Free circuit schematics - wiparat.com

With some modification, I create my own version of FM Telephone bug complete with stripboard layout.
Theortically, the frequency range is from 88MHz to 94MHz but you may experiment with the coil structure to tune it.
Source : http://leoricksimon.blogspot.com

This circuit will allow you to place a phone call on hold and if you wish to have them listen to music while they are on hold. The circuit operates as follows: The RED wire from the phone jack is typically positive and the GREEN wire is negative or ground. When you want to place [...]