cat door uses metal detector as key

[[email protected]]

How about making a metal detector which detects an iron pendant on a
cat collar to unlock the cat door?

Seems to me a BFO metal detector could easily detect a pea sized iron
pellet, especially since there will be no dirt, and the pellet will be
about 6 inches from the coil.

So besides trial and error, how do I figure out what coil size and
inductance I need for the search coil?

Anyone got any simple BFO circuits that would be good for this?

This might actually work and work well!

 

[Rick]

How about making a metal detector which detects an iron pendant on a
cat collar to unlock the cat door?

Seems to me a BFO metal detector could easily detect a pea sized iron
pellet, especially since there will be no dirt, and the pellet will be
about 6 inches from the coil.

So besides trial and error, how do I figure out what coil size and
inductance I need for the search coil?

Anyone got any simple BFO circuits that would be good for this?

This might actually work and work well!

http://geotech.thunting.com/cgi-bin/pages/common/index.pl?page=metdet&file=projects.dat

 

[Jasen Betts]

How about making a metal detector which detects an iron pendant on a
cat collar to unlock the cat door?

Seems to me a BFO metal detector could easily detect a pea sized iron
pellet, especially since there will be no dirt, and the pellet will be
about 6 inches from the coil.

there's lots of cats with metal jewelery.

 

[Eeyore]

How about making a metal detector which detects an iron pendant on a
cat collar to unlock the cat door?

Seems to me a BFO metal detector could easily detect a pea sized iron
pellet, especially since there will be no dirt, and the pellet will be
about 6 inches from the coil.

So besides trial and error, how do I figure out what coil size and
inductance I need for the search coil?

Anyone got any simple BFO circuits that would be good for this?

This might actually work and work well!

Probably cheaper to use an RFID tag these days !

Graham

 

[Adrian C]

How about making a metal detector which detects an iron pendant on a
cat collar to unlock the cat door?

Feed the cat something mildly radioactive and use a geiger counter to
detect it. The cat has nine lives so no need to be so accurate with the
half-life dosages...

 

[James Arthur]

there's lots of cats with metal jewelery.

A bunch of proximity detectors use a free-running
oscillator. The sensed object causes losses in
the sensor's coil, killing the oscillations.

A lossy LRC network on the cat collar would give
a unique frequency-selective RFID, easily sensed.

Cheers,
James Arthur

 

[Jim Yanik]

A bunch of proximity detectors use a free-running
oscillator. The sensed object causes losses in
the sensor's coil, killing the oscillations.

A lossy LRC network on the cat collar would give
a unique frequency-selective RFID, easily sensed.

Cheers,
James Arthur

like a grid-dip meter,with a ferrite bar in the cat collar.
Maybe a "doormat" with a coil in it,when the cat walks onto the mat,the
ferrite bar detunes the coil.Or a coil around the door itself.

 

[Rich Grise]

How about making a metal detector which detects an iron pendant on a cat
collar to unlock the cat door?

If you have a cat infestation, there is nothing you can do that will let
in only one cat and exclude the rest, except something like a cat-lock,
a la air lock, that has room for only one cat. Otherwise, once your cat
unlocks the door, the others will be right on his ass. Of course, if you
forcibly shut the door, you might wind up with this scenario:

:->

Cheers!
Rich

 

[James Arthur]

like a grid-dip meter,with a ferrite bar in the cat collar.
Maybe a "doormat" with a coil in it,when the cat walks onto the mat,the
ferrite bar detunes the coil.Or a coil around the door itself.

Yes, exactly right. The ferrite's a nice idea.

James Arthur

 

[[email protected]]

Yes, exactly right.  The ferrite's a nice idea.

James Arthur- Hide quoted text -

- Show quoted text -

I suppose the required size of the ferrite (fit on cat collar) and the
distance to measure (6 inches) determine what kind of grid dip meter I
will need. Any suggestions on frequency range and how big the ferrite
will be?

 

[James Arthur]

I suppose the required size of the ferrite (fit on cat collar) and the
distance to measure (6 inches) determine what kind of grid dip meter I
will need. Any suggestions on frequency range and how big the ferrite
will be?

Nope, sorry. 50KHz-1MHz?

For the ferrite-detector, metal-detecting circuits might be
a useful starting point. There's an old metal-detecting
ckt using a single CMOS XOR gate package to make two
oscillators, one fixed, one controlled by the sensor
loop. The two outputs are combined by an XOR gate, making
a beat note you can easily detect. That might be good enough.
It was in Electronics Magazine, years ago IIRC.

For the other sort, the grid-dip type, you'd make a series
L-C-R that absorbs at your frequency for the cat collar.
That's the tag. The receiver would be a weakly oscillating
oscillator that the tag's presence would kill, and a diode
detector that detects when the oscillator quenches.

I've looked at many such circuits on the web before--Google
should give many examples. Possible search phrases:

<"Proximity sensor" coil wound oscillator circuit>
<inductive proximity sensor>
<metal detector schematic>

e.g.
http://www.google.com/search?hl=en:...mity+detector"+oscillator+circuit&btnG=Search


This proximity sensor uses an oscillator that's detuned by
someone getting close to a whip antenna. A frequency-selective
detector (a crystal) detects the detuning. (Oct. 1969, pages 1-4)
http://www.radioconstructors.info/gaf2/gaf.html#oct1969

That one's not selective against other cats, but you could put
the antenna in a particular place and teach the cat to
operate it...

The possibilities are many.

Cheers,
James Arthur

 

[James Arthur]

I suppose the required size of the ferrite (fit on cat collar) and the
distance to measure (6 inches) determine what kind of grid dip meter I
will need. Any suggestions on frequency range and how big the ferrite
will be?

Here's an app note on the grid-dip type:

AN2679 Smart inductive proximity switch
http://www.st.com/stonline/products/literature/an/14241.pdf

It's a short-range circuit, for industrial sensing. They use a
uC and an A-to-D, but the RF and diode detector parts
may be interesting. You'd use a sensor coil, looped around
the cat door.

Here's a gate-dip oscillator:
http://www.gregsbasicelectronics.com/circuits/radiofrequency.htm

Here's a BFO metal detector from Don Lancaster:
http://web.telia.com/~u43200663/tech/pocketmetaldetector.htm

HTH,
James Arthur

 

[Charmed Snark]

Jasen Betts expounded in

there's lots of cats with metal jewelery.

Cat bling.

 

[Charmed Snark]

[email protected] expounded in

I suppose the required size of the ferrite (fit on cat collar) and the
distance to measure (6 inches) determine what kind of grid dip meter I
will need. Any suggestions on frequency range and how big the ferrite
will be?

If you're willing to use higher frequencies, you could do a
foil pattern on a PCB, and use that as a tag on the collar.

Snark