This project is both an experience and a very interesting listening system. With the help of an amplifier and a headset we can transmit sound through the wall. These sounds can either be from a radio program, music from any media or sounds picked up by a microphone, in which case we will have an efficient spy system.
Induction signal transmission systems are used in spying because they eliminate the need for wires between the location of the microphone and the listening. In this way, when the microphone is discovered, it will not be easy to find out where the spy is with the phone, since there are no wires to follow.
Our experience shows how this inductive ring transmission system works, and can be performed simply in demonstrations or more elaborately in a true espionage system.
The basic idea is to apply the output of a good power amplifier to a large coil in order to obtain signal radiation by magnetic field. For an amplifier of about 10 to 30 W the distance can be on the order of a few meters or even through a wall.
How it works
The principle of operation is simple: applying the output signal of an amplifier to a coil made with ordinary wire, we have in its turn the production of a magnetic field with the same characteristics of the sound of the amplifier.
To capture the signal all we need is another coil, with many turns of wire, on which we can either connect a headset or a small amplifier powered by batteries.
Assembly
In figure 1 we have the complete diagram of the system and in figure 2 the aspect of the assembly.
The coil 1 consists of 10 to 15 turns of common wire (22) connected to the output of the amplifier. If the wire is thick we need to limit the current at the coil output, as shown in figure 3, using a resistor of 10 ohms x 10 W.
The receiving coil can be obtained by disassembling an old output or power transformer and withdrawing its core. We connect the high impedance (crystal) earphone to the coil with the largest number of turns.
In figure 4 we have the way to use the system in the transmission of signals through a wall.
For listening to conversations place the amplifier with a microphone hidden in a room and position the coil close to the wall. On the other side, position the receiver coil to hear the signals. Test first to get the best placement of the microphone and coils.
Material list
L1- Transmitter coil - see text
L2 - Receiver coil - see text
X1- Crystal earphone or piezoelectric capsule
