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 configuration with common CMOS gates was found in an American publication from the 90s. The...
This circuit is the receiver for the previous transmitter modulated by an audio signal. The...
This circuit, suggested by an Electronics World of 1969, keeps a blinking LED for a while when a...