cat door uses metal detector as key

there's lots of cats with metal jewelery.

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...

On a sunny day (Sat, 13 Dec 2008 15:42:17 +0000) it happened Adrian C wrote in :

Eh, Schrödingers cat could even be walking in the door dead ;-)

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

On a sunny day (Sat, 13 Dec 2008 18:24:10 GMT) it happened James Arthur wrote in :

Weight sensor, check weight on going out. Must be about the same on going in (same cat, but without carrying some mouse).

James Arthur wrote in news:e5T0l.1630$ snipped-for-privacy@nwrddc01.gnilink.net:

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.

Won't work, cat goes out, takes a dump, brings back mouse for a meal.

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

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

James Arthur

be

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:

e.g.

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)

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

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

AN2679 Smart inductive proximity switch

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:

Here's a BFO metal detector from Don Lancaster:

HTH, James Arthur

Jasen Betts expounded in news:gi038e$ve$ snipped-for-privacy@reversiblemaps.ath.cx:

Cat bling.

snipped-for-privacy@wwc.com expounded in news: snipped-for-privacy@k36g2000pri.googlegroups.com:

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