Window fan using a temperature differentiator

[jacob]

First, I have the idea of triggering a window fan based on rule like IndoorTemp>OutdoorTemp+X, where X is selectable, figuring that such thing is within even my electronics skills.

Then, I figured, maybe someone already came up with such a product. And indeed that has happened, but it was superexpensive ($200) and doesn't appear to be sold anymore. All I'm finding is what appears to be simple thermostat controls with a rule like IndoorTemp>X. This is useless to me. Something like OutdoorTemp<X would be partially useful though.

Perhaps my google fu is failing me?

[BRUTE]

arduino and write it?

[Lucky C]

Get an indoor thermistor to set Vi and an outdoor thermistor to set Vo.
Set up a Schmitt Trigger circuit where the input level is Vi relative to Vo.
https://en.wikipedia.org/wiki/Schmitt_trigger

So in the circuit diagrams, you'd have Vi and Vo as the comparator inputs rather than Vin and GND. Or you could offset one of the voltages with a voltage divider to get Vi>Vo+X.

Set your Vi and Vo using some cheap thermistors aka resistors that change resistance over temp. Again a simple voltage divider will work here: DC supply voltage > thermistor > fixed resistor > GND. The node between the thermistor and fixed resistor gets you Vi or Vo.

At least that's what I'm thinking would work as a simple analog solution. You can probably just do it on a breadboard so you can mod as you go, then just keep it on the breadboard since they're cheaper than having PCBs made.
https://www.digikey.com/product-detail/ ... ND/4156445

Also for actually turning the fan on/off, you can run the AC cord through a solid state relay, controlled by the output level of your comparator circuit.

[jacob]

I deduct one point(*) from brute and give ten points(**) to Lucky C.

(yes, of course I'm keeping score. Not sure why ... but I bet it'll come in handy at some point.)

(*) Complete overkill, even for a brute.
(**) I'd naively have jury-rigged a LM741 with a transistor + relay, but this seems much smarter than my experimental physics 101 level circuit design knowledge. I did use to make my own circuit boards with iron chloride, a lacquer pen, and a drill press.

[Lucky C]

If it works out well, I'd be interested in seeing the finished details!
Feel free to shoot me a message if you have any other analog design questions.

[Sclass]

+1 brute. Overkill but easy, cheap and robust. Clone arduinos can be had cheap on eBay. Use a semiconductor temperature sensor like an LM35 for a linearized temperature reading. Thermistors need coding to get straightened out.

Oh yeah, there is this cottage industry of Chinese vendors on eBay selling prebuilt boards around old ICs to do generic jobs. Like there is a cheap 555 timer board among many others. I just found a LM393 comparator board. Just search that on eBay. It is $1.80.

If you want to get tricky you can look for an op amp board (they undoubtedly sell one probably based on something old like an LM358) that you can sum/difference/offset your temperatures with.

I bought a one shot circuit complete with relays for some home automation for $1. Beat building something myself.