We found this circuit in an Australian publication from 1991. It is a kit of the time with input for a guitar pickup. The range is a few tens of meters.
The circuit in the figure is based on an LM101A; diodes can be replaced with equivalents. The...
This circuit, with magnetic sensor, uses an operational amplifier of the time. The circuit is from...
Found in a 90s Popular Electronics, this circuit checks if a remote-control transmitter is...