I basically need a person detector. Sort of...
Some friends and I have been playing laser tag a lot recently, We try to team up in Free For All but always end up shooting each other due to darkness. I was wondering, is there any way (Cheap) To build something that each person can have, one person has a device that sends a signal to the other person when about 20 feet away, and on the other persons device, it would vibrate or buzz or something to indicate that the other person is coming. So basically when one person is 20 feet away, all other devices vibrate. One sender, and the rest receivers. And they would have to be pocket sized. Is there any way of doing this?
My original plan was to use ElectroMagnetic Fields, one person has the detector, and the other, an EM Pump, But I could not find a pump design powerful enough for 20 foot range.