We found this simple protection with zener diode in a 1967 Motorola documentation. The zener diode is 18 V, but the components can be changed for operation with other voltages.
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...