A bug detector is a device that detects hidden wireless electronic devices such as wireless microphones, spy cameras, Wi-Fi devices, GPS trackers or any gadget that emits some kind of radio frequency (RF).
The proposed design can be specifically used as:
- Wi-Fi Signal Detector Circuit
- FM Transmitter Signal Detector Circuit
- Wi-Fi Spy Camera Detector Circuit
- Wireless Mic Detector Circuit
Overview
Also called anti spy RF sniffer, these are usually used to scan and detect hidden electronic surveillance, that may be installed to secretly monitor a "target" or an opponent and secretly learn about their plans.
Bug devices are mostly used by detective agents, police, and secret agents for tracking the behavior of a suspected criminal, or a personal client.
The bug detector circuit presented here is exclusively developed by me, and can be used for detecting, pinpointing any hidden wireless device or unwanted surveillance planted in a room.
The hidden spy devices could be inside beds, cupboards, tables/chairs, flower pots, or in fact anywhere a normal individual would least suspect.
Identifying such hidden unwanted surveillance system can be impossible without using costly and sophisticated equipment. However, the circuit idea presented here is not only cheap to build, it also accomplishes the job with utmost perfection.
The complete circuit diagram can be seen below:
Video Test Result
NOTE: the sensitivity of the circuit can be adjusted to much higher levels either by increasing the 2M2 resistor value, or by adding two more op amp stages in series with the above design, since we already have two extra op amps in spare inside the IC.
Circuit Description
The circuit is basically built using the quad op amp IC LM324. Although the IC has 4 op amps in-built, only two op amps are actually implemented for the bug detector application.
The A1 and A2 stage are identical and both are configured as high gain inverting amplifier circuits.
Since the two amplifiers are joined in series the total gain is highly enhanced making the circuit highly sensitive to RF interference.
The circuit is so sensitive that it is easily able to detect all types of electrical noises even thunder lightning interference.
I was surprised when I saw this bug detector circuit easily picking up signals from my wireless Wi-Fi device from a distance of 2 feet. Actually, while the unit was placed on the bed, I found the LED blinking abnormally as if the circuit was unstable and malfunctioning. I was quite disappointed.
Then I picked it up and put it some distance away from the bed, and the LED just shut off. I tried placing it again on the bed and the LED started blinking again. I still couldn't figure out the reason, and thought may be the bed was acting like a large antenna and causing the disturbance.
However, finally I realized that this was happening because my internet WiFi unit was also placed on the same bed at some distance away.
I removed the WiFi device from the bed and the bug detector LED was simply shut off again.
Next, I did a number of repeat tests and was convinced that the unit was actually detecting RF, and the LED blinking wasn't due to an unstable or malfunctioning condition.
Once confirmed I built the final bug detector circuit and presented it here for your reading pleasure!
Parts List
- R1, R4 = 2.2 Meg
- R2, R3 = 100 K
- C1, C2 = 0.1 uF PPC
- A1, A2 = 1/2 LM324 op amp
LM324 pinout details can be found below:
For further inquiries or information please use the comment box below.
About the Author
I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!
Any ideas on how/why it works at all?
From purely theoretical point, it shouldn’t work, because…
1) C1 shorts antenna input to ground in terms of AC (RF input). In case of “ideal” capacitor, 0.1uF would be pretty much negligible impedance for anything that looks like RF.
2) Gain-bandwidth product of LM324 is nowhere close to wi-fi or cellular, it isn’t RF device on its own.
3) There is no obvious explicit RF detector circuit either. So where does RF turns into something LM324 can process, how and why?
As reasonable guess, “real” parts would have plenty of parasitic parameters, say, most 0.1 uF caps would have some “resonance frequency” far lower than cellular or wifi. Past that point they rather behave like inductance instead, due to parasitic inductance of leads/plates overtaking capacitance action. However it isn’t obvious and if one means it have to be inductor, using “explicit” inductance with specified parameters is probably better idea? Then, I can imagine strong RF field can manifest itself by detecting on either ESD protection of inputs, or on input BJT junctions or so, however it nowhere close to being obvious and it would be my best guess I can do, could be incorrect.
p.s. also drawing + and – on opamp inputs in schematics could be good idea. Say I do have LM358 or 2904, at first glance it seems this circuit needs just 2 opamps, these should do as well, unless circuit relies on some extremely unusal parasitic properties of LM324 I’ve failed to foresee. In case of your circuit I have to scroll between IC pinout and diagram to “translate” it into to LM358-based version. Should there be + and – I would rather just redraw circuit by attaching functional pins the way circuit means it – at the end of day I’ve got curious why the hell THIS works at all. So if it actually does, I would definitely bug few “advanced” RF engineers to properly decode how and why it detects anything at all – I bet it can puzzle even seasoned RF engineers 🙂
Thanks for compelling to investigate this circuit more deeply 🙂
Actually there’s nothing complex about the functioning of the circuit. The op amps are honestly doing what they have been designed to do.
Referring to A1, the non inverting (+) pin3 is tied to ground, and pin2 is also at ground potential via the 100K resistor but is not as grounded as pin3. This makes it highly sensitive to any electrical disturbance even at mV range.
It has nothing to do with the frequency of the RF, it is merely detecting the electrical potential that hitting pin2.
Since the RFs may be fluctuating from across positive and negative cycles, the negative cycle which are more negative than pin3 potential are quickly caught by pin2. causing the output pin1 to go high. This high feedback quickly reaches pin2 back shutting it down, and the cycle is continued causing the LED to flash.
The 0.1uF is actually introduced to blunt of the sensitivity and prevent the circuit from detecting the unwanted disturbances emanating from other possible RF.
I am sorry redrawing the diagram may not be possible because this schematic has been indexed by Google, replacing it may not be good for the article, for the future articles I’ll surely consider adding the +/- to all op amp circuit.