> If the beam were to be interrupted, there would be no pulse

A friend has a very long factory with a retail section at the front. More
often than not evrybody is out the back working on commercial stuff

I made her a door beam to cover the three double doors width of the
shop frontage.

The transmitter is a 556 driving an IR LED at 100Hz with a 38kHz carrier.

The receiver is a 1SU60-type IR sensor and PIC. When the PIC detects
a beam break (timer not reset by a pulse before rollover), it sends a coded
RF signal about 50m to another PIC at the back of the factory part of the
building which drives a doorbell.

Originally I used an old telephone line which had been left in the ceiling
as a hard-wired link but found it prone to false triggering from the
machinery they use. The RF link is just a pair of those $2 434MHz
modules. Been working like a charm for many many years

Joe
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