This circuit was found in the September 1971 Radio Electronics magazine. The buzzer can be replaced by a relay according to the supply voltage. R1 adjusts the sensitivity.
This bargraph-type circuit employs a not very common integrated circuit today, but can be adapted...
Found in a 90s Popular Electronics, this circuit keeps an LED on when a phone is in use. Can be...
Maxim Integrated's MAX17682 integrated circuit is available at Mouser Electronics, a...