How would I build a two way proximity detector with changing frequency based on location to its pair?

 I am looking for a quick instructable that will make two devices that change pitch based on proximity to its pair. It doesn't seem like it would be that hard, but I am not good with stuff like this (with soldering on boards and even knowing what all those things do) but I know enough to know that thats what its going to take.

Thanks for your help. 

[edit: not really an edit, but more of an after thought, a post script if you will -- Think Geek makes a shirt like this but it uses lights as it gets closer... I want to use sound and frequency)

The ThinkGeek shirts use a radio transmitter to do its magic. The sound effect would be a simple add-on. You need two RF transceivers to do this, and access to the signal strength output

I suggest something like this they're available around the world too.
Looking at the datasheet you can see this thing has an interface that needs driving, but you could do all that with a lilypad arduino.

Set the output power to be minimum, and then you read the signal strength meter to drive your audio.

Re-design6 years ago
Google "theremin" for a start.
teamcoltra (author)  Re-design6 years ago
http://www.thinkgeek.com/tshirts-apparel/interactive/a090 I am looking for something more like this. But with sound instead of hearts. I know that this has also been done on this site as part of a valinteines day thing... but it was like a heart shaped box that lights up when the sensor gets closer.

It should not be that hard to do this with sound instead of light fairly cheaply.