Maybe a better description would be that the desire is to: - Enable an exhaust fan if the internal temperature is greater than the=20 external temperature by about 5 degrees && if the external temperature is greater than about 70 degrees. The fuzzy numbers are there because there is no real need for high=20 accuracy. This is an exhaust fan and if it is off by a couple of=20 degrees that is OK. The use of hysteresis is to prevent hunting or=20 oscillation if the temp difference is 5 degrees or the outside=20 temperature is 70 degrees. An Arduino is overkill. What I was fishing for was an elegant=20 minimalist solution. A PIC12F67, or such, is a couple of bucks and the=20 remaining circuitry is another $15 so for less than $20 it could be=20 "done" but I just keep thinking that there has to be a better solution. -Phil- PS Presently I am leaning towards the PIC/1-wire with a 5v relay/FET=20 solution. As you mention lots of flexibility and easy to do. -------------------------------- Long ago when men cursed and beat the ground with sticks, it was called witchcraft.. Today, it's called golf. On 7/14/2013 12:43 PM, Denny Esterline wrote: > This is one of those "best" questions which really don't have a "right" > answer. > As I read your spec... > Outside temp > 70 F AND (Inside temp + 5 F) > Outside temp THEN FanOn > There are several purely analog mechanisms that come to mind. > > Problem is, your spec isn't that clear. :-) the "~70" and > "probably...hysteresis" leave this vague enough that I wouldn't want to d= o > something that doesn't have quite a bit of flexibility. That could be pot= s > and jumpers on an analog board, but personally, given this spec, I'd lean > strongly in favor of the small PIC solution. I'd also steer away from > thermistors. Given the one-off nature of the project, digital temperature > sensors will save you _much_ more time than the couple extra dollars they > cost. Make it much easier to remote mount the sensors as well. > > Frankly, this sounds like a perfect project for an Arduino. I believe you > can get all the necessary building blocks off-the-shelf. > > -Denny > > > > On Sun, Jul 14, 2013 at 7:52 AM, Phil Keller wrote: > >> A friend of mine was wondering if I could build a simple device for >> him. He is looking for something that senses the outside and inside air >> temperatures. If the outside temperature is greater than ~70 and if the >> inside temperature is 5 degrees greater than the outside temperature it >> would enable an exhaust fan. (We would probably need some hysteresis on >> the temperatures to keep the fan from oscillating ON-OFF.) >> >> I can see how this could be done easily with a small PIC, a couple of >> thermistors, a FET & relay and a 5v power supply. >> I could also do it with a few comparators, logic gates, FET & Relay, >> thermistors and a power supply. >> >> Both of these solutions seem overly complex for the problem. Does >> anyone have a more simplistically elegant solution? >> >> Thanks >> -Phil- >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .