This circuit triggers a relay when a light beam that hits the phototransistor is cut off. The circuit is from Electronics Now and the supply voltage depends on the relay voltage.
The integrated circuit used is a voltage reference from Motorola. This configuration was obtained in...
This circuit is from a Radio Electronics from October 1970. The circuit uses transistors from that...
This circuit was obtained from a 1977 magazine, but can still be mounted using BC548 for TR1 and...