Unlike most greenhouse gardeners that use theirs year-round, mine will only be used for about 3 months out of the year. Doing so, keeps me from having to worry about all of the variables that have to be addressed - like humidity, cooling, heating, etc. Nope, this one is gonna be used differently. To fit my application, it only needs to do one thing in the early part of the year - protect the tomato seedlings after they outgrow the propagation chamber next April. BTW, the hardening - off characteristics will be a plus, too. Also, it may get used for rooting cuttings and such, but I haven't put much thought into that, yet. Anyway, based on my climate and intended use, I only need one thing. Ventilation.
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!