This circuit was found in a 1965 General Electric documentation. It is intended for reception of the aviation strip, but inside an aircraft or nearby (which is now prohibited).
This circuit was found in a 1987 magazine. It measures monitors the heartbeat through a...
This crystal-controlled circuit was obtained from a 1980s documentation. The power supply should be 5...
This circuit is from 1957 Raytheon documentation. The phone must be high impedance with 600 ohms...