Friday, January 6, 2012

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.

31 comments:

  1. But I do not see a detector diode. Without a detector the signal entering the LM324 will be RF. LM324 cannot amplify RF, if I am right. Have you tested the circuit?

    ReplyDelete
    Replies
    1. Detector diode is not required here as the signals are very strong and do not require any processing, extraction etc.
      They are picked easily by the hi gain opamps.

      The circuit has been thoroughly tested by me.


      Delete
  2. Sir ,

    In the text you have mentioned : The RF disturbances are fed to the non inverting input of the opamp

    while in the circuit diagram , the RF signal is fed to the inverting input of the opamp

    Please clarify ASAP

    Regards

    ReplyDelete
    Replies
    1. follow the diagram, the diagram is correct.

      Delete
    2. Sir ,

      The diagram is certainly correct , but you will appreciate that my question remains unanswered whether the text in question is right or wrong ?

      Please reply ASAP

      Regards

      Delete
    3. I have corrected the text, you may check it.

      Delete
  3. how many meters can it detector the signal

    ReplyDelete
    Replies
    1. depends on the mobile signal strength....

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    2. Sir ,how can i modified the circuit to received or to monitor freq. 2612000khz up to 2700000khz? any other modification,I'am sure you can make it please.Thank you sir benjaminchan34@yahoo.com

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    3. That's won't be possible with the above circuit since it does not include a frequency locking stage, the above circuit is a very simple design capable of sensing all types of RFs within range

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    4. I need to dig up a piece of my sewer pipe as it keeps blocking where an old outside toilet used to be , its under my garage somewhere about 6ft down , i was wondering if i could strap an old working mobile phone to the drain rods and use the circuit above to locate the phone hence locating where the pipe is under my garage , save me digging up the whole floor . Could this work ? or if you have any other ideas please could you let me know

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    5. It could be feasible, but first you would have to set up the above circuit such that it becomes sensitive to the particular cell phone RF only within a restricted distance, say for example within a meter range or as preferred.

      The cell phone type is also crucial because some cell phones leak good amount of RF while some absolutely don't.

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    6. R2 can be adjusted to the required degree for optimizing the sensitivity.

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  4. Ill give it a try as i don`t live far from a Maplins and i know my way around a piece of vera board , i don`t relish the alternative of digging an whole slab floor up even if it could just give me a vague idea where abouts the pipe is . Could i make R2 a variable resistor ?

    ReplyDelete
    Replies
    1. you can make R2 variable, just make sure to connect a fixed resistor in series with it, the value of the fixed resistor can be anything between 1k and 100k.

      Delete
  5. does the antenna need resonator to select a particular frequency

    ReplyDelete
    Replies
    1. The circuit will detect all kinds of frequencies coming under the RF range, and does not have the capability of detecting particular frequencies. Antenna is an ordinary wire length.

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  6. can we inductor in place of L1 coil....and if we can use inductor then what should be the value of inductor.?

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  7. can we use inductor in place of coil L1... and what should be the value of inductor?

    ReplyDelete
  8. Do you have cell phone signal booster ?
    inside of my flat signal is not available so is this possible to boost signal from outside and give it to room using antenna and booster ?

    please provide booster circuit

    ReplyDelete
  9. can you give GSM signal booster circuit ?

    inside of my flat i dont get signal for any provider but outside i am getting full signal so now i want to get signal from outside using antenna and boost it and spread it inside of my flat

    ReplyDelete
    Replies
    1. I am sorry, I do not have it presently.

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    2. but is this possible to develop the circuit own ?

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    3. are they available in the market, and do they really work?

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    4. Hi Shadrach,

      It looks to be a very complex and sophisticated circuit, it could be impossible for an ordinary hobbyists like us to build it at home, no chance according to me.

      Delete
  10. hi sir!!! long time no chat.

    Can you give a specific coil L1, the coil with the longest range to detect?

    ReplyDelete
  11. What is the size of the capacitors? I used 0.01 (micro Farad) and it is not working.

    ReplyDelete
    Replies
    1. the capacitor value is not so critical any other value will also work if you have done everything correctly.

      to confirm you can take the circuit near the mains socket or wiring, the LED should begin glowing

      Delete

Readers are requested not to include external links while commenting. For consulting a diagram, upload it on Google Drive and provide the link here.

Readers are advised to proceed with the construction of the presented circuits only after understanding the concepts from the core. Not adhering to this can lead to failures and frustrations.