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Looking to build a conductive matt that is split in two with a insulator in between them to allow detection of when a dog or cat walks over it and completes the circuit through their body, but as both can avoid things that they can sense and don't understand, would need to be a current (not voltage, within reason) to my understanding that 1) Doesn't allow them to detect it, 2) Isn't so low that noise is an issue in false positives, no?

Have done much google searching with varying results on what current humans can sense (which I presume would be similar for the aforementioned different animals), but not what kind of current one should stay away from due to false positives.

A tried and true circuit to connect to these pads to output a logic level on/off output would be appreciated also.

user66001
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    Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Nov 17 '21 at 21:47
  • What exactly are you trying to do? To detect an animal passing? There are probably better ways to accomplish this. – Eugene Sh. Nov 17 '21 at 21:57
  • @EugeneSh. - A mat on the ground, where a dog would walk over, that has two conductive plates (not yet determined what they are going to be made out of) end on end, with a small insulative surface in between. Need to detect when 2 paws are on one matt, and the other 2 are on the other matt. – user66001 Nov 17 '21 at 23:08
  • use a ghost detector circuit - stacked darlingtons. 5v won't hurt a pet, even with wet paws, so don't worry about current. – dandavis Nov 18 '21 at 02:17
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    Look into a pressure-sensitive mat as an alternative sensing modality. Much simpler and more reliable! The current conducted through the animals pads, paws, and body will vary wildly with pressure, moisture, and dirt. – Mark Leavitt Nov 18 '21 at 04:28
  • @dandavis - Hurting the pet it an issue, yes, but more concerned about spooking them with a strange sensation. Any idea what current (I understand that to be the important electrical characteristic from a sense perspective) would allow detection with the least amount of false positives with current discrete circuit designs? – user66001 Nov 18 '21 at 10:01
  • @jsotola - I do "own" a dog. Your comment could have just been about dogs paws being very dry, but whatever. I have read that human fingers can be dry also, and be detected by a restive circuit. – user66001 Nov 18 '21 at 10:01
  • @MarkLeavitt - Thanks. Are we saying the difference between air resistance, and resistance of any type of conductive surface like a dry, wet, dirty paw is not greatly different, hence detectable? Would very much prefer not to use pressure sensors if I can help it. – user66001 Nov 18 '21 at 10:03
  • it's ohms you want to measure, not current. a dog could be anywhere from 500k to maybe 5k with wet paws. if it all ran at a safe 5v, that would be from 1mA to 10uA. Not all current sensors can reliably tell the difference between 10uA and 0uA. a cheap ($1) radar sensor module would make more sense to me. – dandavis Nov 18 '21 at 10:54
  • @dandavis - Unfortunately for reasons that are out of scope of this question, I can only use a sensor that is either current or resistivity based. If you can provide help with finding a circuit with either of these attributes (I am aware they aren't separate from each other, and changes to resistance changes current) happy to hear back. – user66001 Nov 18 '21 at 16:23
  • go with measuring resistance. look for ohmmeter circuits that can cover 10k - 1m ohms, then use a MCU or window comparator to turn the reading into action. – dandavis Nov 18 '21 at 18:12
  • @dandavis Thanks. So these circuits would have to put some current into the circuit to read resistance, no? If so, we are still talking about uA, correct? The concern is just that as soon as my pet feels something they don't like, they aren't going to go near anything that looks, or smells the same again. Hoping to avoid this, not switch out the resistance pad with something new each time I test. I guess finding someone who has tested what a pet can feel isn't going to see this question easily. – user66001 Nov 18 '21 at 18:55
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    i can't feel a DMM measuring resistance on my tongue, how could a dog feel it on paws connected by several inches of body? In the name of science, I just tested to see if my cat can feel a 9v battery on a damp paw; no reaction to speak of, despite the "high" voltage and close terminal spacing. I feel you're way over-thinking this. It also takes more than one stimulus exposure to condition a response. – dandavis Nov 18 '21 at 19:41
  • @dandavis - Thanks for the test. Will look into ohmmeter circuits, as suggested. – user66001 Nov 18 '21 at 22:31

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Detecting a resistance change between two conductive plates is easy, but some precautions should be in place, such as in-series resistors to limit the maximum current in case a shorted condition is present (i.e., the two plates touch each other). To detect a change in resistance, I suggest the Wheatstone Bridge circuit, such as this Light Detector. The actual resistance values don't really matter in this circuit. Instead of the LDR light sensor, just replace with a fixed resistor of the same value as the others, then wire your two plates in parallel with this resistor. Any conduction between the two plates will throw off the sensing resistor and cause an imbalance in the bridge circuit, thus the detector to activate. I suggest starting with 100K resistors and lowering the operating voltage to 5V since you don't need to energize a relay coil.

If you need extra paw detection accuracy, then you can break up the front plate for a left/right side, and the back too, for four plates total. If the plates are large enough then this might work, even if both front paws are one one place you get at least some detection.

Another idea, besides resistance change, is to use a membrane switch. Just purchase two roll-up keyboards like this and use as your plates. Then connect to a computer to detect any key presses. Your software could be written to detect dog paws vs human foot.

icodeplenty
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