This circuit uses a resonant reed relay to recognize the frequency of an input signal. Adaptations can be made to obtain functioning as a shield. The circuit is from 1965 documentation.
This receiver with two common JFETs was found in an old English documentation. The phone is...
This TTL circuit with the 7493 makes the binary count of the pulses sent by a flip-flop with the...
This circuit was found in a Popular Electronics from the 90s. The counter circuit is not common...