Hidden Active Cell Phone Detector

The main scope of project is to sense the presence of an activated mobile phone from a distance of one-and-a-half meters to prevent the use of mobile phone in the examination halls.

Certain places where uses 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 system can detect any active cell phone i.e. when someone is trying to make a call or receive a call or sending a message or receiving a message. In any of the above mentioned conditions a buzzer will sound, in the presence of an active cell phone in the radius of one and half meters.

In this project an RF detector is used along with a tuned LC circuit which is formed for detecting signals of GHz frequency range a band traditionally used in mobile phones as the transmission frequency from 0.9 to 3 GHz.

A capacitor is used to form a part of the LC circuit as C while the lead of the same forms the L to receive RF signals from the mobile phone. The project uses 8 numbers 1.5v batteries in two containers for 12V dc availability to the remote control and includes a small buzzer to make the gadget pocket-size. When the mobile phone is activated the RF transmission signal is detected by the detector and starts sounding a beep alarm and the LED blinks. An active cell phone detector is also used to detect the use of mobile phone for spying and unauthorized video transmission.

Block diagram:

Hardware used:

·         Op amp

·         555 timers

·         Transistor

·         LED

·         Capacitors

Resistors

·         Piezo buzzer

·         Switch

·         Battery

·         And finally an antenna

Block diagram explanation:

The above block diagram representation of the hidden active cell phone detector consists of an operational amplifier, a multi-vibrator and a buzzer.

·         Operational amplifier:

Antenna signal is given as input to an operational amplifier, which is an 8 pin IC. The operational amplifier (op-amp) is also referred to as a voltage amplifier which is used for amplifying a voltage. It is an integrated circuit with differential inputs i.e. it has two inputs of opposite polarities. The output signal of the op-amp is very high while compared to the input signal and the op-amp is also designed for performing arithmetic operations in an analog circuit

·         Monostable multivibrator:

The output from the op-amp is fed to a monostable multi vibrator. The monostable multi vibrator is a one shot multivibrator which is used to generate a single pulse .In this one state of output is stable and other state of output is in unstable mode. It is triggered when a logic low signal is given to the input and a high logic signal appears at the output.  When not triggered, the output is normally low and when triggered, it will go high for a short length of time depending on the values of the resistor and capacitor components.

·         Piezo buzzer:

 When the output from the multivibrator reaches the buzzer, the buzzer rings to indicate the active action of the mobile phone. A piezoelectric element may be driven by an oscillating electronic circuit or other audio signal source driven with a piezoelectric audio amplifier. Sounds commonly used to indicate that a button has been pressed are a click, a ring or a beep.

It most commonly consists of a number of switches or sensors connected to a control unit that determines if and which button was pushed or a preset time has lapsed, and usually illuminates a light on the appropriate button or control panel, and sounds a warning in the form of a continuous or intermittent buzzing or beeping sound. The effect of piezo buzzer is related to a change of polarization density with the material, a buzzer is placed for signaling the device at the output.

                       

CIRCUIT EXPLANATION:

The circuit can sense the incoming and outgoing calls, SMS and video communication even if the mobile phone is kept in the silent mode. The instant the system detects the 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. Here the circuit use a disk capacitor is used to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed with some spacing between the leads to get the desired frequency signal. The disk capacitor along with the leads acts as a small loop antenna to collect the RF signals from the mobile phone. Op-amp IC is used in the circuit as a current-to-voltage converter with capacitor connected between its inverting and non-inverting inputs. The output CMOS transistor is capable of swinging the output voltage to within supply voltage terminal. Capacitor should have a lead length with lead spacing of 8 mm. carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimized by trimming the lead length of for the desired frequency. 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.

Project Functioning:

The concept of this project to sense whether mobile phone is active or not and detect the use of mobile phones in restricted places. If any cell phone is operated by a person and the moment the person comes closer to the mobile the buzzer will ring based on the principle of detecting the megahertz oscillation that takes place inside the mobile phones when the phone is in active mode. It detects any active moment of the cell phone and the buzzer will ring whenever the  number is dialed on the cell phone, or while sending or receiving any text messages etc.

CONCLUSION:

This project has been designed successfully so that the pocket-size mobile transmission detector can sense in the presence of an activated mobile phone from a distance of one and- a-half meters. 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 un-authorized video transmission.

FUTURE SCOPE:

 This project can be enhanced further by increasing the detection of range for long distance of cell phones to few meters for overseeing wide rang of area in future; the range can be increased by interfacing the preamplifier devices like JFET and MOSFET in between the capacitor and the IC.