Which protocol / sensor to use for my project

Hi folks.

I'm working on a project which essentially would warn someone if their dog (for example) got too close to let's say, a particular item.

I was thinking NFC tags but to my knowledge the max range is 4 - 6 inches, while I'm looking for 4 - 6 feet ideally.

Infared is of course a possibility, but I need it so an alarm of some kind would only go off if a certain "culprit" goes near the item, not just anyone.

I was thinking maybe xbee? Any thoughts would be appreciated.

Thank you.

Xbee is not a sensor! If you want then to communicate with another device they are a solution but a little expensive. A couple of months ago I had used a CISECO UART receiver. http://shop.ciseco.co.uk/xrf-wireless-rf-radio-uart-rs232-serial-data-module-xbee-shape-arduino-pic-etc/ They perform very well. For a distance sensor I would sugest you a Ultrasonic Sensor. http://www.ebay.com/itm/1pcs-Ultrasonic-Module-HC-SR04-Distance-Measuring-Transducer-Sensor-for-Arduino-/261009210866?pt=LH_DefaultDomain_0&hash=item3cc55c7df2 It can give you the distance with the object and the sensor. You could use that to trigger an event when something gets close ...

Yes, I'm sorry. Xbee is not a sensor.

Could an ultrasonic sensor be paired with another sensor?

Could an ultrasonic sensor be paired with another sensor?

you mean 2 ultrasonic Sensors? Yes uou can attach 2 ultrasonic sensors and read both.

Meaning that you can "pair" two ultrasonic sensors with each other exclusively? What I mean is, the sensors would only "go off" when in proximity to each other?

I'm looking to pair two sensors together so they "go off" when in proximity to each other.

Thank you.

Anyone have any ideas as to what sensors could be paired exclusively with each other?

Bluetooth devices need to be paired and depending on what version you get can have a limited range.

Thank you. I'm going to do some research / prototyping and see if I can figure this out. Thank you :)

I think a ring of ultrasonic sensors at the location, or one on a rotating mast, would work best. No messing with signal strengths to try and calibrate to a distance that would likely vary with battery life in a dog's collar. Have it scan two height levels - if low only received, got an animal - if low & high, then have an adult. Add a thermal check for low heights, animals are generally warmer than humans, yes? I know cats are anyway. A sensor was just discussed in the forum recently, sparkfun carries transistor sized self contained sensor.