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Electronic Eavesdropping Devices Detector


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I am new user in ur forum I am very enjoy but many years ago reading ur information in any forum specially "audioguru" hello audioguru.
I am make pcb eavesdropping device detector but not work no sound. i am testing all component but dont listening any sound of buzzer.
no short circuit  what can i do ???????????????????????????

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Hi Eplanet,
Welcome to our forum. ;D
I fixed the schematic for this project. I did not check its pcb.
Did you substitute any parts?
Does your cell phone operate on a low microwave frequency? My cell phone uses some frequencies much higher than this circuit can detect.

With a cell phone transmitting near it and the sensitivity control turned to max, measure and post here the DC voltages at the collector of TR1 and at the output of each opamp.

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Hi Eplanet,
I think your project is working but you can't hear it.
The opamps make a voltage-controlled-oscillator and your input voltage to them is 5V which is good. There should be a triangle wave at the output of A2 and the outputs of A3 and A4 should be a square wave.

I think your transistor TR3 is not working. Replace it then change C2 to a 1k resistor.
Also change R18 to 3.3k.

I think the opamp A1 might have problems with some opamps because it has a DC gain of 1M/220= 4545 but its input offset voltage could be 8mV then its output would be saturated.
It needs to have some kind of regulated offset voltage adjustment.


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Hi Eplanet,
In your personal message to me you say your circuit makes a continuous tone.
As I explained, opamp A1 has a very high DC voltage gain and it amplifies its own input offset voltage.

It is too bad that the circuit doesn't have an adjustment to cancel the offset voltage. A regulated supply would be needed to do it.

Try decreasing the value of R6 to decrease the gain. If 220k is used then the gain is 1000 and the output voltage without a signal will be 1.1V. The max sensitivity will be reduced 4.5 times.


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hello audioguru
thank you for your help to me.
but not have result up to now because my circuit have alarm continously.
please check it :
collector  BFR90 : 2.91v
IC :
pin1 :1.29v
pin7: 1.93v
pin8 : 1.65v
pin14 : 2.04v
R6: 1.1M
and with C2: 4.7
P1:volume  5k
antena: 50cm wire
and pin2 of LS connecting to +9v because in pcb not have this connect but in schematic diagram connected.
for pin1 that have 1mv what can i do ?

thanks for all and all the best

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Hi Eplanet,
Before pin 1 of the IC was 5.0V, now it is 1.29V. What did you do to change it?

Pin 1 is supposed to be 0V without any signal. When a signal is weak then the voltage will be small and the tone will be a low frequency growl. When the signal is stronger then the frequency of the tone will be higher.

You know what? Maybe your project is picking up the signal from a nearby radio or TV station. Remove the antenna to see (hear) if the frequency of the tone is reduced by the weaker signal.

If your project is not picking up a signal but there is a tone then your opamp has an input offset voltage that it is amplifying. Replacing the opamp will change the input offset voltage but it could be worse or it could be the opposite polarity which will reduce the sensitivity.

I suggested reducing the value of R6 to reduce the gain of the opamp then its output voltage will be lower. Did you try it?

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Did the frequency of the tone change:
1) When you removed the antenna?
2) When you turned down the sensitivity control?
3) When you replaced the MC3403 IC?

What resistor value did you try for R6?

Do not connect the speaker terminal 2 to the positive supply. The pcb is different from the schematic because R4 is from the speaker terminal to the positive supply, and you don't want to short it.

Try changing the circuit to be a normal AM detector like in my sketch:


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  • 4 weeks later...

hello audioguru
please tell me that when i build a Eavesdropping device(this circuit) for start after build what hapen after press the on swich (for start).

I don't think the project works:
1) The value of R18 is too low.
2) C2 should be a 1k resistor.
3) The AM detector circuit and DC amplifier's input look weird.
4) The DC amplifier amplifies its own DC input offset voltage which messes-up the voltage-controlled audio oscillator.

If it works correctly then it will pickup all FM and TV stations and taxi cab radios in town.
If there is no RF then the VCO should be quiet. With a low amount of RF then it should make a low frequency growling sound. Its pitch should get higher with stronger RF.
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what is the meaning of offset exactly?

Input offset voltage of an opamp is an error in the balance of its inputs which is amplified by the opamp and causes a DC output voltage when the inputs have the same voltage. Most opamps have a max input offset voltage of 8mV. Single opamps have terminals where a trimpot can be connected to cancel input offset voltage.
Modern expensive opamps are trimmed with a laser when they are made for extremely low input offset voltage. They usually need a dual polarity supply. 
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With no radio signal picked up by the circuit, the output pin 1 of opamp A1 is supposed to be 0V so the voltage-controlled-oscillator that is made with the other opamps has no sound output.

Your A1 opamp amplifies its own input offset voltage and makes its output voltage high enough for the voltage-controlled oscillator to make sound.

The RF transistor and detector diode don't affect the offset voltage. They don't operate at high enough frequencies to pickup modern cell phones.

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  • 2 weeks later...

i am making a eavesdropping devise and it is work but this project sensitive to anything very high. specially to any metal and sensitive to computer
and for this project 9v battery don't good working.
please tell me what can i do ?
i wont sensitive to any metal i want sensitive to mobile and eavesdropping devices.

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Yes, the Eavesdropping Devices Detector project has an untuned input so it picks up all high frequency signals. A computer makes high freqiency signals so it will detect them.
A piece of metal doesn't make high frequency signals so the circuit should not detect anything from it.

The supply current is very low so a 9V alkaline battery should last for a long time.

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