Electronic Eavesdropping Devices Detector

[MP1]

I am curious at the thinking that kit-makers put errors in their products purposely -- and if the kits would not work -- would that defeat their intention to sell the kits

 

[webharsh]

hi
i like the circuit . can u suggest amodification so that it can detect IR frequencies.

 

[audioguru2]

Hi Webharsh,
Welcome to our forum. ;D
The circuit amplifies and detects very high frequency radio signals. More amplification won't allow it to detect IR which is light that cannot be seen.
To detect IR you need a light detector such as a photodiode, phototransistor or IR receiver IC.

 

[webharsh]

so can we connect a photodiode with the circuit , to detect IR signals ? if yes then how ?

 

[audioguru2]

so can we connect a photodiode with the circuit , to detect IR signals ? if yes then how ?

Of course not. This is a high frequency radio circuit. IR is light, not radio and is modulated at low frequencies.
For years, IR remote control uses an IR receiver IC.

What do you want to do with IR?

 

[webharsh]

actually i want  a circuit that can detect IR frequencies .

 

[audioguru2]

IR is a form of energy, it doesn't have a frequency that is tuned. Anything with some heat in it transmits IR. The sun transmits a lot of IR as do stoves and fires. Your body transmits some IR.
You must say how strong is the IR you want to detect.

IR is analog modulated with sound and is used for wireless music and language translation.
IR is also digitally modulated with codes for remote control.
Which do you want to detect?

 

[webharsh]

thanks . the one which is modulated with sound .

 

[audioguru2]

This one has the sound frequency-modulating an IR carrier of about 100kHz:

View attachment 38985

 

[webharsh]

hi . thnx for ckt . u call ur ckt eavesdropper detector but the ckt detect any radio freq in range 1-1000 mhz.?????

 

[audioguru2]

I don't know how sensitive is the Electronic Eavesdropping Detector. It might just pickup every radio and TV station, and taxi cab radio in town.

 

[eplanet]

hello
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 ???????????????????????????

 

[audioguru2]

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.

 

[eplanet]

hello audioguru
folowing ur question about collector and output of op-amp :
tr1 collector : 3.9v
op-amp: pin1: 5v pin7: 0.3v pin8: 0.3v and pin14: 7v
what sall i do ?

 

[audioguru2]

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.

View attachment 40405

 

[audioguru2]

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.

View attachment 40406

 

[eplanet]

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

 

[audioguru2]

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?

 

[eplanet]

hi audioguru
i am reducing value of R6  but i dont get good result. dont change it.
do u thing in my circuit any component was damage?

 

[audioguru2]

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:

View attachment 40409