LED indicator, but which can be modified to activate a...