Light-beam alarms
In the Chapter 1 survey of modern optoelectronic devices and techniques brief mention was made of light-beam systems that can (among other things) be used as the basis of intrusion detectors and alarms. The present chapter expands on this theme by showing practical ways of making invisible IR light-beam alarms; the chapter starts off by looking at some basic principles.
Intrusion alarm basics
A simple invisible-light-beam intrusion detector or alarm system can be made by connecting an infra-red (IR) light transmitter and receiver as shown in Figure 11.1 Here, the transmitter feeds a coded signal (often a simple squarewave) into an IR LED which has its output focused (via a moulded-in lens in the LED casing) into a fairly ...
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