In this 2001 circuit we have the way to trigger an SCR with a phototransistor. The circuit appears elsewhere in this section, obtained from publications from different eras.
We found this circuit in a 1968 Electronics World. The potentiometer should be set to trigger the SCR...
This circuit was found in a 90s Popular Electronics. The circuit uses 4 bicolor LEDs to indicate...
This small FM transmitter has a sensitive modulating stage and is therefore suitable for listening...