Mobile Bug Detector

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 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.