This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit uses a diac and the transistor can be general-purpose.
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...