Long distance proximity detector
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!