Proximity switch (CB22444E)

We found this circuit in a 1992 Popular Electronics. The circuit has an optical sensor with LED and phototransistor. The other components are common.

Antenna tuner (CB6982E)
This circuit for receiving or transmitting low power was found in old American documentation. It is...
Noise canceler (CB8774E)
Suggested by the 90s ARRL Handbook edition, this circuit aims to reduce or cancel noise at the...
Adapted from capacitance (CB25301E)
This circuit allows you to read capacitances on a common analog instrument. It is part of an...