The purpose of this circuit is also the same as above, that is for catching weak sound signals or for hearing some hidden conversation, however the caught voice signals here is transmitted into the air instead of forwarding it through wire links.
The sent signals can be received over any standard FM radio, tuned accurately to the respective frequency.
The circuit is basically a small RF transmitter built around a single transistor.
The circuit functions quite like a Colpitts oscillator incorporating a tank circuit for the generation of the required oscillations.
The frequency mainly depends on the positioning and the values of the inductor, C1, C2 and C3. The coil turn distance and diameter may be manipulated a little for optimizing best response over the FM receiver.
A small antenna in the form of a 3 inches wire may be attached at the shown point for making the “bug” highly responsive and generate distortion free signals.
For the 741 circuit, the speaker is only positioned and used as the receiver and may be placed in some other room, where the eavesdropping may be intended to be carried out.
The linking of the speaker from the amplifier circuit may be done through wire connections, preferably by using thin wires and escorting the entire length up to the speaker in some hidden way, probably by laying it under the carpet or across the corners of the room.
For the wireless circuit everything becomes pretty simple and you just have to hide the transmitter circuit in some suitable place, like under the table, couch, sofa etc.
Parts List:
| Symbol | Description |
| R1 | 33K |
| R2 | 100K |
| R3 | 470 Ohms |
| C1 | 10 pF |
| C2 | 27 pF |
| C3 , | 27pF |
| C4 | 102 disc |
| C5 | 10uF/10V |
| Mic | condenser mic |
| T1 | BC547 |
| L1 = 3 to 4 turns of 22SWG super enamel copper wire, 5 to 7 mm diameter, air core | |
| | |
Circuit Diagram:
