How to Make a Cell Phone RF Signal Detector Circuit – A Simple Science Fair Project

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. 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.

Circuit Description:

The circuit is basically a simple high gain non inverting AC 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.

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.

The amplified signal is fed to the input of the next stage which is just a replica of the previous stage. 

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.

Construction Clues:

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.

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.

 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.

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