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.
Cell Phone Detector circuit diagram
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
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
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.