# Does this exist (proximity circuit)?

1. Jun 18, 2008

### IHateMayonnaise

More than likely somebody has already done this, however I do not know what to search for or where to start. Hopefully I can find some insight here.

I am looking for some sort of proximity circuit. Specifically, it should consist of two separate components: A "homing beacon," i.e. a stationary circuit broadcasting some signal, and a device that picks it up when it is in range. Both should be battery powered. It needn't work over large distances..say more than about a couple feet or so. Has this been done before? And also, is it legal (from the FCC's perspective) to have this system for my own personal use? Thanks all!

IHateMayonnaise

2. Jun 18, 2008

### mgb_phys

It's fairly easy to do with a garage door opener system using one of the unregulated low power bands (eg 419Mhz)

For a bit more money but less work you can use an off the shelf system like BlueTooth where the base would automatically notify you if a new client had come into range.
I worked on a protoype 'man overboard' system using BlueTooth where the base constantly pinged all it's clients (attached to each crew member) and set an alarm if one of them couldn't be contacted - presumably because it had gone out of range or had stopped transmitting.
It's was the only offshore system where making it fail when immersed in sea water was a design goal!

3. Jun 18, 2008

### IHateMayonnaise

That's cool, thanks for the thoughtful reply!

Bluetooth sounds pretty much perfect, except that it's a bit overkill. It only needs to work for a couple feet, and outside of this it's actually important that it doesn't work. Here is the big picture: if the traveling device moves more than two feet (more or less...doesn't need to be exact) from the transmitter, it activates another circuit. Sort of like an alarm.

I understand that the transmitted distance depends on obstacles, atmospheric conditions, etc.., but then again the distance sensitivity doesn't have to be perfect - it just needs to do something if it moves more than 2 or 5 or 8 feet away. A good idea would be to be able to vary the intensity of the signal, so this can be tested. Also, if anyone has a better idea of how to achieve this goal, please, share. This is the first thing I thought of so I'm going with it.

4. Jun 18, 2008

### stewartcs

How about a shock collar? :rofl:

CS

5. Jun 18, 2008

### mgb_phys

That was the idea behind the BlueTooth man-overboard, it was intrinsically fail safe since if it failed or lost signal that was the alarm. If you are doing a theft prevention or child-wandering-off type system you need to think along similair lines. Where the loss of signal is the alert rather.

Do you need to track multiple independant clients from one base station? If so you either need different frequencies or to send some sort of ID data.
Take a look at the Atmel AVR micro, there is an rf data example in the dev kit - I think it's called the butterfly.

6. Jun 18, 2008

### asynchronous13

Have you looked into RFID? The passive version only detects out to a few inches, but the active/powered versions can detect to several feet.

7. Jun 18, 2008

### IHateMayonnaise

That sounds pretty much exactly what I'm looking for, thanks! How difficult would it be to fabricate one (if they're not too complicated) or about how much would one cost? I am jumping straight to google to see if i can't find these things out, however a second opinion never hurts :)

8. Jun 18, 2008

### edmondng

there are 3 types of rfid, passive, semi-active and active. Passive rfid harvest the energy from the coil so it only works when it is close to detect the energy (inches). You will need active rfid. Try digikey.

9. Jun 19, 2008

### asynchronous13

For hobby-style RFID check Phidgets or Parallax. Prices from $40 to$65 for the reader, the cards are \$1-2 /each (for passive). Active costs more, but I don't know a brand off-hand.

10. Jun 19, 2008

### NoTime

I think you can still get small sonar modules (they used to be used in cameras).
One of those might work.
Depends on if only identified objects should be detected.

11. Jun 19, 2008

### IHateMayonnaise

Thanks for the replies all - I think I found out what I needed. There is actually an RFID device that does exactly what I am looking for - the Gotcha! Child Monitoring System - the parent wears a device and if the child travels outside a predefined radius it beeps incessantly. Thanks for the help everyone!