Very short range wireless transmitter

[silentb0y]

I am in need of a way to make to simply make an Led light up when to people gets close to eachother.
My plan was to make two tiny circuitboards (one for each person) and when the two circuit boards gets close An Led have have to turn on at both circuitboards.
The range should be no more than 10 meters.
I have many solutions myself but they are all way overkill ( arduino with bluetooth module or rf transmitter) for the type of project.
What would the simplest way be for making to circuitboards that light up when they get within 5 to 10 meters of eachother.
The solution should also as small as possible.
Thanks in advance

[Rerouter]

tiny little RF source, and a detector, when the amplitude of the RF is exceeded, the light turns on, could be done with a compairtor, or if you wanted to get fancy, could try and get away with a darlinton transistor and diode for the detector,

the only trick here is to prevent each persons rf source from setting off there own detector, which would happen if they stood close enough to something reflective, like a bit of ungrounded metal,


if you wanted to increase the complexity a bit, use 2 frequencies, one for each person, and use a bandpass filter so the detector only detects the other persons signal strength is exceeded,

[Marco]

Just use some not too precise oscillator to ping the signal with a low duty cycle and turn detection off when transmitting.

How selective do they have to be? Is it a problem if they light up when someone presses a keyfob?

[Zero999]

How exact does it have to be?

If the 10m range is highly critical then it gets complicated. You need a transmitter and a transponder. Transmit the signal to the other person who's wearing the transponder, wait for the return signal. If it takes too long for the signal to be returned, then they're more than 10 meters away. Electromagnetic radiation travels at 299 792 458m/s so from that you can work out how long it will take for the signal to travel 20m (to the transponder and back). You also need to take into account the delays in the transmitter and transponder responding. This won't work where they're obstacles as the radiation will have to take a longer path.

Using ultrasound is an easier option because it's much slower 340m/s so the delays will be longer and therefore easier to measure but it's more dependant on temperature than the speed of electromagnetic radiation, so not so accurate. The frequency is much lower too, a wavelength of 10mm is only 34kHz, rather than 30Ghz which is much easier for electronics and a shorter wavelength gives greater accuracy, although it'll be more sensitive to obstacles.

If it's just a rough guess then monitoring the signal strength can be used but it's even more dependant on obstacles and you need to make sure the transmitter has a precisely known power output, antenna gain and the same for the receiver's antenna and detection threshold.