Smoke PUMPS
That's the one!
If you want to do without a servo, you'll need an electronic switch. You can buy them or make one from radio shack parts, but either way, it's gonna cost more in time and money than a cheap servo. I just use a servo with a micro-switch (also available at Radio Shack). Just mount the switch so that the servo hits it. (Make sure it's a push-on, release-off switch)