Mobile Bug Detector


his handy, pocket-size
mobile transmission detector can sense the presence of an activated
mobile phone from a distance of one-and-a-half metres. So 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. The circuit can detect both
the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode.

The moment the bug detects 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. 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.

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.

Mobile Bug Detector Circuit

Mobile Bug Detector Circuit Diagram

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 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 optimized
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 meters.

Author: D. Mohan Kumar – Copyright: EFY Magazine


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