Home >>  Electronics  >>  an alarm raising detector for an encroaching intruder

i have to build the ciruit for the title given, there is a transmitter which is transmitting in all direction, and there is a receiver which only recieves when someonw approaches to the system,in doing so an alarm raises..so the project goes like,when someone approaches to the system from anywhere,the rays from the transmitter are reflected to the receiver and it triggers and alarm..