Telephone pickup preampliifer
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.?
Notes.?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.
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