This circuit is from a 1992 PE Hobbyst Handbook. The circuit fires an old electronic flash using an SCR. The circuit can be adapted for other types of applications.
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...