Trying to figure out what type of controller, temperature sensors would be needed. Wanting to operate a fan with a temperature differential between a greenhouse (GH) and attached building monitoring air temp in each.
When GH gets above 85F and attached building drops below 85F, a 110V, 4.7A, 2010 CFM inline fan circuit engages.
Separate fan circuits parameters: Also want to monitor ground temp maybe with K thermistors?? for an underground fan circuit.
Also if GH is above 85F and heat is not needed in either attached building or underground, fan exhausts to outdoors.
Will be using 3 different fans for different circuits as I have not located an automatic diverter for the application.

 

hi dan,
From your description I would expect that you want to log/record the control times and data.??
not just monitor them.
Eric

 

hi dan,
From your description I would expect that you want to log/record the control times and data.??
not just monitor them.
Eric

Yes that would be great as eventually we would like to write physics papers on the systems (I am not the physicist)

 

I would use LM35 temperature sensors, which produce a voltage proportional to temperature, and then comparators such as LM339 to execute the "logic" of your system. Start by mapping out the complete logic; how many temperatures to measure and defining how each fan behaves under every set of temperatures. (You'll need that anyway, no matter what circuitry you use.)

The comparator outputs can control MOSFETs, which in turn can operate relays to send power to your fans. The circuitry can all run on 5-12V DC, but the relay can then control the AC to the fan motors.

 

Yes that would be great as eventually we would like to write physics papers on the systems (I am not the physicist)

I thought you would say no to that question!

In that case, I'd look at a LabJack U3HV for data acquisition and control. I have one and love it. It connects to a computer by USB and you can control it using Excel or whatever programming tool you like. Of course there are other options.

 

To simplify some you could use the temperature comparators to directly drive SSRs (solid state relays) which then control the AC fan power.

 

Yes that would be great as eventually we would like to write physics papers on the systems (I am not the physicist)

In that case I would consider the LabJack U3HV as wayneh's post #5.
E
http://labjack.com/catalog/u3-hv

 

Logging data is not my strong suit, but I have a thermostat.
This chip provides the comparison voltage so you don't have to build it externally.
Still, an LM35 and an op-amp is a perfectly good circuit.

http://forum.allaboutcircuits.com/blog/lm723-as-a-thermostat.532/