Found in a Popular Electronics from the 90's, this circuit detects the approach of objects by the reflection of the radiation emitted by the LED. The relay is a 12 V sensitive type.
This circuit using components that are still common in our market was found in a 1987...
The varactor or varicap diode tunes the signal for listening to SSB signals on a common receiver. The...
This circuit that also serves to control LEDs and other loads is from a 1960 publication. The...