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.

Divider by 5 Counter (CB25363E)
This TTL circuit was found in the December 1990 issue of Radio Electronics magazine. The circuit has...
Full wave dimmer with SCR (CB7219E)
Found in an Electronics World of 1968 this circuit can use any SCR and diodes depending on the...
Wireless CW Monitor (CB4179E)
Placed near a telegraphic transmitter (CW) and with a small antenna or pickup, it reproduces the...