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Hidden Proximity Sensor Answered

Hey guys, I am trying to find a method of creating a hidden proximity sensor that would work similarly to a laser i.e. if the line of sight is broken then an output can be achieved, but without actually using lasers. There are a few problems, the sensor will be in a dark space and will have some smoke blown through the line of sight (lasers would be seen). I want the sensors to be hidden, so they will have to be mounted out of sight, but as they will be used in a relatively open space, i imagine this will mean on the ceiling. This will mean that they will have to detect all the way to the floor, so the range will probably have to be at least 8 feet. Does anybody have any ideas? i'd really appreciate any input. Neal


. Infrared source/detector?

Thanks for the reply NachoMahma. Will an infrared source/ detector not be visible if it has smoke blown through the infrared beam?

. The IR itself is invisible. Using UV might cause smoke particles to fluoresce, but that shouldn't be a problem with IR.

It shouldn't be. I'm thinking that they go off when a heat source moves into view of it. Smoke shouldn't obfuscate that. I may be wrong though.