electronic project problem

i am making a project  and i want a  way to make two devices that one of them has an LED on it and when the other devices goes out of a certain range 30m lets say the LED either comes on or goes of and i dont want them to be  connected.It's for a school project and i wanted it to be small so I could attach them on to bicycles let's say and then if my brother goes 50m ahead of me the LED will come on/go off on his device so that he is aware of the distance between us I know nothing about electronics at the moment, this might put you off helping that's okay
 i hope that makes sense to somebody and that someone has an idea. thanks

Comments

[steveastrouk]

The simplest idea would probably be to have a low power radio on one device and a receiver on the other, when the radio signal drops too low, the LED goes off.

If you want it to be accurate, then add an ultrasonic transmitter and a receiver. When the transmitter pings, use the radio to signalthat a pulse has been sent, to start a timer at the receiver, which waits for the pulse.

[vitusvenon1]

It's for a school project and i wanted it to be small so I could attach them on to bicycles let's say and then if my brother goes 50m ahead of me the LED will come on/go off on his device so that he is aware of the distance between us I know nothing about electronics at the moment, this might put you off helping that's okay

[blkhawk]

There is a project for an electronic beacon that could help you.

[iceng]

Will this be used on sunny days and line of sight ?

A

[rickharris]

How much do you want to spend?

how good are your electronic skills.

For me I would use a light sensor in a tube and a bright LED on the object. I assume the object orientation will be known.

OR Use a micro processor and ultrasonic TX RX to gauge distance away.