I am disappointed with the automatic vent openers. The few that did work would only open a few inches. I was worried about cooking my plants, or even worse, the kids' remaining fish! So I set off to go build a better mouse-trap.
I'll have to warn you, I run an IT department, so I know how to program and tinker with electronics....
I wanted to completely automate the greenhouse so I didn't have to keep an eye on it. At my disposal, I had an old laptop from work. I then purchased a couple of USB temperature cards and a relay card (dlpdesign.com). Next I found some used windshield wiper motors (surpluscenter.com) and started tinkering.
I wrote a controller program that was able to monitor the temperature of the greenhouse. Based on the temps, the vents are able to open & close on their own. It's easier to watch how they work, then to explain it, so here's a link to YouTube: Click for Video
Here are a couple of closeup pics of the vent opener:
The controller is also able to plot a history of temperatures. Here's a 24 hour snapshot.
Blue=tank water temp
Black Up Arrow=vent opening (one set opens at 73, the other at 75)
Black Down Arrow=vent closing (one set closes at 72, the other at 74)
Yellow=amount of sunlight entering
So far, I've very happy with the results. I'm thinking about using the same controller to control a hydroponics system next year. Stay tuned for future posts!