Here is a simple circuit based on IC uA 741 that can be used to pickup voice from telephone lines without even contacting the line. If a high impedance magnetic core is placed near a telephone instrument or near one of the telephone wires, it picks up inductively both sides of the telephonic conversation without any electrical contact. A preamplifier based on IC uA741 is used to pickup these signals. The circuit is nothing but an inverting amplifier whose gain depends on the feedback resistance R4.The Op-Amp produces the necessary gain to drive the headphone. The circuit can be used as a telephone bug too.
Circuit diagram with Parts list.
- Assemble the circuit on a good quality PCB or common board.
- You can use 9 to 24 V DC for powering the circuit.I prefer 12V.
- The coil L1 can be made by winding 3000 turns of 0.4mm insulated copper wire on a plastic former.
- If the pickup is bad increase the no of turns to 5000 and try.
- The POT R4 can be used for gain adjustments.