Found in a 90’s Radio Electronics, this circuit uses a component from the time, difficult to find today. It generates 12 tones in different octaves.
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...