We found this cable tester in a Popular Electronics from the 90s. The circuit uses a connector according to the cable to be tested. Adaptations can be made for different types of cables.
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...