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I'm a mechanic Jim not an electronics engineer!

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topsoil

New Member
Hi, I'm not great at designing electronics but I can build kits with instructions and fix broken electronics. I'm fairly mechanically inclined and can design and build all sorts of cool stuff. I'm also a "small farm" farmer.

Why am I telling you all this? Because I tend to design thing that would work better if there was a circuit dedicated to it operation instead of making do with what I can find, and I'm hoping that the wonderfully electronically inclined folks here might be able to help me with such endeavors.

Here is an example:

I have a cold frame that will open and close depending on temperature. I'm using a small gear head motor to operate it and two base model household thermostats to control the motor. One therm. is set to heat the other is set to cool, and when the heat one reaches a set temp it opens the cold frame, when the cold one drops below a lower set temp the cold frame closes.

I imagine that this could be handled with a simple circuit that could be made for $10.00 instead of all the expense of the thermostats.

My question is... Is this the place to ask for this kind of help?

thanks in advance.

topsoil


p.s. definition: Cold Frame- a box with a clear cover placed over sprouting vegetables in the early spring to keep them from freezing or over heating.
 

BrownOut

Banned
You could probably make it cheap with an RTD type thermo sensor and a comparator, some drive circuitry, etc. But if you have a design already working, I'd just go with that.
 

MikeMl

Well-Known Member
Most Helpful Member
Do you need to sense temperatures at two different locations, or is it that you want two different temperatures at the same location to generate two separate outputs?
 

Hero999

Banned
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topsoil

New Member
Thank to all of you, I guess this is the right place to ask these types of questions.


MikeMl:

Temperatures are sensed at the same location with two separate outputs.

The cold frame is a box 3 feet square about 15 inches high with a glass top that opens and closes. When the temp reaches 80 degrees the top needs to open so the seedlings don't cook. When the temp drops to 60 it needs to close so the seedlings don't get too cold. The sun is the only heat source so there is no heating element or anything of that sort being operated, just a 12v gear head motor (from a car window opener) being controlled forward or reverse as needed.

BrownOut:

I don't know what a RTD type thermo sensor is, and although I have a system that is working it is clunky and relatively expensive. I need about 20 of these cold frames each controlled separately. but I need to keep the cost of each one under $50.00 or they aren't worth the expense.

Hero999:

Those are really cool, I've seen them before and considered them. My understanding is that they don't work well in clod frames though, because of how quickly the temps change in such a small area. They apparently don't respond well and tend to open and close dozens of time in an short period of time. Thanks though.

-topsoil
 

Mike_2545

Super Moderator
Thank to all of you, I guess this is the right place to ask these types of questions.




Hero999:

Those are really cool, I've seen them before and considered them. My understanding is that they don't work well in clod frames though, because of how quickly the temps change in such a small area. They apparently don't respond well and tend to open and close dozens of time in an short period of time. Thanks though.

-topsoil
I have used those in a few coldframes before, they work well, just make provisions to catch them in the event of high winds. Like a short chain or rope to limit the opening. A few were mangled because the wind blew the lids off.
 

mneary

New Member
At Lowe's they have a mechanical heat/cool thermostat Item #: 155770 for $17.14 which you probably can't beat with home made electronics.

[edit]oops I just noticed it has a heat/off/cool switch so it might only have one element :(
 
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Hero999

Banned
Here's an electronic solution.

The switches are limit switches.

Use an LM311 for U1.

It's a power guzzler with a huge standby current of >50mA.

If I wanted to power this from a battery, I'd use MOSFETs, and a micropower comparator, that way it could be run from a solar backed up battery.

You might be able to drop the battery requirement but then you need to be sure it will close the window at sunset, an additional voltage sensing comparator could be used for this but there won't be much light at sunset so you'll need a grossly overrated solar panel.
 

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