The values of the indicated components in this circuit of a 1967 Electronics World, are for a frequency of 512 kHz. The power can be between 6 and 22 V and the FET is a general-purpose type.
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...