How Cell Phone Detector Works

What is Cell Phone Detector?

We are most familiar with cell phone active detectors. The cell phone detectors are mostly hand and pocket-size mobile transmission detectors. It can sense the presence of an activated mobile phone from a distance of one and a half meters. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc.

Use of Cell Phone Detector:

It is also useful for detecting the use of mobile phone for spying and un-authorized video transmission. Certain places where use of mobile phones are not allowed like exam hall, temple, offices and theaters, in those places to detect and restrict the use of mobile phones this proposed system is very helpful. This must detect the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The illegal use of cell phones is a growing and dangerous problem in correctional institutions worldwide. These devices are a significant threat to prison security and circumvent the monitoring processes in prisons, while helping inmates commit new crimes both inside and outside the facility.

How Cell Phone Detector Works:

The moment the bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases. An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones.

Frequency Range of Detector:

The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuit detecting gigahertz signals is required for a mobile bug. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.

When mobile phone is active, it transmits the signal in the form of sine wave which passes through the space. The encoded audio/video signal contains electromagnetic radiation which is picked up by the receiver in the base station. The transmitter power of the modern 2G antenna in the base station is 20-100 watts. The mobile phone transmits short signals at regular intervals to register its availability to the nearest base station. Distance to cellular base station is the most important environmental factor. Generally, the nearer a cellular phone is to a base station or transmitting tower, the weaker will be the signal that needs to come from the phone. Range of frequencies of different categories are, AM radio frequencies between 180 kHz and 1.6MHz,FM radio uses 88 to 180 MHz, TV uses 470 to 854MHz.waves at higher frequencies but within the RF region is called Micro waves. Mobile phone uses high frequency RF wave in the micro wave region carrying huge amount of electromagnetic energy.

Block Diagram and Working of Cell Phone Detector:

The circuit uses a 0.22μF disk capacitor to capture the RF signals from the mobile phone. This part should be like an aerial, so the capacitor is arranged as a mini loop aerial. In short with this arrangement the capacitor works like an air core with ability to oscillate and discharge current. The output of transistor is within 10 mV of either supply voltage terminal. The lead inductance acts as a transmission line that intercepts the signals from the mobile phone. An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones that’s why circuit detecting gigahertz signals is required for a mobile bug.

Op-amp is used in the circuit is act as a comparator. It may come with MOSFET inputs and bipolar output. The input contains MOSFET transistors to provide very high input impedance very low input current. It has high speed of performance and suitable for low input current application. Hence the result is in very low input current and very high speed of performance. It is used in applications like ground referenced single supply amplifiers, fast sample hold amplifiers, long duration timers etc.

IC-555 is a highly stable controller capable of producing accurate timing pulses. With the monostable operation the time delay is controlled by one external resistor and one capacitor. With an astable operation the frequency and duty cycle are accurately controlled by two external resistors and one capacitor. These are used in applications like precision timing, pulse generation, time delay generation etc.

Cell Phone

  • One lead of the capacitor gets DC from the positive rail and the other lead goes to the negative input of IC1. So the capacitor gets energy for storage. This energy is applied to the inputs of IC1 so the inputs of IC1 are almost balanced with 1.4 volts. In this state output is zero. But at any time IC can give a high output if small current is induced to its inputs. When mobile phone radiates, due to high energy pulsations, capacitor oscillates and releases energy.
  • When the mobile phone signal is detected then the output of U1 becomes high and low alternately according to the frequency of the signal. This triggers mono-stable timer U2 through.
  • And the TR pin of 555timer goes low then pin3 of timer becomes high. When pin3 is high then buzzer will ring.
  • You may use a short telescopic type antenna. The unit will give the warning indication if someone uses mobile phone within a radius of 1.5 meters.

Advantages of Cell Phone Detector:

  • Smaller in size
  • Detection of hidden cell phones

Applications of Cell Phone Detector

It is useful where the use of mobile phone is prohibited like

  • Petrol pumps
  • Gas station
  • Historical places
  • Religious places
  • Court of laws
  • Examination halls
  • Spying and unauthorized video transmission
  • Military bases
  • Hospitals
  • Theatres
  • Conferences
  • Embassies

37 Comments

  1. francoise umubyeyi says:

    sir am so greatful for your help but I tried building up this circuit and the it gives the output immediately after supplying it what’s the cause?

