> On the outside of the wood cat tunnel I would wind some
> turns of copper wire that would be tied into an oscillator circuit. The
> shorted turn would either kill the oscillator or shift its frequency
> allowing the door to open.
> I can build just about anything electronic or mechanical and have lots
> of parts on hand but I am not very skilled on scratch designed circuits
> like this. Would somebody here be willing to help me with some pointers.
> I can work from an hand drawn schematic. I should be able to supply
> power from a wall wort if needed.
To get started, look for circuits for grid dip meters, theremins, or (best of
all) metal detectors.
You should be able to get a reliable frequency shift with a shorted-turn sort
of collar at
pretty much any reasonable frequency. I'm imagining about a 30-turn coil as
the sense
coil (that the cat walks through) resonating with a couple of capacitors in a
Colpitts
configuration (that avoids having to make a tapped coil).
Actually, thinking about it, I'd hang a resistor in the power supply to the
oscillator to
measure the current draw - when a shorted turn shows up, the oscillator should
draw
more power, creating a larger voltage drop across the sense resistor. A simple
RC
filter and or peak detector after that would let you use a simple comparator to
detect
the change (detecting a change in a DC level seems easier than detecting a
change
in frequency).
As Ray pointed out, RFID would work too, with a tag in the collar or the
in-the-body kind.
There are commercial versions of this, as well as RFID sensors available from
adafruit
and the like.
There was also a project that used a video camera and profile recognition to
let in a specific
cat - and only if the cat wasn't carrying an animal in its mouth! It's a
little over the top, but does
refer to a ready-made cat door ("Cat Mate") with a solenoid to unlock it, which
might be useful
if you don't want to build your own door mechanism.
http://www.quantumpicture.com/Flo_Control/flo_control.htm
- John
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