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?
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 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.
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:
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?
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.
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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)
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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...
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.
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