Small Heat Vent: Bimetalic Coil

I'm building a portable cold frame; it's a 2x2 frame about 1.5 feet high, 4 feet long, and 3 feet deep. Sides and top will be covered with double thic k polycarbonate sheeting left over from building the greenhouse a few years back. My wife's idea is to put it out in the garden and start some seeds u nder it to get a jump on the growing season. Garden places sell things like this but want a fortune for them.
The problem is that on a very sunny day, it can get hot enough inside there to cook and kill the growing vegetables. I can put in a small, manually op erated vent, but would like to have something automatic, that opens when it gets too hot and closes when it gets cold again. I've googled around but c an't find what I need. I'm looking for something about 2 inches square that I can mount though a wall of the construction.
The people on this group have such a wide range of experience that it's pos sible that someone can give me a link to these devices, if they exist. Than ks for any help you can give.
Paul
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On 2/11/2013 2:41 PM, Pavel314 wrote:

Perhaps the bimetallic choke coil from an old carburetor is sensitive enough for your purposes. Here's a couple of links that may help with your project. ^_^
http://www.crestmfg.com/
http://incubatorproject.wikispot.org/Bimetallic_Coils
TDD
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On 2/11/2013 2:41 PM, Pavel314 wrote:

high, 4 feet long, and 3 feet deep. Sides and top will be covered with double thick polycarbonate sheeting left over from building the greenhouse a few years back. My wife's idea is to put it out in the garden and start some seeds under it to get a jump on the growing season. Garden places sell things like this but want a fortune for them.

there to cook and kill the growing vegetables. I can put in a small, manually operated vent, but would like to have something automatic, that opens when it gets too hot and closes when it gets cold again. I've googled around but can't find what I need. I'm looking for something about 2 inches square that I can mount though a wall of the construction.

Think you'll need a lot more area than that to do any good...look at one of these...
<http://www.johnnyseeds.com/p-8136-univent-automatic-opener.aspx
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; it's a 2x2 frame about 1.5 feet

That's rather pricy aat $49.
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; it's a 2x2 frame about 1.5 feet

That's what I was thinking. A passive 2" square vent doesn't sound very big. And with a flap of some kind, etc, the effective area is further reduced.

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On 02/11/2013 06:02 PM, snipped-for-privacy@optonline.net wrote:

My thoughts on a 2"x2" opening would involve increasing the size to 4"x4" (or even 6"x6") and installing a little computer fan into a frame to fit. It would be trivial to built a thermostat to turn the fan on with a thermister and a small signal transistor and a couple of resistors, or you could go the lazy route and use a prebuilt electronic thermostat.
You would, of course, need to supply a source of low-voltage to the cold frame if you decided to go this route, but it would have the benefit of being automated.
Jon
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On 2/12/2013 11:01 AM, Jon Danniken wrote:

Why not one of those solar powered vents you can buy fits the top part of a car window? The things are inexpensive enough to play around with then take apart, modify and use the parts to build whatever you like. ^_^
(Amazon.com product link shortened)
http://tinyurl.com/bc6k2go
TDD
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On 02/12/2013 09:18 AM, The Daring Dufas wrote:

(Amazon.com product link shortened)

Hey, even better, as you wouldn't need to supply power or do any designing, and since it is the sun that is causing the overheating, you'll always have sunlight for the solar cell.
There do appear to be quite a few poorly-made units out there, though; this model is more expensive, but seems to have better reviews:
(Amazon.com product link shortened)
Jon
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On 2/12/2013 12:18 PM, Jon Danniken wrote:

(Amazon.com product link shortened)

I remember finding one of those solar window vents at a thrift store and it was a dollar. I'll cruse the thrift stores for items I can take apart to experiment with and it costs very little to come up with parts for projects. ^_^
TDD
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On 02/12/2013 10:27 AM, The Daring Dufas wrote:

Heck yeah! If it's only a dollar, you can probably replace whatever the most "breakable" thing is on the unit, and reuse the rest of the kit as a project.
Jon
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On 2/13/2013 11:32 AM, Jon Danniken wrote:

One time I was looking through a bin in the toy section of items that hadn't been put out yet and I found a prosthetic breast. When the gal working there saw it she freaked and took it. I was disappointed that they wouldn't let me buy it because I wanted to hollow it out and put it over a mouse to make a "Computer Hooter". A soft GUI pointing device that is cushioned and easy on the hand. ^_^
TDD
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Try checking out automatic foundation vents....You may have to do some adapting, but they pretty much do the function that you need.
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On Monday, February 11, 2013 7:37:48 PM UTC-5, Richard Heath wrote:

apting, but they pretty much do the function that you need.
That's the sort of thing I'm after but they're a bit large for this applica tion. I was looking for something smaller. We're going to try a manual vent , one on each end to allow cross-ventilation. Since my wife goes out every morning and evening to adjust the greenhouse heater anyway, she can just ad d this task to the list.
Paul
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On 2/11/2013 12:41 PM, Pavel314 wrote:

feet long, and 3 feet deep. Sides and top will be covered with double thick polycarbonate sheeting left over from building the greenhouse a few years back. My wife's idea is to put it out in the garden and start some seeds under it to get a jump on the growing season. Garden places sell things like this but want a fortune for them.

cook and kill the growing vegetables. I can put in a small, manually operated vent, but would like to have something automatic, that opens when it gets too hot and closes when it gets cold again. I've googled around but can't find what I need. I'm looking for something about 2 inches square that I can mount though a wall of the construction.

possible that someone can give me a link to these devices, if they exist. Thanks for any help you can give.

Think about this... If you poke a chimney pipe thru the top, can you get enough convection to do the deed? Now, imagine an S-curve in the chimney so the air travels down before going up...where down may be a very small distance.
By judicious use of white and black paint, can you create a system where there's insufficient convection to get past the down section unless there's sun on the pipe?? Maybe even paint the sides of the pipe differently so the angle of the sun is part of the equation?
I know zip about photosynthesis, but can you build a system that lets visible light thru, but blocks the IR?
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