This transistor class B 5 W amplifier was found in a 1969 Mullard documentation. The transistors are from that time, and modern equivalents may eventually be used.
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...