Yep, even when the outside temperature is 30-40 degrees, the temperature inside my little greenhouse can quickly reach 90-100 on a sunny day. This would require a small fan to remove the heat, and if it was thermostatically controlled - that would be great, too!
So, here's what I came up with : A small 12v dc fan that required less than 200 milliamps of current, a cheap temperature controller, and a power source that could be used anywhere on the property. Since I already had a gel-filled battery from an electric scooter (12v, 35 amp hour capacity) and a small fan (cpu fan, 12v/190 milliamps) from a junk pc, all I would need is the means for controlling the circuit. With only $6 worth of electronic components, the controller was assembled from the following diagram :
The way this controller works, (without getting too technical) is R1's (thermistor) resistance value changes as the surrounding temperature increases/decreases. As the thermistor is heated up, it's resistance decreases until the threshold voltage determined by R4 (8,200 ohm resistor) is reached. At this time, the output of the operational amplifier (IC-1) goes from high to low, causing transistor Q1 to be saturated - allowing current to flow through the emitter to the collector. Since the fan already has a negative signal supplied to it, Q1 basically acts like a switch - providing the positive signal to energize the fan motor. Does that make sense?
R4 is a fixed resistor, and should make the fan come on when the temperature reaches 88 degrees, but i'll probably tinker with some other resistance values to get it to come on at maybe 80.
The battery should be able to operate the little fan for 184 hours straight before going dead, which is over a week. Based on daytime/nighttime temperatures, the battery should last for at least 3 weeks before it needs charging again. That's not too bad....
I figure the fan is rated for about 30 cfm, which will provide about 2 complete air changes per minute within the greenhouse. That should keep it from getting too hot inside, as well as control moisture as it collects on the inside of the plastic. Now when you see it, you'll understand the reasoning for the design concept. Hehe.
Told ya it was gonna be weird. Ha!
EG
9 comments:
EG,
Man you are a regular "Macgyver". I'm sure this project will be outstanding.
That's so crazily awesome it's amazing. I'm glued to my chair for the next installment.
Is this English? There are too may letters and numbers mixed together. HA! I guess I'll just have to wait for the pictures.
Sounds like quite the fan set up! They certainly can heat up fast in the sun. I had romaine growing in my cold frame near the end of Feb, when the temp was in high 20's outside.
Ok now that my head has stopped spinning... I want one! :-)
My greenhouse is only up from January to early April. I finally bought a small fan over the summer to use this next season. I guess I could set it on a timer.
I wouldn't call it weird, just a circuit diagram and explanation. But then again I understood it about as much as when my daughter showed me her senior paper on quantum computing. They both assume knowledge that I don't have.
That's a lot of work for a little fresh air. I'll be using my pvc hoophouse again starting in late January. Like last year, I'll have to monitor the weather and open one end if it's going to be too warm. I only killed one container of seedlings last year. The sun in early spring does a lot of damage too. I use a white bed sheet as a shade cloth to harden off fresh seedlings started in the basement.
Good luck. I'm already thinking about tomatoes too.
eekaliving - I've just been exposed to alot of technical information for most of my life. No big deal..
ribbit - really? I'm glad you like it so far. This should be a good greenhouse when finished.
Thomas - yes, it's english - just sophisticated english.
Dan - they sure can. It's amazing what a little sunlight can do.
Liisa - Sounds like you need a catholic priest, instead of a greenhouse. Ha! I bet a timer would do pretty good for your climate.
Daphne - Quantum computing? Ouch! That made my head hurt...hehe
Tom - in reality, it's not alot of work at all. I always like to use everything at my disposal to create the most advanced setup possible. Good idea on shadecloth to help control the temperature.
I'm with Daphne. I know what it was and what it will do, I just couldn't talk my way through the diagram or repurpose batteries or do much on my own. I'd probably end up using some kind of balloon thingy that would expand with temperature to open a window!
But I'm not gonna do greenhouse, no how no way. Enjoy!
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