A simple electronic circuit project is discussed here that may be built by any school student for displaying in a school fair science exhibition. The proposed circuit is a high gain opamp amplifier which detects slightest of RF disturbance that might be created by various electrical systems. The circuit is basically a simple high gain non inverting AC amplifier, built around the IC LM 324. Looking at the figure we see actually the the circuit is a repetition of four identical circuits in series.
So we would only want to study the basic concept of the any one of the stages consisting just one op amp. As mentioned in the earlier part of this article, the opamp is configured as a high gain non inverting amplifier, where the input is received at the pin #2which is the inverting input of the op amp. The RF disturbances in the air is received by the antenna and fed to the inverting input of the opamp which is  amplified by the circuit to some specified level depending on the value of the feed back resistor across the output and the inverting input of the opamp.
Increasing the value of this resistor increases the sensitivity of the circuit, however too much sensitivity can make the circuit unstable and induce oscillations. Here the relatively weaker signals from the first stage is further enhanced and made stronger so that now it may be fed to the third stage for repeating the actions that is for further amplification until the last stage whose output illuminate an LED, displaying the presence of even the minutest possible RF disturbance in the air. The discussed circuit of cell phone RF signal detector, sensor is very easy to build and requires minimal knowledge of electronic for going about with the procedures. Take the IC first, and carefully insert its legs inside the PCB holes through proper alignment. Now as per the diagram start connecting the resistors and capacitors one by one to the pin outs of the IC, remember that from the component side of the PCB, the pin out will be just the opposite to what it is from the track side, so be careful with the pin out designations and connections. Once it is made, it’s all connecting the board to a 9 volt battery and confirming the results.
For this you may make a call from your cell phone or just call to know your balance report, the LED in the circuit should hopefully start responding to the cell phones generated RF signals.
Alternatively, you may try clicking your kitchen gas lighter very close to the antenna of the circuit; the LED could be seen flashing with the clickings of the gas lighter. Here is a simple engineering project circuit of handy mobile cell phone detector (sniffer).
This circuit can notice the use of GSM mobile phone in mobile restricted areas such as examination hall or confidential rooms etc. The sniffer keeps monitoring the RF (Radio frequency) level in the region and gives warning if the RF level increases due to mobile phone activity. The circuit is able to detect all forms of mobile phone activity even when it is in silent mode. Since the voltage from the sensor diode is too weak, three stages of amplification are required to give warning indication through speaker.

The pre amplified signal is fed to next amplifier stage built around IC TL071, it is a low noise JFET input op amp with low input bias and offset current.
Here the TL071 is designed as an inverting amplifier; the inverting input of op amp receives the pre amplified signal from the first part.
The amplified signal is then passed through a capacitor and diode; with the help of this arrangement we can introduce more stages to extend the range. The next potentiometer is used to vary the signal amplitude, then it is fed to amplifier stage built around IC LA4440, it is a two channel audio power amplifier with inbuilt dual channels for stereo and bridge amplifier applications.
When the mobile phone is activated within the range of eight meters, a loud motor sound is heard through the speaker. The RF reception and performance of the circuit is dependent on many factors such as output power from the mobile device, orientation and position of the cell phone.
1 – a simple piece of wire for the design you’re talking about, can he effectively detect the frequency of the two band GSM 900MHz and DCS 1800MHz? Hey i try to find the 1N3491 diode everywhere from last 6 months, but failed again and again.
Sir,can I proceed with this circuit for my mini project?I managed to get all the components..Could you please give me a design of this circuit? CircuitsGallery introducing 'The NEW CG PROGRAM STORE', from where you can buy embedded program codes. Our YouTube channel provides collection of various video demonstration of Circuits, Engineering Projects, Simulations etc.
This is a mobile phone sniffer circuit that can detect the signals being used in the GSM (Global System for Mobile Communication) band at about 900 MHz. Is there a possibility to get a more detailed discription on the circuitry operation to my email? Thank you I was about half way through designing something very similar using almost the same components. Well, i also want to ask,what modification will b needed if i need to detect the postion of the cell phone. Sir, I had tried your mobile detector project , but it detect only when the call was comming.
This is a diode-based version of a 1920s crystal radio circuit and the same warnings apply: It can pick up ANY strong RF signal nearby. Hi I am electrical engineering student doing design I have taken a cell dector as my project and It is working but I want to put and 16by2 lcd display but i cannot interface a microcrontrller with the lcd display.Please can please give me ideas or websites I can get this information. Hi I am electrical engineering student doing design I have taken a cell dector as my project and It is working but I want to put and 16by2 lcd display but i cannot interface a microcrontrller with the lcd display.Please can please give me ideas or websites I can get this information. Does it mean that nobody can help in designing circuit to detect cell phone on standby mode?

Cell phone being the major generator of RF interference is easily picked up by this circuit and can be seen through an LED illumination at the output of the circuit. Only two of its op amps may be incorporated, however for making the circuit extremely sensitive, all four of its opamps have been rigged in  series. This pocket-sized mobile communication detector or sniffer can sense the existence of a mobile device. It is a high frequency (Microwave frequency) low power transistor with huge current gain (approximately 15dB at 0.5GHz) and bandwidth. It has good ripple rejection of 40dB, small residual noise, built in over voltage and surge voltage protection, and pin to pin protection. Since the signals are digitally encoded, it can detect only the signal activity, not the speech or the message contents. I wouldn't place my hopes in this circuit's working as advertised except when it is held *next* to a GSM phone -- it needs a strong signal to work.
I experienced this in most ICU's of the hospitals to prevent the frequency of the signals to interfere with the frequencies of the medical machines.
So it can be used to avoid the use of mobile phones in private rooms, examination halls, etc. It is also useful for detecting the use of mobile phone for spying and other related activities. The circuit can detect incoming and outgoing calls, SMS, MMS, GPRS usage and video transmission even when the mobile phone is set aside in silent mode.
The instant bug detects RF (Radio Frequency) transmission signal from a mobile phone, it produces a beep sound alarm. There are two separate detector units.  Every detector unit consists of a dipole antenna, a choke and a diode. The wire type is not critical but its better to choose a fairly thick wire  that will not bend too easily. Since the forward voltage of a silisium diode is high, it won't give a sufficient result in this circuit. When a silisium diode is used the circuit also works but the detecting area becomes very very narrow.

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