Looking for Cheap Proximity Sensor (For Cat-Flap)

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Hi,

I have just imported an "Electronic" Cat-Flap. This is a door with
a latch that only allows one own cats to use the door. However I
was extremely disappointed  with the "Electronics". This consists of 3
read switches with a 9V battery holder and a  solenoid which unlatches
the flap when a fairly large magnet is brought within about 20mm of
the door edge. The magnet is about 10x10x20mm. I want to replace this
primitive setup with something a bit better. What cheap proximity
sensor can people recommend ? I am looking for something that will
give at least one bit of info within 100 to 200 mm from that cat-door.
The device on the cat side should not need a battery.
If I can get an unique ID as well, then this will be a bonus.
Any suggestions ? If I have to use a small MCU (AVR or 8051), it would
not be a problem. Even if I can get something that will work with a
much smaller magnet, would at least be better than the current setup.
What devices are used in these coded keys one gets with many cars
these days ?

Regards
   Anton Erasmus

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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Most prox sensors I'm familiar with are non-discriminatory: They would
open for the neighborhood dog just as well as the cat, which probably
isn't what you want (if so, just use a normal cat flap).

RFID tags could do more than you need, though I'm not sure you (or I)
would consider them cheap.  TI sells a mid-range reader module good to
about 175 mm for less than $200
(http://www.secureorderprocess.com/ti/product_detail.asp?product_id93 %),
though you may want the eval kit, which is a bit more expensive ($645,
http://www.secureorderprocess.com/ti/product_detail.asp?product_Id89 %),
but includes some transponders (tags) and other helpful stuff.  

Longer-range readers are available if you have a lot more money to
spend.

The tags themselves can be very cheap, but tend to be sold in volume.
I'm not sure where you could buy just a handful (other than the eval
kit).

HTH,

                               -=Dave
--
Change is inevitable, progress is not.

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
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I can see it now.  Entrepreneur buys thousands of identical tags,
and markets system.  Becomes popular, and three households on the
same block buy in.  Households immediately become common to all
three sets of pets.  Cats, dogs, ferrets, maybe the odd skunk.
All householders add additional collars and invisible fences.
Pets, weighted down by innumerable special purpose collars, die
early of physical exhaustion.  Lawyers enter the field, armed with
innumerable writs, liens, depositions, etc.  Owners move out.
Remaining pets become feral as collars wear off.  City ordinances
are passed.

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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[...re: RFID...]
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But you can't.  Each tag (ISO 15693) has a unique 64-bit identifier.
Even if the numbers do eventually repeat, 18.45e18 is a Really Big
Number(tm).  Finding two tags with the same ID would be... unlikely.

Many tags also have user-writable areas of up to 64k bits.  You could
put your name and address in there.  

In fact, companies like AVID will inject RFID tags under your pet's
skin (Now that's a *real* embedded system...), though these are
generally just ID numbers with the real ID information kept in a
cental database.

Regards,

                               -=Dave
--
Change is inevitable, progress is not.

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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Repeat are you serious?

If they could manufacture tags at a rate of 1,000,000 a second, and ran their
machines 24 hours a day, 7 days a week, it would take over 584,000 years to
start repeating identifiers.

Finding two tags with the same ID would indeed be unlikely.  And won't really
be a problem unless 584,542 years from now you still have the same, *really* old
cat. ;-)

-Zonn
--------------------------------------------------------
Zonn Moore
Zektor, LLC
www.zektor.com

Remove the ".AOL" from the email address to reply.

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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Not really.  But some people (Benneton? for example) are talking about
putting one of these transponders in every item they sell, so they're
talking volume production.

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Well, that assumes 1 manufacturer using all 64 bits.  I don't know,
but I'm guessing that each manufacturer gets a block of numbers to
use.  

If I don't know this for a fact, I sure don't know the size of the
manufaturer ID, but let's assume 16 bits.  That leaves 48 bits for
281.5e12 unique IDs.  Using your 1e6/minute, and assuming your math
was correct, that still gives each mfr almost 9000 years.  Should be
adequate...  At least until some newer technology replaces it.

Regards,

                               -=Dave
--
Change is inevitable, progress is not.

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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I wasn't really serious either, I just happened to have a calculator in my hand
when I read the post.  I knew it was tongue in cheek by the "Really Big
Number(tm)" trade mark. ;-)

But as long as I have that calculator, let's talk volume...

If 100 tags weigh an ounce, and only 48 bits are used per manufacturer, then
2^48 tags would weigh 87,960,930 tons.  Now, that's volume production! :-)

-Zonn
--------------------------------------------------------
Zonn Moore
Zektor, LLC
www.zektor.com

Remove the ".AOL" from the email address to reply.

