Looking for a short range detection and identification

Question

I am currently building a smart cat door. One of its functionalities is that it keeps the door locked until our cat comes close to it, then it unlocks the door.

I tried using RFID, the RC522 because I read that it can work fr a range of 1m. But after testing, the max range I could get out of it was less than 10cm.

After doing some research I found out that the protocol used in the RC522 can't do more than 10cm.

Another solution I came up with was to use Bluetooth instead, but the problem with Bluetooth is that it has a long detection distance for our application (with class 2 having a range of 10m). This will mean the door will detects the cat even if she is just setting in it's vicinity and we don't want that.

Is there any other sensor combination that can accomplish this task?

• It doesn't have to be RFID or Bluetooth.
• It has to detect the cat at a short range, lets say no >2 meters and no <1 meter.
• It is preferable if the tag/module that goes on the cat is unpowered, or requires low power.

Thank You

Answer 1

Why such a huge range?

When I built my smart cat door 40 years ago I used RF. I used a 10nF polyester capacitor, and wound a few turns of wire around it, and potted it in resin, including a small hook to attach to the cat collar to make a dongle for the cat. Then built an oscillator at its resonant frequency. I built a flat search 'coil' using veroboard (for stability) with copper wires soldered between the strips, total size probably 25x50 mm. With the cat's head in contact with the flap, the dongle was within 100mm of the bottom of the cat flap, where the search coil was. That's enough coupling to increase the loss of the coil, especially since the orientation of the cat approaching the flap ensured the collar, and hence the dongle, was coaxial with the search coil. Rectify the voltage across the coil, trigger on a dip. The point about using a resonant dongle was it could be relatively small and light.