How to create a simple electronic infrared security system

'This is an ultra compact intruder alarm system based on an intrusion caused by the interruption of an infrared light beam which is emitted from the infrared LED on the transmitter card, cannot be seen by our naked eyes. There is a receiver card which stays off till the infrared light falls on it. But when an opaque object comes between the receiver and the transmitter card, circuit gets completed and the alarm starts ringing.