    1. Tarun Agarwal says:

      Hi Francoise
      This circuit can be used for detecting mobile phones used for spying and unauthorized transmission of audio and video.
      It can be used to detect stolen mobile phones

  2. Sir plz tell me tat is dis ckt works on any cells or limited to such phones

    1. Sir I’m eagerly waiting for ur reply because I want to do dis project sir so I kindly give me some guidelines about this

    2. Tarun Agarwal says:

      Hi Dileep
      Here it is, please go through the required link of Hidden Active Cell Phone Detector project-https://www.edgefxkits.com/hidden-active-cell-phone-detector
      please visit our domestic website https://www.edgefxkits.com/
      For more details please contact to Mr Sathish on +91 8885507011 or you can email us on info@edgefxkits.in

  3. Sir is this circuit works for 4g phones ?or it can any cells plz clarify the dought sir

    1. Tarun Agarwal says:

      Hi Dileep
      For any technical queries or for customization of projects please email us on team@elprocus.com

  4. Mudsir Ali says:

    1) HOW CAN A CAPACITOR CAN WORK LIKE A TINY DIPOLE ANTENNA ?
    2) HOW CAN A CAPACITOR CAN DETECT A RF SIGNAL , AS WE ALL KNOW CAPACITOR IS CHARGE STORING DEVICE AND IF THIS CAN DETECT RF SIGNAL EVERY CAPACITOR CAN DETECT IT ?
    3)HOW CAN A CAPACITOR HAS ITS OWN FIELD AND HOW IT’S DISTURBED BY A RF SIGNAL ?
    4) WHAT IS THE PURPOSE OF HAVING SUCH A LOW VALUE CAPACITOR?
    5) WHAT IS THE PURPOSE OF DEFINING LEED LENGTH AND THE DIFFERENCE BETWEEN LEED LENGTH ?

    1. Tarun Agarwal says:

      Hi Mudsir Ali
      I sincerely appreciate your kind response regarding my article. As per your requirement, I can suggest you that please go through our website link you can get exact information
      https://www.elprocus.com/factors-should-check-before-selecting-capacitors/
      https://www.elprocus.com/difference-between-capacitor-and-inductor/
      https://www.elprocus.com/construction-of-capacitor-with-working/
      https://www.elprocus.com/bypass-capacitor-its-functions-and-applications/
      https://www.elprocus.com/capacitors-types-applications/
      For any technical queries or for customization of projects please email us on team@elprocus.com

  5. the mobile is not activated at that time how to detect the mobile

    1. Tarun Agarwal says:

      Hi K. Suresh
      For any technical queries or for customization of projects please email us on team@elprocus.com

  6. Hi, what type of antenna is used in this circuit? And in what type of store can I buy it? Thanks!

    1. Tarun Agarwal says:

      Hi Yass
      For any technical queries or for customization of projects please email us on team@elprocus.com and our team help you.

  7. sir,
    My faculty asked me what will be the use of mobile bug if it detects such small range?
    how can we increase the range?
    if it is in airplane mode n students are copying from it can it detect ? if not why?
    then what will be the use of mobile bug.

    1. Tarun Agarwal says:

      Hi Rishitha
      For any technical queries or for customization of projects please email us on team@elprocus.com

  8. Asalmualaikum sir , this mobile detector is only used for detecting incoming n outgoing calls and msgs but i want to make a project where we can detect mobile if somebody is recording videos or capturing photos.i ll be very grateful if u help me to complete this project sir.iam eagerly waiting for ur reply

    1. Tarun Agarwal says:

      Hi Ruhi
      We don’t have the projects based on your requirement but we have based on embedded, Solar, Robotic and etc
      please go through the below link https://www.elprocus.com
      For any technical queries or for customization of projects please email us on team@elprocus.com

  9. arpita gupta says:

    sir, we made a circuit as per the connection given in the circuit diagram for mobile detector. but we are facing some problem .
    it’s not working . It is unable to detect mobile phones.
    so,kindly help me .

    1. Tarun Agarwal says:

      Hi Arpita Gupta,
      May I know order number, Date of Order, Name, Model no and Kit type…?
      So, that our technical team will get back to you.
      Or mail all your queries to team@elprocus.com and our team will immediately solve your problem

  10. sadaf yaqub says:

    Assalam-O-Alaikum, sir i want to ask you why you use 2 led in project cell phone detector? plz reply this is also my project , i want to make a project of yours because i like it plz plz reply ………………
    thanks…………..

