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.

Power CMOS Shield (CB19763E)
Bipolar transistors are types of general use like BC548, while the power MOSFET can be of any type...
Garage Warning (CB6041E)
We found this circuit in an old documentation. Its purpose is to turn on warning lights when the...
Humidity and Rain Alarm (CB17475E)
This simple humidity alarm circuit includes a timed warning oscillator whose time is set to R4....