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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their
to

Walmart in the US is going to require all suppliers
to have them in their goods when manufactured
or for small items in the packaging.

In clothing it is sown into the lining.

(see gilette stories that have been in the news
gilette in UK has fitted RFID tags in all their products as part of a test)

http://lists.indymedia.org/pipermail/imc-cambridge/2003-August/000275.html
http://www.doxpara.com/read.php/security/rfid.html

Looks like the retail chains here in Australia
atre going to go the same way.
Except possibly you will have to buy the tags from the store
as they will implement their own system.

For cats and dogs here they all have to be microchipped.
Chip is inserted with a "large" needle.

Alex



Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
Here's the link to Flo Control:
http://www.quantumpicture.com/Flo_Control/flo_control.htm

You could, perhaps, build something similar based on this hardware, using
custom software:
http://www-2.cs.cmu.edu/~cmucam /

Tanya



Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
Retinal scanner or kitty facial recognition would be my reccomendations.
You can mount either the laser scanner or camera on the side of the cat
door, then run the scanning algorithm on the PC in your house.

Another option would be a wireless ID unit embedded within the cat.  RFID
tags sound nice, but what happens if the cat loses its collar and its RFID
tag on a cold, winter night.  It'll be stuck outside and freeze to death!
No, you definately want to mount the electronics inside the cat.  Its the
only way to be sure.


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Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
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In the Netherlands, you can have your pet injected with an RFID tag.
Wouldn't be too hard to read that one out.

Meindert



Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
On Fri, 19 Dec 2003 22:40:23 +0100, "Meindert Sprang"

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My cat actually have an RFID Tag already. (I got her at the SPCA, and
they put one in as standard practice). The reader they use has to be
very close (Next to the skin) to read the tag.
What sort of circuitry would be needed to read the existing RFID tag
over a distance of 200mm ?

Regards
   Anton Erasmus




Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
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My guess is that this is necessary because of the orientation of the tag. It
will most likely be lying flat inder the skin and therefore in the worst
possible orietation for the reader. I would think if you have a coil around
the cat flap, your distance may be much higher.

Meindert



Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
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No, it's usually an MFID tag. Requires close proximity because it
operates as the secondary coil in an imaginary air-cored transformer.
The reader puts a carrier frequency into the primary winding, which
powers the tag through induction. The tag modulates an ID code onto
the secondary winding, which changes the current drawn by the primary.
Microchip publishes some good app notes on the topic.

RFID systems fall into three categories, distinguished by operational
frequency. The long-range systems used on train cars, shipping
containers etc. operate in the n-GHz band. Long range, expensive,
performance greatly affected by atmospheric conditions. The types used
for animal chipping are much lower-frequency and have an effective
range of just a couple of centimeters.

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
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Get a terrier.  Guaranteed to detect a cat 100-200mm
from the door and doesn't need batteries.




Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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I believe that the average terrier would have almost no false negatives when
used as a cat detector.  But are false positives a problem?  aka barking for
no apparant reason?

Kelly



Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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Now if you can suggest a cheap way to interface this terrier to an
AVR, then it might work :-)

Anton






Re: Looking for Cheap Proximity Sensor (For Cat-Flap)

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Many dogs and cats already have unique grain-of-rice sized implanted
passive RFID tags. (My cat does.) Used with national registries, they
can help reunite lost pets with their owners, and possibly also
provide medical information.. As I recall there are at least three
different companies doing this in the US, and no doubt their tags are
all mutually incompatible.

Jim McGinnis

Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
           snipped-for-privacy@intekom.co.za "Anton Erasmus" writes:

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[%X]

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Sounds like an ideal application for an RFID tag. Small, inductively
powered from the receiver and, if you have more than one cat (or other
pets) would know which one was entering/leaving (useful status info).

--
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Re: Looking for Cheap Proximity Sensor (For Cat-Flap)
Hi Anton,

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You might think the shape-recognition idea was frivolous, but I have
seen at least one fielded project using simple shape recognition of
the animal's silhouette to correctly allow the designer's cat to enter
[within certain time periods], while rejecting the cat if he has
something in his mouth or if it's some other animal. The principal
reason for building the device was to prevent the cat from bringing in
mice, sparrows, etc.

The web site for the project shows lots of sample photos, rather
sinister looking skunks and so on (all in silhouette) captured by an
IR-camera.

Unfortunately I can't find it in a couple of moments' search with
Google, but I KNOW it exists. Maybe it was mentioned here last year
sometime. It seemed very cool.

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