search4electronics: Mobile Phone Detector

Introduction :-
This handy 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. Therefore 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. Mobile phone detectors can be used to detect the signal on both the incoming and outgoing calls SMS and video transmission even if the phone is kept in silent mode. The moment we press the call button on our mobile phone discontinuous beeps on buzzer are heard with LED blinking and both of them become continuous as the call is connected and continues until the signal transmission stops. 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 which is fulfilled here.

CIRCUIT DIAGRAM:-

List of components:-

1. C1,C2 22pF

2. C3 0.22µF

3. C4 100µF/16V

4. C5 47pF

5. C6,C7 ,C8 0.1µF

6. C9 4.7µF/16V

7. LED1

8. PZ1 BUZZER

9. T1 BC548

10. R1,R3 2.2M

11. R2 100K

12. R4 1K

13. R5 2K

14. R6 15K

15. IC1 CA3130

16. IC2 NE555

Working:-

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.

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 optimize 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.

Applications:-

1. This detector circuit can be used to detect and prevent use of mobile phones in prohibited areas like examination hall, office or some meetings etc.

2. It can be used to find our mobile phone if kept in silent mode in a hard to look area.

3. This can be used by many companies to locate appropriate areas for its tower stands.

4. It is also useful for detecting the use of mobile phone for spying and unauthorized video transmission.

JUGAAD CIRCUIT:-