Thermostat to control 12v DC fans.

[j3r0m3]

Hey people,

This is my first post after a search for an electronics forum. I'm quite the newbie, but do have an edge for electronics and have learnt quite a lot since flying RC helicopters and using Lipo technology.

I'm growing tomatoes in a custom built grow box with a CFL 250w light.

I need some kind of temp/fan control to keep the temp consistent.

I don't know where to start with thermostats? Anyone be able to give me some advice on what to start looking for.

I'd like an LCD readout with a thermostat, to control 3 12v fans, be able to set a min/max temperature - so when it reaches the max temp it will start the fans and then when the temp drops to min it will cut them off.

Does this sound realistic?

Thanks in advance

 

[Hero999]

What's the point in using fans? They're just going to blow the warm air round and round.

 

[j3r0m3]

The grow box has three fans:
One on the left side at the bottom to draw air in and two on the right side at the top to draw air out. So it creates a flow and removes the rising hot air whilst bringing in fresh colder air...

What else would I use besides fans to control the temp? I'm not too fussed if its 100% accurate, but would be a nice little project to keep me busy and help the grow.

 

[Andrew Leigh]

Hi,

Sounds like it could end up being inefficient, invest 250W's worth of power into the grow box and then when too hot then suck the heat out through fans. Unless of course you plan on switching the light on and off to regulate the heat?

You don't say what temperature you want to be at?

What about making variable shutters in place of the fans then having a LV element in the base of the box. Natural convection will occur assuming you need fresh air and your element will turn on and off as required to stabilise the temperature.

A thought .....

Andrew

 

[bychon]

Programmable thermostats with LCD displays are available. They are sold for use on central air conditioners. You will have to provide them with a 24 VAC transformer and have them control 24 VAC relays. One limitation is that they don't have adjustable span...that is, they start where you set them, and quit 2 degrees later. If that will satisfy you, buy one.

 

[Reloadron]

Hey people,

This is my first post after a search for an electronics forum. I'm quite the newbie, but do have an edge for electronics and have learnt quite a lot since flying RC helicopters and using Lipo technology.

I'm growing tomatoes in a custom built grow box with a CFL 250w light.

I need some kind of temp/fan control to keep the temp consistent.

I don't know where to start with thermostats? Anyone be able to give me some advice on what to start looking for.

I'd like an LCD readout with a thermostat, to control 3 12v fans, be able to set a min/max temperature - so when it reaches the max temp it will start the fans and then when the temp drops to min it will cut them off.

Does this sound realistic?

Thanks in advance

This is one of those projects that can be as simple or complex as you want to build into it. Based on your initial post I'll tell you what I would do.

I would just use a simple thermistor coupled with a comparator like a LM339. I would go find a reasonably priced indoor/outdoor little digital thermometer just to monitor temperature inside and outside the grow box. Taking the outdoor sensor bulb and shoving it in the box. Then adjust your comparator circuit to turn on your fans at a given temperature reading based on your digital thermometer. Add some hysteresis of a few degrees which you can experiment with. Let the comparator drive a MOSFET like an IRF 510 that can easily drive several small fans with 12 volt power.

Yes, you could find a PIC and program it and add a display and that would work just fine. My thought was keep it simple and something that will do what you want.

Just My Take
Ron

 

[Hero999]

It's pretty easy to make a thermostat from an LM311, which has the advantage of being able to directly drive a small relay.

Craig's Thermostat Circuits
LM311 Light/dark detector/thermostat

 

[bryan]

Huh, tomatoes, are you sure...

Hey people,

I'm growing tomatoes in a custom built grow box with a CFL 250w light.

 

[BrownOut]

Hehe... Take a look at this. That might work for you.

 

[j3r0m3]

Huh, tomatoes, are you sure...

I'm sure they're tomatoes, they smell a bit weird though...

 

[BrownOut]

Maybe it's tomacco! https://www.tomatocasual.com/2008/04/22/the-tomacco-one-simpsons-episode-comes-to-life/

 

[Dragon Tamer]

It sounds like the light is used for photosythesis and not nescessarily to heat the box. If you have the box inside, the heat is not necisarily a bad thing to have, however if you exceed a certain temp it will hurt the tommato plant. The fans are a good idea, and could easily be done with something like an op-amp like brownout suggested, or you can use a uC with PWM wich would be the best solution if you want a display to work with the circuit.

 

[j3r0m3]

Heat is an issue as the room in which it is located has four servers and a few other bits that produce quite a bit of heat, we've no Air-Con in there and just use oscillating fans to blow the air out the windows (or around haha).

I've been searching and I've found **broken link removed** which is perfect and quite a good price... Taking the fun out of it though :S

Would I be better off buying the above or continue to consider making it myself, and learning some new skills... ARGH Decisions!

 

[Dragon Tamer]

I would consider making it yourself because then you can make exactly what you want and it will be easier to get the design to work with what it needs to, in this case the fans. That model that you posted looks old and I doubt that it will have directions on how to connect it properly to the fans to make them work.

I would suggest just making an analog fan controller just because it is simpler and won't take as much time. If you go with the digital thermostat then that will require a microcontroller that needs to be programed to function properly.