    1. Tarun Agarwal says:

      Hi Sadaf Yaqub,
      And also we use 2 led because, one is for power supply and another is for Mobile detector.
      For any more technical queries or for customization of projects please email us on team@elprocus.com

  11. HOW CAN WE INCRESE THE RANGE OF DETECTION?

  12. is it possible to detect the signal generated during bluetooth and wifi transmission using this cell phone detector circuit ?

    1. Tarun Agarwal says:

      Hi Rauf, No it is not possible to detect the signal generated during bluetooth and wifi transmission using this cell phone detector circuit.
      For any technical queries or for customization of projects please email us on team@elprocus.com

  13. sir,i will be greatly thankful to u if u can provide me the documentation of this project with circuit diagram . i want to implement it,.. Thanks

    1. Tarun Agarwal says:

      Hi Amit,
      We are so sorry to inform that we cannot provide you the documentation and the source code unless you buy a product from us.
      For any other technical queries or for customization of projects please email us on team@elprocus.com

  14. SIR,
    MOST RESPECTFULLY I WANT TO STATE YOU THAT I AM AN ENGINEERING STUDENT.
    I WANT TO TELL YOU THAT I HAVE IMPLEMENTED YOUR CIRCUIT AND IT IS WORKING EFFICIENTLY BUT WHEN I PRESENTED THE CIRCUIT IN FRONT OF FACULTY MEMBERS OF MY COLLEGE , SOME QUESTIONS ARE ASKED TO ME WHICH HAVE BOUND ME TO ASK YOU FOR THE PURPOSE OF YOUR GUIDANCE.

    THE QUESTIONS WHICH ARE ASKED ARE WRITTEN BELOW –

    1) HOW CAN A CAPACITOR CAN WORK LIKE A TINY DIPOLE ANTENNA ?

    2) HOW CAN A CAPACITOR CAN DETECT A RF SIGNAL , AS WE ALL KNOW CAPACITOR IS CHARGE STORING DEVICE AND IF THIS CAN DETECT RF SIGNAL EVERY CAPACITOR CAN DETECT IT ?

    3)HOW CAN A CAPACITOR HAS ITS OWN FIELD AND HOW IT’S DISTURBED BY A RF SIGNAL ?

    4) WHAT IS THE PURPOSE OF HAVING SUCH A LOW VALUE CAPACITOR?

    5) WHAT IS THE PURPOSE OF DEFINING LEED LENGTH AND THE DIFFERENCE BETWEEN LEED LENGTH ?

    SIR ALL FACULTY MEMBERS WANT TO KNOW THE BASIC IDEA OF DETECTION .

    SIR I HAVE TRIED EFFORTLESSLY TO GATHER INFO. ABOUT THIS BUT MY ALL OF THE EFFORTS WERE INVAIN, I AM WRITING YOU WITH LOTS OF HOPE , KINDLY HELP ME ON THIS PROBLEM AS I SHALL BE HIGHLY THANKFUL TO YOU.

    WAITING FOR YOUR KIND & FAVOURABLE RESPONSE.

    1. Tarun Agarwal says:

      Hi Tej,

      It appears you have missed the explanations in the documentation section at page 79.
      1. It is forming a loop antenna and not a dipole antenna
      2. It is a disk ceramic capacitor. The 2 plates of the low value capacitor picks up the GHz range signal
      3. As the capacitor is not fetching any active power it gas no field being a passive component
      4. As the frequency goes high pole dia goes lower and lower
      5. Lead length and spacing form a resonance at GHz frequency for feeding maximum signal to the circuit

      Please note you have to go through more on the antenna subject for more detail that we don’t teach

    1. Tarun Agarwal says:

      Hi Karthi,
      Thanks for the compliment.
      For more technical information and updated project ideas, please visit our blog again.
      For any queries or for customization of projects please email us on team@elprocus.com

  15. SIR ,

    MOST RESPECTFULLY I WANT TO STATE YOU THAT I AM AN ENGINEERING STUDENT.
    I WANT TO TELL YOU THAT I HAVE IMPLEMENTED YOUR CIRCUIT AND IT IS WORKING EFFICIENTLY BUT WHEN I PRESENTED THE CIRCUIT IN FRONT OF FACULTY MEMBERS OF MY COLLEGE , SOME QUESTIONS ARE ASKED TO ME WHICH HAVE BOUND ME TO ASK YOU FOR THE PURPOSE OF YOUR GUIDANCE.

