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detector

Cell Phone Detector aka Mobile Bug

This handy mobile bug or cell phone detector, pocket-size mobile transmission detector or sniffer can sense the presence of an activated mobile cellphone from a distance of one and-a-half metres. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is also useful for detecting the use of mobile phone for spying and unauthorised video transmission.

The circuit can detect both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. 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.

Cell Phone Detector Circuit Schematic

mobile bug cellphone detector

An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones. 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.

Here the circuit uses a 0.22μF disk capacitor (C3) to capture the RF signals from the mobile phone. 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.

and another video from Sufyan Ghori

Op-amp IC CA3130 (IC1) is used in the circuit as a current-to-voltage converter with capacitor C3 connected between its inverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input to provide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor is capable of swinging the output voltage to within 10 mV of either supply voltage terminal.

Capacitor C3 in conjunction with the lead inductance acts as a transmission line that intercepts the signals from the mobile phone. This capacitor creates a field, stores energy and transfers the stored energy in the form of minute current to the inputs of IC1. This will upset the balanced input of IC1 and convert the current into the corresponding output voltage.

Capacitor C4 along with high-value resistor R1 keeps the non-inverting input stable for easy swing of the output to high state. Resistor R2 provides the discharge path for capacitor C4. Feedback resistor R3 makes the inverting input high when the output becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin 8) and ‘null’ inputs (pin 1) of IC1 for phase compensation and gain control to optimise the frequency response.

When the cell phone detector signal is detected by C3, the output of IC1 becomes high and low alternately according to the frequency of the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7. Capacitor C6 maintains the base bias of transistor T1 for fast switching action. The low-value timing components R6 and C9 produce very short time delay to avoid audio nuisance.

Assemble the cell phone detector circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. As mentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for the desired frequency. You may use a short telescopic type antenna.
Use the miniature 12V battery of a remote control and a small buzzer to make the gadget pocket-size. The unit will give the warning indication if someone uses mobile phone within a radius of 1.5 meters.

271 Comments

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  • Bob Whitley

    I tried this with my daughter for a science project but couldn’t get it to work. I tested the transistor, capacitors and resistors with a multimeter and they were good, except that I couldn’t test the pico capacitors since the multimeter isn’t that sensitive. So I replaced the pico capacitors and the CA3130 and NE555 just to be sure, and it still doesn’t work. Any recommendations? I using a CA3130E but as far as I can tell that should still work. Any help or ideas would be appreciated!

  • Abhilash

    I have made this ckt and it is working. but it is only up to 15-20cm.what should I do for increasing the distance.

  • prince

    led and buzzer gets on even if no mobile phone is present
    what to do now????
    reply asap….

  • Abubakar Suleiman

    I need latest update of the circuit if available

  • rathod

    i have done this project earlier..actually i want to increase the range of my device actually i need range of 10 meter radius….or else range of a class room for 70 students…
    please would u help me…

    its so mandatory guys… ..

    help me!! out guys

  • sai krishna

    its not working and what type of anteena should i use

  • Inder

    Can anyone tell me how to connect a whip antenna on the PCB in this project. Please tell me the connections…

  • manoj

    sir,i m working on this project,but i
    was asked to increase the range
    uptp 5 metres,n i m not able to get
    it,kindly grace me,in response to my
    problem

  • Kaustubh

    Best article…all info is vry useful…thnkz alot…!!

  • Kaustubh

    Sir…can you tell me which type of antenna can I use instead any expensive one..? Can I use slant telescopic antenna..??

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