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

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