This circuit uses a transistor to trigger the alarm from a low quiescent current sensor. The relay is 12 V. The circuit is from Radio Electronics magazine, April 1975.
This circuit, found in a 1986 documentation, uses an optical sensor to count pulses and, in its...
This circuit, found in a 1997 publication, activates a lamp or a monitoring camera when the bell...
We found this circuit in an English publication in 1981. It can be improved by exchanging T1 for a...