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 circuit is from an 80's documentation. The transistor can be any general purpose PNP and the...
This powerful pushbutton uses a power MOSFET. We found it in a Popular Electronics magazine from...
This shield circuit for lamps and other charges was found in a Popular Electronics from the 90's....