This detector circuit detects an RF signal ranging from 860 MHz until 965 MHz which is previously had been discussed in the chapter before which is in GSM band. When a mobile phone been detected, the buzzer will sound and the LED will blinking and this will alert the other people that someone at the particular area had using their mobile phones.
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.
IC NE555 timer is highly stable controller capable of producing accurate pulses. With a monostable operation the time delay is controlled by one external resistor and one capacitor. With an stable operation, the frequency and duty cycle is accurately controlled by two external resistor and one external capacitor.