This handy, pocket-size mobile transmission detector can sense the presence of an activated mobile phone from a distance of oneand-a-half metres.So it can be used toprevent 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 signal transmission.
Here the circuit uses a 0.22μF ceramic 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.
When the mobile phone signal is detected by C3, the output of Op-ampIC1 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.
Courtesy: D.Mohankumar
