This circuit was found in a 73 magazine for radio amateurs of the 90s. The circuit uses two types of indicators, the diodes are for general use. The power supply is 5 V.
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...