Exhaust Fan that turns On at an adjustable temperature and Off again at a lower temperature?

OK, I've looked all through the postings, and I just can't find what I'm looking for. I've built an enclosure for my pet snake, and I would like to add an exhaust fan that comes on at a certain temperature, and then shuts off again when a lower temperature is set. I don't wish to use Arduino because I'm guessing that there is a fairly simple (cheap) method to do this using a temperature sensor, a 12V DC computer fan, and maybe a potentiometer (so I can adjust what the temperature is held at). Although I have the ability to solder, and I can read circuit diagrams....I lack the basic knowledge to create them for myself. I already did an internet search, and didn't come up with what I was looking for either. I'd rather not spend $60-80 for a store bought unit. Any help would be appreciated.?

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mathews8 years ago
You will require: thermistor, resistors, 2 potentiometers, 2 op-amps, NAND gates, power transistor Use a thermistor for the temperature sensor as part of a potential divider with a resistor. Connect the midpoint of the potential divider to one op-amp -ve input and to the +ve input of the other gate. Use the potentiometers on their own to create two more potential dividers and feed each of these to the remaining op-amp inputs. Create a SR-Latch with the NAND gates, and connect an op-amp to each input. Then connect the power transistor to an output of the SR-Latch and use it to power the motor (or a relay to power the fan). All the other information you will need will be on the internet, and the parts list should be no more that £5.
orksecurity8 years ago
Your stated problem problem is that you want some hysteresis in the behavior -- you want it to come on above temperature x, then stay on until it drops below temperature y, where y
Re-design8 years ago
Here's what you need. It's listed for 110v but it's just a switch you can use it on less if you want. It requires no power for it's operation, just power to the fan. I use one in my dog kennel in the summer to keep the air inside from getting too hot. The dial is not 100% accurate but once you get it set where you want it it's great.

If you really need one for his safety, why design it yourself.
mvingtargt (author)  Re-design8 years ago
It's not a safety concern, and I haven't moved her into the "new" enclosure yet. It shouldn't get hot enough in there to harm her, but there are optimum temperatures for comfortability and longevity. She's already 11 years old, but I'd like to get at least another 9 years of company. :) I couldn't find a link to the controler you were refering to though. I'd like to check it out to see if it would work for my application if you wouldn't mind posting it again. Thanks.
mvingtargt (author) 8 years ago
This is the type of answer that I think I'm looking for ;). Unfortunately, even after thouroghly reading p. 6 of the pdf.....I still have no idea what they're talking about. :p Therein lies the root of my problem. I can fabricate an IC, but my technical knowledge of them is....low (or nonexistant if you prefer). Could someone please help me by drawing me a circuit diagram, and explaining in plain (AKA dumb person) english how to program it. Thanks
lemonie8 years ago
The usual deal is to have a thermostatically-controlled heater. If you've got a lamp in there the snake should be able to move away if it's warmed-up enough. Is there a reason why you're thinking of a fan? L
mvingtargt (author)  lemonie8 years ago
Her current setup has that kind of set up. But it's an open cage design. The new enclosure is more "aquarium" style, and I don't want to take any chances with lack of air circulation, or with the entire enclosure overheating. Basically, it's just a safety measure.
How much ventilation do you have? I'm thinking that if you need forced airflow switching it by temperature isn't logical. You'd want a timed ventilation or low continuous wouldn't you? No you don't want an overheat, but I'd guess you've got a lamp in there which would be on a timer / on/off and maybe it's that that needs the switching rather than a fan? L
Overkill?
idk. not ime
All you need is a cheap thermostat and a PC power supply.