Proximity between two devices?

Discussion in 'The Projects Forum' started by svdsinner, Apr 9, 2012.

  1. svdsinner

    Thread Starter Active Member

    May 17, 2011
    I'm looking for a way to build a multipart system where a central receiver circuit could indentify if one or more transmitter circuits are within roughly 3 feet of the transmitter. There would need to be up to 10 transmitters, with at least 4 different "codes" that the receiver could differentiate between.

    (Similar to RFID tech, but I need more range, the transmitter can be active rather than passive if needed, and only reasonable accuracy is required.)

    The application is an automatic feeder for my hobby farm, where my miniature cattle and my horses can get food, without the other animals stealing it from them. (If you just leave food out, the big horse will steal all the miniature cattle's food) The animals would have the transmitters hanging off of their collars/halters. The feeder needs to identify that an animal is checking the feeder, then a micro-controller will decide if the animal needs food (or has eaten enough for the day), which kind, etc. and dispense as needed.

    The detection circuit is the part I'm struggling with.
  2. wayneh


    Sep 9, 2010
    Are you sure? I thought RFID was good for at least several feet, making it pretty much ideal for this.
  3. panic mode

    Senior Member

    Oct 10, 2011
    my colleague and friend used RFID for tracking multiple motorcycles passing through gates to keep track of scores during the race. sensing range about 5m using tiny tags placed on metal of the frame or on helmets.
  4. svdsinner

    Thread Starter Active Member

    May 17, 2011
    Of course not, that's why I'm asking questions :)

    If money is no object, you can buy RFID systems that have ranges of over 50' and can simultaneously track hundreds of tags in its range. Such systems cost thousands of dollars.

    Affordable RFID (<$100-ish), however, seems to be limited to swipe-the-card-near-the-sensor range.

    I would be thrilled to be proven wrong if anyone knows of any affordable solutions with 36" of range.
  5. svdsinner

    Thread Starter Active Member

    May 17, 2011
    BTW, in case it might spark some ideas, there are several things I can get away with in my scenario that might make things easier.

    • There are other cues that an animal is nearby that could trigger the system. From there, the receiver would only have to know which transmitter is closest. (IOW, I wouldn't have to monitor for absolute proximity, just relative proximity)
    • The transmitter will move through a known area of space that could have line of sight to the receiver while the animal is approaching. That means things like IR or ultrasound could be used in the solution.
    • Since this will be used in a rural area with limited competition for unlicensed RF, I could use multiple channels/frequencies in the unlicensed spectrum rather easily.
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Since the transmitter side has apparently not much limitation in size you could get away with a inductive coupling. These are normally used in the 125kHz range but for the very near range, mainly limited due to the small size of the transmitter coil.

    If you imagine a transmitter coil of at least 1m in diameter you should be able to switch it with 125kHz (in series resonance), at 3 feet you should then be able to power up a receiver coil which in turn can power a cheap RF transmitter module...
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
    you could drill a hole in their hoof & epoxy in a RFID capsule. Then put your scanner under a platform in front of the feeder that they have to stand on. That will get you your "(<$100-ish) swipe-the-card-near-the-sensor range". Put guides or stall walls leading up to the feeder so they have to stand in a specific spot, with their foot on the scanner.