Best way to detect motion

What is the recommended approach for motion detection with .NET Gadgeteer? I need to detect that my cats have entered their newly crafted automated kitty litter machine. I can think of three ways, each with associated problems:

  1. Use the PIR sensor but is it going to be offered again?

  2. Use the Light sensor but would it pick up room light, not just cat movement?

  3. Use the Accel G248 sensor but wouldn’t it pick up vibration from motors while they are running?

Let me know if there are specific instructions on how to use these techniques.

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@ tacomman, I would think an off-the-shelf PIR could be easily modified to allow it to be incorporated into your design. It might involve a little creativity on your part to ensure that the supply voltage is correct and that the output signal from the PIR is correct to feed into your Gadgeteer project, i.e. either 5V or 3V3 depending on which input you use - I can’t recall if all inputs are 5V tolerant so please check before you destroy your Gadgeteer board… A simple transistor as a switch could be used for that, or even an resistive potential divider would work. Then it should just be a case of setting up a pin, possibly through an extender module to fire an interrupt on your main board.

Failing that a possible alternative approach might be a simple pressure pad in front of the area you are monitoring that could similarly interface to an interrupt.

Hope that helps.

Given the group here I thought I’d also add some more eccentric solutions in case you’re interested:

[ul]Out fit the cat in question with an RFID tag and use this RFID tag [url][/url] [/ul][ul]Use an IR distance module (no longer available from GHI)[/ul][ul]Add a radio collar to the cat and measure that[/ul][ul]Teach it meow at the correct pitch before it enters its kitty box[/ul]
I know the above are perhaps a little more involved than some of my other suggestions, but I think these might get more people talking about how to help with your solution… :smiley:

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PIR would work, but remember that they only detect heat+motion. A still object is the same as no object. Who hasn’t had to wave their arms in a meeting room or bathroom stall to keep the lights on? I wouldn’t want to surprise kitty just because s/he didn’t move.

Ultrasonics would work too, but the cat might actually hear the pulses and be deterred from using the box.

I think I might be inclined to use a Sharp IR distance device. It works in the men’s room, so it should work for kitty too, presuming s/he is suitably IR-reflective (certain fabrics are invisible to IR - maybe that goes for fur too).

Perhaps one of these A time of flight range finder


Apparently they are used in cellphones to determine if the phone is close to your head.

Yes, but it is trickier to interface with 2.8v and no tolerance for 3v3 or 5v, a more complex data interface, and only 10cm range. The Sharp sensors are longer range 5-60cm and much less fiddly to interface both in terms of supply voltages and output format.

Have you thought of hacking an electronic bathroom scale? You can pull the signal from the load cell and run it through an Instrumentation Amp and then into an ADC input.
Added benefit you can track your cat’s weight, or when the litter needs changed. :wink:

Do both - there’s no such thing as “too much data” :slight_smile:

I’ve always liked the doppler radar modules for detecting motion as they can work through non-metalic obstacles including brick walls. The downside is that if the cat stands still it becomes invisible.

@ skeller - the problem with the electronic scale is they are meant to be loaded and then unloaded otherwise they start to suffer from creep with a continuous load.

I have the Sparkfun Load Cell Amp as one of the breakout boards that I’m converting to Gadgeteer Modules, but I can’t say when I’ll get to it.

Well if push comes to shove, you can always use optical flow with camera.