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.

Crystal Variable Oscillator (CB13851E)
With the circuit shown in the figure, the frequency of a 5 MHz oscillator controlled by crystal can...
Wein Bridge Oscillator (CB10775E)
This circuit was obtained from Popular Electronics in 1993. It consists of a low frequency...
Power sensitive relay (CB22293E)
This circuit was found in a documentation from the 60s. The drive transistor is a power...