Mobile bug

How the circuit works?
     * Ordinary LC (Coil-Capacitor) circuits are used to detect low frequency radiation in the AM and FM bands.
    * The tuned tank circuit having a coil and a variable capacitor retrieve the signal from the carrier wave.
    * But such LC circuits cannot detect high frequency waves near the microwave region. 
    * Hence in the circuit, a capacitor is used to detect RF from mobile phone considering that, a capacitor can store energy even from an outside source and oscillate like LC circuit.

Use of capacitor
* A capacitor has two electrodes separated by a ‘dielectric’ like paper, mica etc.
* The non polarized disc capacitor is used to pass AC and not DC.
* Capacitor can store energy and pass AC signals during discharge. 0.22 capacitor is selected because it is a low value one and has large surface area to accept energy from the mobile radiation.
 * To detect the signal, the sensor part should be like an aerial. So the capacitor is arranged as a mini loop aerial (similar to the dipole antenna used in TV). In short with this arrangement, the capacitor works like an air core coil with ability to oscillate and discharge current.

How the capacitor senses RF?
    * One lead of the capacitor gets DC from the positive rail and the other lead goes to the negative input of IC1. So the capacitor gets energy for storage. This energy is applied to the inputs of IC1 so that the inputs of IC are almost balanced with 1.4 volts. In this state output is zero. But at any time IC can give a high output if a small current is induced to its inputs.
   * There a natural electromagnetic field around the capacitor caused by the 50Hz from electrical wiring. When the mobile phone radiates high energy pulsations, capacitor oscillates and release energy in the inputs of IC. This oscillation is indicated by the flashing of the LED and beeping of Buzzer. In short, capacitor carries energy and is in an electromagnetic field. So a slight change in field caused by the RF from phone will disturb the field and forces the capacitor to release energy.

PIN CONFIGURATION OF IC 

(1) ICCA 3130

(2) IC NE 555 TIMER

APPLICATION

    * 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.

LIMITATION

    * RANGE OF THE CIRCUIT
    * The prototype has only limited range of 2 metres.But if a preamplifier stage using JFET or MOSFET transistor is used as an interface between the capacitor and IC,range can be increased.

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