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.
For reference only, this is the first IF amplifier stage of a common transistor radio from the...
This circuit from July 1975 Radio Electronics uses any operating system with a symmetrical source....
This setting is valid for other transconductance operational amplifiers. The circuit is from...