Mobile bug Posted by Edwin antony | at 7:45 AM

 

This 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 signalsrom 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 generalpurpose
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.