Hi All,
I have a water fountain for my cat (fountain pictured below) which I would like to control via either a simple pressure mat, a cat-friendly switch, or an Arduino. I'm familiar with Arduinos but have pretty limited knowledge of electronics, so I'd like to take this as an opportunity to learn.
Fountain
The fountain has a simple 12V water pump which simply sits at the bottom center and is mechanically attached via rubber suckers.
A cable then exits the fountain assembly and terminates in a male barrel jack which plugs into a 240v > 12v adapter with a female barrel connector.
Arduino control
In the case of Arduino control I'm presuming I would need a relay (happily I have a few of these Sparkfun relay PCBs built up). However I'm not sure what's the most efficient way to wire everything up; presumably the neatest thing would be to power the Arduino and the fountain via the 12V supply. Either that or power the Arduino via battery but let's go with mains > 12V for now.
In my head I am taking a male 'generic power supply' adapter like those seen on the left here:
and soldering two cables to that (before shrink wrapping them), and connecting these to some kind of circuit board/breadboard.
Then I am tapping off the 12V rail once for the Arduino voltage in and once for the pump (via the relay). Ground goes to a generic ground rail that will be used for the whole board (right?).
Great so now I have power to my Arduino but the pump is dead.
I need to wire the relay's ground to my ground rail, and the Arduino's 5V output to the relay's VIN.
Then I need to wire an Arduino logical output to the relay's 'controller' in.
So my question is: is it safe to connect the relay's (5V) ground connection, and the pump's (12V) ground connection to the same rail, or do I need to have some kind of protection in there?
That's the only question I can think of for that scenario. Presumably the only other thing is whatever sensor I use to detect the cat which could be IR or pressure mat or motion sensor or whatever.
I know an Arduino is probably overkill here (and I really can't afford more gear) but it would be nice to know how much the cat drinks each day and / or control the thing remotely to encourage her to come near it and drink.
(btw at the bottom of this post I have a screenshot of what I currently imaging the layout/circuit diagram would look like for this).
Switch control
But for argument's sake, what would I need to make this with the bare minimum parts? Presumably the most basic thing would be an insulated pressure mat switch that closes the 12v connection when the cat steps on it? How would I add a delay to it (maybe put a capacitor after the switch?)?
Any and all advice welcome and appreciated 
The fountain:
The PSU:
The Sparkfun relay PCB:
A sketch of what I think this would look like in Fritzing (my first time doing this so be gentle!)