Switch for hobby project

[buttercup]

I want to be able to detect when my cat enters an area. The area is 24 inches square.

Could I rig up a home-made capacitive switch? I could form a sandwich of two sheets of aluminum foil, separated by a sheet of insulative plastic foam material (squashable). If the cat put her foot on the sandwich, the capacitance would change, and I gather that that change can be detected by some kind of simple circuit.

You can see my level of expertise from the fact that I ask the question.

Can someone point me to a website /youtube etc where such things are described in very simple terms, and/or with detailed step by step instructions. Many thanks.

 

[yellowbot]

what you are looking for is a pressure plate switch.. not sure how simplified you need the instructions to be or the materials you have but if this link doesn't help you, then google: DIY pressure plate switch

http://www.instructables.com/id/Use-a-DIY-Pressure-Plate-Switch-to-Automate-Your-H/

 

[benstt]

I I want to be able to detect when my cat enters an area. The area is 24 inches square..

Does the cat need to survive? Lots of options.

I haven't used a capactive plate, but sounds like you're on the right track.

 

[geeky]

depending on where the area is, an off the shelf motion detector aimed at the right angle might get the job done. It would help to understand more broadly what you are trying to accomplish?

 

[buttercup]

Thanks for taking the trouble to provide input. Sorry I didn't make it clear.

I need an electrical signal that gives me a yes/no if my cat enters the area, 2ft square.

I thought of a touch pad, but I cannot find a commercial one that senses a force of e.g 1lb (which would be my cat putting one paw onto the pad, which is what I am seeking.) A motion detector is not the thing.

It occurred to me that two sheets of aluminum foil, separated by a thin sheet of squashable plastic foam, would make an excellent capacitor. It seems to me that a cat's paw force of e.g 1lb compressing the two "plates" closer together, just locally, would provide a highly detectable change in capaitance, which could easily be harnessed by a circuit of some kind, to create a yes/no signal. So, I'm thinking, I would have have an excellent ready-made capacitor which would surely provide a detectable signal, which would be very simple and reliable, and can be tailor-made for the my area, -- plus, costs next to nothing.

Holding the two Al sheets apart with a layer of foam solves the mechanical problem that might occur with a touch-pad, that the upper sheet would simply sag down against the lower sheet, giving an always-on signal. Separated by the layer of foam, my two foil sheets can never touch -- even though the area of my sensor is four square feet or more.

But I have no idea how to do the circuit. I gather a capaciitve switch works by constantly charging/discharging the capacitor plates, and sensing when the charge/discharge time changes. But I have no idea how to put that into effect. If I could buy a ready-made circuit or unit, that would be great -- but I guess the sensing circuit would have to be tuned to the particular parameters of my capacitor.

Very easy? Impossibly difficult? Help?

 

[nofrill]

I guess you want to catch the cat entering this area when you are not keeping watch or at home, correct?

When you are not keeping watch or at home, is your cat the only living being moving around the house? If this is not the case, I guess your "touch pad" method would not work either, if something else other than your cat steps on it.

If the above is correct, would you consider setting up a webcam and hook it up to a PC / laptop that runs a motion detection software, which sends a text message or email to your cellphone or computer? The software will take snapshots or record video when there is motion, so that you can review it when you return home. To be precise on the area where your cat may tread, you can either set up the webcam high up so that it points directly downwards at the area of interest, or get a webcam with zoom ability to localize the area of interest.

 

[Garrett]

But I have no idea how to do the circuit. I gather a capaciitve switch works by constantly charging/discharging the capacitor plates, and sensing when the charge/discharge time changes. But I have no idea how to put that into effect. If I could buy a ready-made circuit or unit, that would be great -- but I guess the sensing circuit would have to be tuned to the particular parameters of my capacitor.

Very easy? Impossibly difficult? Help?

I think you are getting too complicated. You can simply use the foil as the conductor and have a non conductive top/bottom with low current. If you need logic, you can go Arduino. There are a lot of sources for Arduino motion sensors if you want to get fancy.

You could also put a discrete switch in the sandwich and hook it up to a Raspberry Pi.