Thread of comments
Mobile Bug, Mobile signal detector
This handy, pocket-size mobile transmission detector can sense the presence of an activated mobile phone from a distance ofone and a-half metres. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is alsouseful 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 thesilent mode. The moment the bug detects RF transmission signal from an activated mobile phone, it starts sounding a beepalarm 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 mobilephones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuitdetecting 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 thecapacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor alongwith 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 itsinverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input toprovide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor iscapable 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 mobilephone. This capacitor creates a field, stores energy and transfers the stored energy in the form of minute current to the inputsof 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 highstate. Resistor R2 provides the discharge path for capacitor C4. Feedback resistor R3 makes the inverting input high when theoutput becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin 8) and ‘null’ inputs (pin 1) of IC1 for phasecompensation 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 frequencyof the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7. Capacitor C6 maintains the basebias of transistor T1 for fast switching action. The low-value timing components R6 and C9 produce very short time delay toavoid audio nuisance.
Assemble the circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. Asmentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitorin standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for thedesired 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 thewarning indication if someone uses mobile phone within a radius of 1.5 metres.
An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobilephones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuitdetecting 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 thecapacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor alongwith 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 itsinverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input toprovide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor iscapable 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 mobilephone. This capacitor creates a field, stores energy and transfers the stored energy in the form of minute current to the inputsof 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 highstate. Resistor R2 provides the discharge path for capacitor C4. Feedback resistor R3 makes the inverting input high when theoutput becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin 8) and ‘null’ inputs (pin 1) of IC1 for phasecompensation 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 frequencyof the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7. Capacitor C6 maintains the basebias of transistor T1 for fast switching action. The low-value timing components R6 and C9 produce very short time delay toavoid audio nuisance.
Assemble the circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. Asmentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitorin standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for thedesired 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 thewarning indication if someone uses mobile phone within a radius of 1.5 metres.
Comments
No Comments have been Posted.Post Comment
Please Login to Post a Comment.Popular Projects
|
![[Valid RSS]](http://validator.w3.org/feed/images/valid-rss-rogers.png "Validate my RSS feed")