A cellphone or mobile phone detector is actually a high gain op amp amplifier which detects slightest of RF disturbance from a mobile phone, and illuminates an LED.
NOTE: This concept was first developed by me and later on the idea was COPIED by many reputed website.
Mobile phones today 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.
The concept behind the working of this mobile phone detector is a highly sensitive comparator circuit which is unstable at its input due to high sensitivity, such that it turns ON even with the minutest electrical interference in the atmosphere around it.
Since it is designed to detect mobile phone signals one may misinterpret it to be detecting the GHz signals, actually it's not, and it simply can't.
Even if the mobile phone signals may be oscillating at GHz levels, the signal is still a radio frequency (RF), having the properties of electrical interference.
It is this electrical interference that is picked up by the op amp input, and converted into a DC output, for illuminating the LED
The circuit is basically a simple high gain inverting amplifier, built around the IC LM 324. 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.
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.
NOTE: Using 4 op amp stages can make the design extremely sensitive and the circuit may start sensing all sorts of RF signal that may be present in the atmosphere. Therefore I recommend using only 3 op amp stages in series for this project.
- All R1 = 100K 1/4 watt
- All R2 = 2.2 Meg or any value between 1 Meg and 10 Meg (1/4 watt)
- All C1 = 0.01uF, or 103 ceramic disc or PPC, any type will do.
- A1 --- A4 = LM324 IC
As mentioned in the earlier part of this article, the op amp is configured as a high gain non inverting amplifier, where the input is received at the pin #2 which 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 op amp 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 op amp.
Increasing the value of this resistor increases the sensitivity of the circuit, however too much sensitivity can make the circuit unstable and induce oscillations.
The amplified signal is fed to the input of the next stage which is just a replica of the previous stage.
Why it is so Sensitive
It's due to the 4 series op amp stages which helps to make the circuit highly sensitive an this can pick up cellphone RF from a distance of 10 meters.
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.
After a lot of experimentation I finally realized that creating a long range cell phone detector wasn't feasible. It's because the modern phones have a high grade RF shielding, which allows only very little RF to leak out from the phone. Therefore the RF do not reach too far in the atmosphere making it impossible to detect them beyond a few inches from the phone.
To improve the distance I tried making the circuit more sensitive by adding more stages in series, but that didn't work. Because higher sensitivity meant the circuit started detecting many different existing RF disturbances in the air, which kept the LED flickering all the time.
The Finalized Circuit
The finalized tested design can be seen below, it is exactly similar to a WiFi detector circuit
How to Assemble the Circuit
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. It is built with the following instruction:
After procuring the given components, fix them over the piece of general PCB in the following manner:
Take the IC first, and carefully insert its legs inside the PCB holes through proper alignment.
Solder the leads of the IC.
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.
How to Test
Once it is assembled, it’s all about 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.
Another way of checking the circuit would be to take it near your mains electric board, the LED should light up when brought even withing a feet near to the board indicating the presence of the mains field and confirming the working of the circuit.
Note: The coil L1 can be made from any gauge wire, just a few turns of any diameter between 5 to 9mm will do.
RF Sniffer using a Single Op Amp
While the RF mobile detector circuit was primarily meant to indicate the existence of RF emissions, this circuit is implemented for several different functions, such as testing car security keys and as a bug detector.
The RF sniffer circuit is so sensitive that it can pick up fields as low as to 1 mW at 1 m distance and from around 100 kHz to 500 MHz signals.
Essentially, it is just a broad -band input circuit, a rectifier and meter, nevertheless for achieving required sensitivity an amplifier is necessary and the diodes should be accurately selected.
Germanium diodes are able to operate even at lower forward voltages compared to the silicon types, and frequency response is bigger using point contact devices, therefore point -contact, germanium 0A90 diodes happen to be the best alternative.
A 1 mH inductor over the input minimizes LF sensitivity, as does the feedback capacitor. Adjusting the meter offset is not important, nonetheless it will enable the nulling of unwanted frequencies.
The meter might require series resistance to fine-tune sensitivity. The display reading may not be linear and will only help to indicate the presence of RF and the relative power of the RF.