Cell Phone Detector

The cell phone detector or mobile bug is a handy device which can sense any activated mobile present near a half meter to its range. So it can be used to prevent the use of mobile phones in mobile prohibited zones such as examination halls, confidential rooms etc. This circuit is capable of detecting both the incoming and outgoing calls, SMS and video transmission as soon as it comes under the range of detector even if the mobile is in silent mode. The moment the bug detects RF transmission signal from an activated mobile phone, the LED present on the bug starts glowing and the beep sound continuous until the transmission signal ceases.

Components List

  1. Operational Amplifier : IC2 CA3130
  2. Monostable Timer : IC1 IC 555
  3. Transistor : BC548
  4. Capacitors : 47pF ( C1) 0.1µF (C2 , C3 ) 0.01 µF (C4) 100µF (C5) 4.7µF (C6) 0.22µF (C7) 22pF (C8 & C9)
  5. Resistors : 2.2MΩ (R1 & R6) 1KΩ (R2 ) 12KΩ(R3) 15KΩ(R4) 100KΩ (R5)
  6. Piezo-Buzzer
  7. Switch
  8. 12V Battery

PCB Layout and Eagle files

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PCB Layout:

Components Placement:

Cell Phone Detector Circuit Schematic

  An ordinary RF detector LC tuned circuits are not suitable for detecting signals in the GHz frequency band used in mobile phones. The mobile phones have a transmission frequency ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10cm. So, it is necessary to design a circuit which can detect GHz signals. The disk capacitor 0.22µF (C7) is used to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18mm with a spacing of 8mm between the leads to get the desired frequency. This disk capacitor along with the leads acts as a GHz loop antenna to receive the RF signals from the mobile phone. The one of the main part of this circuit is an Operational Amplifier. Op-amp IC CA3130 (IC2) is used as a current to voltage converter with C7 connected between its inverting and non-inverting input terminals. It is a Gate-protected P-channel MOSFET (PMOS) transistor used to provide very high-input input impedance, very low output current and exceptional speed performances. It is capable of swinging the output voltage to within 10mV of either supply voltage terminals. C7 in simultaneity with the lead inductance acts as a transmission line that intercepts the signals from the mobile phones. This capacitor creates a field, stores energy and transfers it to the inputs of IC2 in the form of minute current. C5 along with high valued resistor R1 keeps the non-inverting input stable for an easy swing of the output to the high state. Resistor R2 provides the discharging path for C5. R1 used as feedback resistor makes the inverting input high when the output becomes high. C5 i.e. 47pF is connected across ‘pin 8’ and input ‘pin 1’ of IC2 for phase compensation and gain control to optimise the frequency response. When the cell phone detector signal is detected by C7, the output of IC2 alters between high and low according to the frequency of the signal indicated by LED1. The other important part of this circuit is a monostable 555 timer which gets triggered by C7. C6 maintains the base bias of transistor T1 (BC 548) for fast switching action. R4 and R4 are used to avoid audio nuisance as these are very low-value components providing a very short time delay. Finally, assemble the cell phone detector or mobile bug on a general PCB as compact as possible and enclose it in a small box to make it portable. As mentioned earlier capacitor C7 should have a lead length of 18 mm with lead spacing of 8 mm. Use the 12V battery of a small size and small buzzer so that the gadget can become pocket size. The unit will give the indication if someone uses a mobile phone within a radius of 1.5 meters.

Datasheet for 555timer and CA3130 :

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