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This handy mobile bug or 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. 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. 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 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.
Assemble the cell phone detector 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 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 meters.
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