While the climate here in Los Angeles is exceedingly mild–it rarely gets much below freezing–springtime can, some years, be too cold to get good germination of summer vegetables such as tomatoes and peppers. This was the case in 2010 when I was not able to get a single tomato seed to germinate until late May. To head off another seedling crisis I built a simple cold frame.
In order to prevent the cold frame from becoming a solar cooker (it can get over 80°F during the day this time of year) I pimped it out with an Univent Automatic Greenhouse Vent Opener. The Univent uses no electricity. As the temperature gets hotter a small piston thingy forces open the window you attach the Univent too. As the temperature cools in the evening, the Univent closes the window. It was easy to install, though the directions it came with seem to have been translated back and forth between several European languages before materializing into English.
The price on Amazon seems a bit steep at $50. I got mine on sale from Peaceful Valley Farm and Garden Supply,
but they no longer seem to carry it back in stock here.
I’m fully aware that my cold frame, with it’s plastic cover, would be way too flimsy for places with real weather. Nevertheless, I can imagine the automatic vent opener being useful in many climates.
ETA: Mrs. Homegrown here: I just wanted to add a clarifying note. This cold frame is The Germinator ™, one of our recent garden improvements. Ordinarily it is covered with wire screen, which lets sun in but keeps critters out. Erik’s plan is to swap out the plastic sheeting with the wire screen as needed.