This circuit is from General Electric from the 80s, using a relay with high current contacts (20 A). The transistor admits equivalent, and any phototransistor can be used as a sensor.
This Rytron transmitter was published in a 1953 book. The circuit can be implemented with a BC558...
Foto-Darlington for applications such as light sensor. In normal applications only the collector and...
This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and...