Electronic Eavesdropping Devices Detector

[AN920]

i want sensitive to mobile and eavesdropping devices.

Many cheap bugging devices work in the commercial FM band so that may cause problems detecting them out of all the other stations.

[audioguru]

Many cheap bugging devices work in the commercial FM band so that may cause problems detecting them out of all the other stations.

The other stations are all the FM stations, all the VHF and UHF TV stations and all the taxi cab radios.
The Eavesdropping Devices Detector picks up a computer's interference so it might even pick up radiation from a radio and a TV.

[eplanet]

hello
this is eavesdropping device detector that i was build it. these green LED's that u can see is VU meters with LM3915.
tell me your idea.

[audioguru]

Hi Eplanet,
Your project looks very nice.
Why are all the LEDs on? Is it picking up a very strong signal?

[eplanet]

hi
i have problem!!!!! all LEDs on because i am near by a computer. it is very bad when i shutting down computer in room all the signal was end. after it unfortunately all over the room even near the bug not gave a signal and only touching antenna with my hand have signal.
unfortunately i didn't know that i am near the computer when i was building this project and i was thinking that this project work. but i understand that i was near the computer  :'( :'( :'( :'( :'( :'( :'(

[eplanet]

hi what is your idea about this circuit?
where is the buzzer in this circuit ??

[AN920]

hi
i have problem!!!!! all LEDs on because i am near by a computer. it is very bad when i shutting down computer in room all the signal was end. after it unfortunately all over the room even near the bug not gave a signal and only touching antenna with my hand have signal.
unfortunately i didn't know that i am near the computer when i was building this project and i was thinking that this project work. but i understand that i was near the computer  :'( :'( :'( :'( :'( :'( :'(

The circuit need a much better high pass filter on the input, to eliminate the low frequency noise that is triggering your circuit. The RC filter is not enough. Another way will be to have a switchable filter for commercial FM band, mobile etc. The circuit also need to me in a metal or screened box so the only signal comes in via the antenna. You can paste tin foil on the inside of your plastic box to screen this. I will look at designing a suitable filter/s that you can try.

[ante]

Here is a simple Bug Finder 20 MHz to 1GHz. The scanned article is from an old German magazine but I

Bug_Finder.pdf

[audioguru]

Hi Ante,
The Bug Finder circuit you posted is nearly identical to our Eaves Dropping Devices Detector project.

[ante]

Hi AG,

Yeah, like I said

[AN920]

OK, here is the modified input circuit to try.

The first picture shows the original circuit and a plot of the gain (blue) and input match (red). It is clear that we have loads of gain (23dB) from near DC. As bugs will normally be in the FM band (88-108MHz) and up, we don't want this gain at the low end to pick up PC psu's, tube lights etc. The red trace shows that the input match (50 Ohm) improves with higher frequencies. At 2GHz the return loss is about -12dB

The last picture shows the modified schematic. Notice that below about 30MHz we have a rapid gain drop-off. Input match is very bad resulting in high attenuation below 30MHz. Above 30MHz we can see the gain increasing and peak around 500MHz. Also note that the input match gets much better over the frequency range.
The best possible with the old circuit was -11dB. Now we are below -13dB after about 150MHz with -17dB at 350MHz.

The BFR93A model that I used should be close enough to the BFR90/1A

Ideally we need more gain, something near 20dB+ to make the circuit more sensitive at the upper end. Another alternative is to use a simple gain-block that will have enough gain up to 2GHz. That together with zero bias schottky RF detector diodes as a voltage doubler should be sensitive enough.

[AN920]

I managed to find a model for the BFR90A. It turns out that it can perform a bit better. More gain overall, but with the previous bias the input match above 1GHz was only -5dB. Running the transistor at 10mA instead of 7mA improved this condition.

First plot shows the problem with bias 7mA

Last plot shows the improvement at 10mA

[eplanet]

hello thanks anyone
do u know  where is the  buzzer in this circuit?

[audioguru]

Hi Eplanet,
I think The Bug Detector With Beep is a radio transmitter that transmits Amplitude-Modulated beeps.

