In use telephone indicator (CB8657E)

Found in a 90s Popular Electronics, this circuit keeps an LED on when a phone is in use. Can be used with extensions.

Cardiac Monitor (CB10961E)
This circuit was found in a 1987 magazine. It measures monitors the heartbeat through a...
3 MHz TTL Oscillator (CB5123E)
This crystal-controlled circuit was obtained from a 1980s documentation. The power supply should be 5...
Headphone Amplifier (CB14735E)
This circuit is from 1957 Raytheon documentation. The phone must be high impedance with 600 ohms...