Close proximity detection circuit help

Discussion in 'The Projects Forum' started by icydash, Apr 30, 2009.

  1. icydash

    Thread Starter AAC Fanatic!

    Jan 14, 2009
    Hey guys! I'm trying to design a circuit that will detect when someone wearing a passive RFID bracelet is within 12 inches (no more than that, the range is pretty specific for a reason) of the circuit, and turn on an LED. My initial thoughts were to use passive RFID bracelets since they are extremely cheap and require little to no circuitry on the person wearing the bracelet (which is very very important to the project), but it's becomming clear that it's extremely hard to get the range of 10-12 inches out of it without using enormous amounts of power on the transmitters' side. Do you guys have any ideas of how I can make a circuit that will read a passive RFID tag at 10-12 inches without immense amounts of power, or of something else I can use instead of RFID for the same purpose?

    The only real requirements are that the bracelet needs to ideally have as little circuitry as is humanly possible (and prefribly doesn't need battery power), and it has to be able to be picked up in a range no more than 12 inches maximum, but definitely at least 8 inches minimum.

    Thanks for your input!

    Last edited: Apr 30, 2009
  2. electronictech

    Active Member

    Apr 1, 2009
    I don't know much about RFID, but I have noticed where I work, the device(transmitter/transceiver) that I bring my key FOB up to in order to unlock the door is not much larger than the key FOB itself. With that said, it might be possible to increase the range of the transmitter/transceiver by increasing the size of its antenna. I am just guessing but I want to believe the antennas are loop you might be able to keep the loop of the key the same, but increase the size of the locks loop and by doing so increase the operating range.

    I'm really not sure, but some experimentation will yeild the best results on how to increase the range.

    Anyways, good luck and have fun!
  3. icydash

    Thread Starter AAC Fanatic!

    Jan 14, 2009
    Thanks for the response. What is FOB?

    I was thinking about using a circuit like the following with a passive RFID bracelet:

    The only problem is I contact the guy and he said the coil would have to be pretty big and you;d need at least 2.7 Watts to get around 6 inches of range out of it, which would put like 1000 volts p-p across the coil, which just isnt safe.

    Does anyone have any other non RFID ideas? The bracelets can be active if they have to be, I'd just rather have the least amount of circuitry as is possible.