This circuit for the 3 to 6 MHz band was found in a 90s Popular Electronics. The components of the network formed by C6 depend on the frequency range to be covered.
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...