Full description here. If you use it in your book, you will need to pay royalties ;-)

http://cappels.org/dproj/Cellphone_Watcher/Cellphone_Watcher.html

 

Can you please tell me how this circuit is detecting the signal

Hi, please read the page start to end and come back again: http://cappels.org/dproj/Cellphone_Watcher/Cellphone_Watcher.html

EDIT: Ops, I didn't see the previous post by Dick Cappels. (so the post by me can be removed.)