Theme parks and outdoor venues sell little personal misters that you pump up intermitently. You might be able to automate something like that, or at least use the nozzle. You could probably find them online or maybe even at the drug/dept. store.
The hardware stores sell misters that attatch to your garden hose. They have 5 or 6 mist heads per 10 ft. coil. That's probably where I'd start, come to think of it.
Now, this is just a thought (never used them in this capacity), but I wouldn't expect enough pressure out of a pond or swamp cooler pump.
Wait a minute, my buddy Dwayne just showed up (a heating & air guy)... I'll pick his brain and edit this post.
Part Two Edit:
If you want to try the garden hose mister, Dwayne says that you need to simulate outdoor faucet pressure, which is around 60 gallons/minute. To do this, he suggests going to your nearest plumbing supply to look into what's called a boiler pump (used to feed boiler units 2 & 3 stories up), particularly a TACO (pronounced "tay-ko") brand somewhere around 1050 rpm. He says they're fairly cheap ($50-ish), and the new versions are self-contained. They come with inputs/outputs that can be easily modified... his suggestion is to use a 1" intake and go right to a 3/4" outfeed (garden hose size) so that the pump isn't strained.
They run on household 110v, so you could easily put a switch or timer in-line with the power.
I would eliminate the pump all together. Your faucet bib outside will provide all the water and pressure you need in one. I would use a sprinkler water valve to turn it on and off either remotely or by motion sensor. You could use x-10 for the remote, or a hacked motion security light. The motion sensor would turn it on for about four seconds in the test mode. I don't think volume of water is your issue. It's pressure from your water source and typical house water pressure will range from 40 psi to sometimes 80 psi. The pump would make it stand alone.
I use an automotive type 12v windshield wiper sprayer. I get different nozzles from Orchard supply or home depot that you use to mist plants. You can usually find the little replacement motor sprayers at auto stores for $10-$15 dollars or hit the junk yard & yank one with the plastic bottle attached & motor.
I built this set up last year for my toilet from hell after playing with it I really like the effect I got from it in the end....
here is the 1st set up I tried and came out good but with a smaller holder for the water it goes quicker... I tried with a large water bottle and it lasted for the 2 nights that we were open and worked out great...
If you look at the setup of a paint gun its simple and will solve your problems
this is what northern has basically created
Air pressure creates a vacume and sucks fluid through the pipe, mixing with air to create a combination of the two
Adding a couple flow controls to these will allow you to pin point the pressure of the air and the amount of liquid being ejected. Just like a good paint gun. regulate the air pressure and volume and the amount of liquid being fed into the system. Believe it or not it requires VERY little air pressure!
The other thing to do it modify a paint for your needs, they can be found relatively cheap, canister and all included. Put a solenoid valve on the end before the air goes into the gun and your done ! Just apply your trigger to turn the valve on or off!
Note: When making liquids make sure to find something that is non staining, last thing you want to do it stain some poor kids costume that decided to get reallllly close to your prop ! Stay away from blood mixtures that use thickening agents, corn starch or syrup for example as they are known to congeal (harden) and cause nothing but problems!
I made a spitting lizard with an air hose and a smaller rubber tube. One end of the small tube was in the water the other was inserted into the air hose. When I hit the switch to send a blast of air it sucked water up thru the smaller tube and shot out a fine mist of air and water.
(the mist was aimed away from the outlet in the pic and this was just a test, the outlet was covered when the lizard was actually in use0
its a very simple set up, the only kind I can do LOL
I poked a hole in the air hose and shoved the smaller hose in it, careful not to push it in too far .. the other end was submerged in water
this is the trigger I use, its not the best but it works for me, because its manual I can choose who to scare/spray. I know you're suposed to use a solenoid switch but I havent figured them out just yet.