Found in documentation from the 1980s, this circuit can have several adaptations to use modern transistors and LEDs instead of the lamp. The power supply voltage can also be changed.
This circuit activates a relay when the phone rings allowing the control of a warning lamp or even...
This circuit was found in a 90s Popular Electronics. The sensor can be any phototransistor. ...
This different power control circuit with SCR was obtained from a 1967 magazine. The SCR can be...