This is a field strength meter found in an 80's Hands On. The circuit operates at frequencies up to 500 MHz and has a buzzer as its indicator. Can be used as a detector for spy transmitters.
This circuit appears in similar versions on this site. This is from a Popular Electronics from the 80's. The...
This two-transistor delay circuit from the 1960s was found in documentation from the time....
This circuit operates with signals from 150 kHz to 1 MHz and makes use of a known NAND CMOS gate....