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  • Mobile Phone Detector

  • Created: Oct 28, 2014

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Summary

This project is a mobile phone transmission detector that can sense active mobile phones within an approximate distance of 1.5 meters. It uses CA3130 integrated circuit that has MOSFET input and CMOS output ideal for single supply applications. In addition, the device uses NE555, a precision timer that is capable of producing accurate time delays or oscillation. This project can detect an incoming and outgoing calls, SMS and video transmission even when the phone is in the silent mode. When it detects an RF transmission signal from an activated mobile phone, the device will automatically beep with a blinking LED and this continues until the signal transmission ceases.

The device is an ordinary RF detector that is suitable for detecting signals from 0.9-3 GHz frequency band, a frequency range for mobile phones with a wavelength of 3.3 to 10 cm. 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. Op-amp IC CA3130 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 mobile phone 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 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 metres.

Mobile phone detector can be used to prevent cheating in examination halls by using cellphones. This may help increase the security level especially in events that need confidentiality. This can also be used for spying and monitoring unauthorized video transmission since it can detect both the incoming and outgoing calls, SMS and video transmission even at silent mode.

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