Found in a 1968 Texas Instruments documentation this circuit uses an SCR from that time. Modern equivalent types can be used for a higher power version.
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...