# Building Telephone Spy Circuit

Discussion in 'The Projects Forum' started by baba16, May 17, 2012.

1. ### baba16 Thread Starter New Member

May 17, 2012
6
0
I am trying to implement this circuit.I found out the following circuit on internet but there was nothing mentioned about how does this circuit work.Values of some circuit elements were missing like What should be the value of IN4884 Diodes? What should be the value of Coil ? What should be the value of Antenna ?

if Genius understands how this circuit is working, then please mention it in your Answer.

I am a beginner so please bare with me this question.

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2. ### DickCappels Moderator

Aug 21, 2008
2,767
667
To answer your questions:
The value of the diodes is that shown on the schematic. its hard for me to read it, but it looks like "1N4X84", where "X" is not legible. You can use 4X 1N4004 through 1N4007 for this.

For the inductor, you can try six or seven turns of solid wire close-wound on a pencil (8 mm or so in diameter), then stretch the coil until your signal gets to the frequency you are interested in.

You don't need an antenna for this because the coil makes a nice antenna by itself. If you decide to try and antenna, try about 8 cm of solid, insulated wire.

3. ### baba16 Thread Starter New Member

May 17, 2012
6
0
Thanks alot buddy. I have zoomed the picture an I found out the value of the diode is IN 4348.

I didn't get your point "then stretch the coil until your signal gets to the frequency you are interested in."

And can you please explain how this circuit work. That would alot to me. BTW many thanks for your kind assistance.

4. ### DickCappels Moderator

Aug 21, 2008
2,767
667
The circuit is in series with the telephone line. When the telephone is off the hook, current flows through this circuit. The 1k resistor in series with the LED works in conjunction with the resistance of the telephone line to limit the average voltage across the circuit but to allow the voltage across the circuit (from the bridge + to bridge -) to fluctuate as a function of the instantaneous current flowing in the telephone line. The current fluctuates as a function of the sound signal that the telephone line is carrying. So ,at this point you have some voltage across the circuit, and that voltage fluctuates as a function of some sound. Hold that thought.

The transistor circuit is an RF oscillator whose frequency is a function of the inductance of the coil and the capacitances associated with it. These capacitances include the 26 pf capacitor, the 12 pf capacitor, the capacitance among the coil windings and the capacitance of the collector of the transistor's collector to its base. The transistor's base-emitter capacitance is a function of the voltage applied to the circuit (specifically between the base and collector). So now you have voltage that varies with the sound signal on the telephone lines across an oscillator circuit and the frequency of the oscillator is a function of the that voltage. That gives you frequency modulation.

The inductance of an air core coil, and from that the resonant frequency this circuit is mainly a function the diameter of the coil, the number of turns, and the length of the coil. Once a coil is wound, the easiest way to change the inductance, and therefore, the resonant frequency (the transmitter's frequency) is to stretch or compress the coil. If you use insulated wire, you can escape problems related to the turns shorting.

If you don't get at least an A- on this, I will be disappointed.

5. ### baba16 Thread Starter New Member

May 17, 2012
6
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Sir I will try my best to get an A. I always try never to disappoint my tutors and friends. And you are my Tutor and friend also. Sir thanks alot for your such kind and nice help. If you allow me can I become friends with you on Facebook.

Best Regards.

6. ### Etronic Active Member

Oct 7, 2011
127
2
This may help you.
The four diodes is 1N4884 its a full wave bridge rectifier. you can get one at digikey.com For fifty three cents.

http://search.digikey.com/us/en/products/MB4S/MB4SCT-ND/965653

7. ### baba16 Thread Starter New Member

May 17, 2012
6
0
Sir thanks alot. Now I have started assembling this circuit, The main Problem that I am facing uptill now is how to connect phone line to input and output, as the phone line has many wires..