[Pahom]

Originally Posted by brasilpb

With the switch open, the capacitor is charged with 1.5V battery voltage. When the key is pressed, the capacitor discharges through the coil of a loop, forming a brief electromagnetic field, which if the detector is sensitive enough it beeps. This value of 15pf was the lowest value where my device can detect its discharge, if your devices are not so sensitive they can experiment with higher values like 1nF, 10nF etc.

Thanks!!!!! An interesting and easy solution.