Metal detector

Looking for idea how to detect proximity of some metal object.

My kid is working on a school project with Arduino, and a goal is to create a door that alows your pet to pass through, but only yout pet, nothing else. So the idea is that a pet could wear a metal collar, or in case of a bird it could be a ring on it's leg.

So, what kind of sensor can detect a small metal ring from 2-3'' away?

what kind of sensor can detect a small metal ring from 2-3'' away?

That is certainly possible, but it takes a fairly sophisticated electronic circuit to do it. Google "coin detectors" for many examples, both commercial and DIY.

A small magnet moving through a coil of wire generates a voltage, but it takes thousands of turns of wire to get a signal large enough for an unaided Arduino to detect.

Try this one:

Bring the tone into the Arduino.

A suggestion. Do it like traffic lights detect cars.

Wind a coil ( 10-100 turns? of wire round the flap, use it as part of the resonant circuit of an L-C oscillator, and measure the frequency. Say a few 10's of kHz.

Any nearby metal object should alter it.


But then a person wearing a ring would be let through the door.

Do it with RFID tags. They can be read at resonable distance and they are very specific - nobody else has the same ID number as the tag on your cat's collar. You can even get unmounted RFID chips, like you see in those security stickers in some stores. That should be light enough for the bird to carry.

Try this one:
Metal Detector Kit Elenco K-26
Bring the tone into the Arduino.

Thanks, I think we will use something like this one.

But then a person wearing a ring would be let through the door.

That wouldn't be a big problem. Door should be small enough so that it is of no interest for humans, and applications could be on places where it is not primary goal to keep humans out, only unwanted animals. For example, chicken koop that lets in only "marked" chicks in/out, but keep predators out.

Thanks for idea with RFID, that might be interesting upgrade of this project.

This is for your chickens -

This is for your chickens -

That's awesome, unfortunately 500 pcs of RFID tags seems a bit too much for a school project :slight_smile:

  • dip grid meter

you can do yourself - simple LC resonance circuit

We decided to go with RFID instead. Easier to find modules, libraries, and application is much more sophisticated.

You can get RFID tags in small quantities from Amazon. The keychain size would fit right on a collar.

Keychain 125kHz RFID tags 5 for $2.88 with Prime shipping

Make sure the pet door frame isn't a complete loop of metal. It is simpler to wind a coil that size for 125kHz than it is for 13.56MHz. You can get pretty close by simply unwinding the loop that came with it, measuring the length, and wind a coil with that much length around the appropriately sized form.

Seeed Studio 125kHz RFID reader UART output

Train dog to hit a pedal that opens the door. Seriously, good engineering is to find simple solutions.

To add Arduino flavor, you may set a certain sequence of hits that open the door, like three hits in a row, one second between them.

But then a person wearing a ring would be let through the door.

Through a pet door? That person would have to be pretty young.