[eplanet]

hello anybody
i want to build a eavesdropping device detector that can detect only bug and good work and better than this project on this site. do u have any good project or idea about it????????????
for example see this attachment:
http://cgi.ebay.com/All-in-One-Counter-Surveillance-Device-RF_W0QQitemZ300113432713QQcmdZViewItem

[eplanet]

hello audioguru
please check this circuit for any problem. if haven't any problem i want to build it.
and please tell me why S1 connecting to 1n4148 and connect to GND?
thank u very much.

[audioguru]

It looks like the same circuit as in our Projects section but its C8 is backwards and it doesn't have a sensitivity control.
It has a few parts missing around the detector diode and the speaker doesn't have a current-limiting resistor in series.

I didn't check the values of all its parts.

S1 probably allows you to connect its 0V to earth ground to make it more sensitive. EDIT: It is the on-off switch from the negative wire of the battery.

It will have the same problems of input offset voltage and sensitivity to interference as our project.

[M.Omer]

1. Could anyone of you please tell me the physical dimensions of the PCB?
2. What could possibly go wrong with this project? This would be my first project and I don't want to have bad dreams regarding this one for the rest of my life.  ;D
Later.

[audioguru]

Hi Omer,
Other members have reported problems with the circuit that are not easily solved.

It gives false indication of a transmitter nearby and it picks up all kinds of interference from computers and monitors, AM and FM stations, TV's, TV stations and taxi cab radios.

[M.Omer]

Thanks! =)
I have some other questions.
1. What exactly do you mean by turning-up or turning-down the sensitivity?
2. In on of the replies, you said that we can replace the R7 by a High-Q parallel LC circuit. What exactly would that do?
3. The physical dimensions of the PCB?
4. Is the bare diagram of the PCb and it's parts is correct here. As I really want to make this thing work, I want to be sure of the basic things.
5. Has this actually worked for someone out here? Fir instance, Maroela and eplanet. xD
6. Is there any way I could increase the sensitivity of this device so that it would detect only the bugs and not the radio stations etc.?
7. I read those improvements AN920 put forward. What exaclty do they do?

Later.

[audioguru]

1. What exactly do you mean by turning-up or turning-down the sensitivity?

P1 is the sensitivity control.

2. In on of the replies, you said that we can replace the R7 by a High-Q parallel LC circuit. What exactly would that do?

AN920 recently showed his modofocation of changing R7 to an inductor and changing the values of C4 and C5 so they all make a highpass filter. Then the circuit won't be as sensitive to computers, their monitors, TV's and AM radio stations.

3. The physical dimensions of the PCB?

In MS Paint, I took the IC pins spacing as a ruler and found the pcb to be 2.1" wide by 2.2" high.

4. Is the bare diagram of the PCb and it's parts is correct here. As I really want to make this thing work, I want to be sure of the basic things.

We don't know if the pcb and the parts list is correct.

5. Has this actually worked for someone out here? Fir instance, Maroela and eplanet.

Nobody has said it works well.

6. Is there any way I could increase the sensitivity of this device so that it would detect only the bugs and not the radio stations etc.?

It is an extremely simple AM radio. It does not have tuning so it picks up all nearby radio signals at the same time. Bugs and radio stations (and TV stations) all transmit radio waves.

7. I read those improvements AN920 put forward. What exaclty do they do?

They make the input a highpass filter to attenuate low frequency interference as mentioned above.

The opamp has an input offset voltage that could be much higher than the signal from a bug. It would need re-design to add a null adjustment.

[M.Omer]

Thanks a lot. I'll let you know of my progress and any setbacks I face during this.
Later. =)

[eplanet]

hello my friends
i built this project for three times  and not good work because i should keep this device near the bug until smooth work.
do u have any idea?

[audioguru]

Hi Eplanet,
The input of the circuit needs the highpass filter shown by AN920 so it has reduced sensitivity to cpmputers, monitors, AM radio stations and other interference.

The rectifier/detector circuit looks strange. Did you try my modification? If there is a sound output when the sensitivity control in my modified circuit is zero then the opamp needs to have its input offset voltage nulled.

[M.Omer]

@ Eplanet,

Dude, you are giving me goosebumps! Three times? =O lol

Audioguru,
Wouldn't the PCB design change if I use the modification given by AN920?