This circuit is from a 1967 RCA publication. The components are of the time, admitting equivalents, including the SCR that determines the maximum power of the load.
The sensors in this alarm are reed-switches. In the diagram we have only one, but several can be...
We found this circuit in a Poptronics from 1998. The circuit can be extended to more competitors...
The circuit shown in the figure uses two of the four operational amplifiers found on the LM324...