This circuit was found in an Italian magazine Radiorama from 1960, but it can be assembled with more modern transistors such as BC558 and powered with 6 V. The transformer can be obtained from an old transistor radio and the L1 coil consists of 500 to 100 turns of wire around a box that houses the device. This box, which seems empty, since the circuit is hidden, receives the signals of a link formed by a few turns under a carpet connected to an amplifier. The sounds emitted come out of the box and someone can speak into a microphone.
