>>Originally I used an old telephone line which had been left in the ceilin= g>>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 I am assuming the telephone line was being used to send a PWM signal? The reason behind choosing a garage door sensor was that the sensor has to = be off-the-shelf. Sai On Friday, January 3, 2014 8:57 PM, IVP wrote: =20 > 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 --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .