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Long distance proximity detector Answered

Hello.  Does anybody have any idea on how to make a very cheap proximity sensor, which could work over a distance of about 100 metres? I want to make a system where LEDs (carried by people!) change colour as they approach a certain point, i.e, white LEDs turn red as they move towards a central point. The problem is I want to have 50+ separate LEDs, and I'm on a very tight budget! Any ideas appreciated. Thanks!


"very cheap" is going to be a problem. You can try reverse geo-cache for big distances but that wont be very accurate once you get close. Long range RFID (active) tags can be very accurately traced by a central unit but i'm not sure about the other way round. Theoretically you could build some sort of radio frequency emitter and a couple detection units that measure the intensity of the specific frequency you are emitting.
I very much doubt you can build anything for under 10 euros a pop (which already puts you at 500 for 50 units) but with a smaller group and a larger budget you should be able to pull it off.

Hi, thanks very much for the advice! I've just realised, that the system would only need to actually work over a smaller distance - if the sensor could just sense when it was about 10m away from the transmitter (or whatever!) then change the LED colour. Would this be any more feasible, or is it just basically the same problem?
Many thanks!

Reduced range should make it more feasible but if you are multiplying anything you buy/build by 50 units it will be hard to keep it "very cheap".

I am afraid my technical knowledge with electronics is limited so I cant really help with the details but the short range should open some alternative possibilities. Maybe you can have something generate some EM noise which you can pick up with a simple sensor or something.

I'd probably have to go with FS' radio emitter system too.,