    THE QUESTIONS WHICH ARE ASKED ARE WRITTEN BELOW –

    1) HOW CAN A CAPACITOR CAN WORK LIKE A TINY DIPOLE ANTENNA ?

    2) HOW CAN A CAPACITOR CAN DETECT A RF SIGNAL , AS WE ALL KNOW CAPACITOR IS CHARGE STORING DEVICE AND IF THIS CAN DETECT RF SIGNAL EVERY CAPACITOR CAN DETECT IT ?

    3)HOW CAN A CAPACITOR HAS ITS OWN FIELD AND HOW IT’S DISTURBED BY A RF SIGNAL ?

    4) WHAT IS THE PURPOSE OF HAVING SUCH A LOW VALUE CAPACITOR?

    5) WHAT IS THE PURPOSE OF DEFINING LEED LENGTH AND THE DIFFERENCE BETWEEN LEED LENGTH ?

    SIR ALL FACULTY MEMBERS WANT TO KNOW THE BASIC IDEA OF DETECTION .

    SIR I HAVE TRIED EFFORTLESSLY TO GATHER INFO. ABOUT THIS BUT MY ALL OF THE EFFORTS WERE INVAIN, I AM WRITING YOU WITH LOTS OF HOPE , KINDLY HELP ME ON THIS PROBLEM AS I SHALL BE HIGHLY THANKFUL TO YOU.

    WAITING FOR YOUR KIND & FAVOURABLE RESPONSE.

    1. SIIMPLE ANSWER IS THT THE LC RESONANCE WITH THIS ARRANGEMENT IS THE OPERATING FREQUENCY (HERTZ)

  16. SIR ,

    MOST RESPECTFULLY I WANT TO STATE YOU THAT I AM AN ENGINEERING STUDENT.
    I WANT TO TELL YOU THAT I HAVE IMPLEMENTED YOUR CIRCUIT AND IT IS WORKING EFFICIENTLY BUT WHEN I PRESENTED THE CIRCUIT IN FRONT OF FACULTY MEMBERS OF MY COLLEGE , SOME QUESTIONS ARE ASKED TO ME WHICH HAVE BOUND ME TO ASK YOU FOR THE PURPOSE OF YOUR GUIDANCE.

    THE QUESTIONS WHICH ARE ASKED ARE WRITTEN BELOW –

    1) HOW CAN A CAPACITOR CAN WORK LIKE A TINY DIPOLE ANTENNA ?

    2) HOW CAN A CAPACITOR CAN DETECT A RF SIGNAL , AS WE ALL KNOW CAPACITOR IS CHARGE STORING DEVICE AND IF THIS CAN DETECT RF SIGNAL EVERY CAPACITOR CAN DETECT IT ?

    3)HOW CAN A CAPACITOR HAS ITS OWN FIELD AND HOW IT’S DISTURBED BY A RF SIGNAL ?

    4) WHAT IS THE PURPOSE OF HAVING SUCH A LOW VALUE CAPACITOR?

    5) WHAT IS THE PURPOSE OF DEFINING LEED LENGTH AND THE DIFFERENCE BETWEEN LEED LENGTH ?

    SIR ALL FACULTY MEMBERS WANT TO KNOW THE BASIC IDEA OF DETECTION .

    SIR I HAVE TRIED EFFORTLESSLY TO GATHER INFO. ABOUT THIS BUT MY ALL OF THE EFFORTS WERE INVAIN, I AM WRITING YOU WITH LOTS OF HOPE , KINDLY HELP ME ON THIS PROBLEM AS I SHALL BE HIGHLY THANKFUL TO YOU.

    WAITING FOR YOUR KIND & FAVOURABLE RESPONSE.

  17. Zenia Saicedo says:

    It is a well-organized text from which detailed information of cell phone detector is learned. As far as I am concerned, cell phone detector has not been widely used yet.

    1. Team ElProCus says:

      Thanks for the appreciation.

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