Proximity detector circuit

Hi all,

I was hoping someone might be able to help me with a little project. I'd like to build a small proximity detector that can detect (flash an led or beep etc) when a copy of the circuit is in a range of about 10M.

I know this would require a transmitter & reciever running continuously within the same circuit. I presume there would also need to be something that would tell it to ignore signals that are too strong, otherwise it would continuosly detect itself.

I was thinking a small bluetooth-type of device would do the job.

The idea is that a group of random people could each carry one with them, while they walk around in a public place & have to find each other. It's sort of a life based game where players chat online & so never actually see each other until the circuit tells them that another player is nearby.

Thanks in advance

They sold those things in Japan several years ago.

Bluetooth seems like serious overkill for that application.

Interesting application. I guess some form of time multiplexing where a given device listens for say 90% of the time and broadcasts for the other 10% of the time and they rely on the fact that the devices are not synchronized so messages are bound to get through in short order. 10 meters is not a demanding distance.

Not a lot of information has to pass between the devices, but a short FSK ascii message (send "HI") might help prevent false positives from interfering sources. A simple PIC with those cheap FSK transmitters/receiver modules might fill the bill. PIC turns on a cheap buzzer when it receives a valid message.

https://www.sparkfun.com/commerce/product_info.php?products_id=872


Lefty

Brilliant. Thanks for that.

I was thinking a lot more about this last night.

For Instance it could use USB to charge a small rechargable battery & have an interface where you could enter your details & the details of who / what your looking for.
eg: I could enter my details as tall dark & handsome & I'm looking for a single tall blonde slim woman. One day I'd get a buzz from the device & would know that she is single, so I'd feel far more comfortable with asking her out. Alternatively she may get a buzz & notice Mr. TD&H.