I forgot to add that the SS Relay box should be labeled with something like "Warning -- Turns on Randomly." If you build this box and can demonstrate that it can turn on, and more uncertainly, turn off the fans, we can control them with either an analog or digital device. But, if all you want is the simple operation you delineate, say, turn the fans when the outside ambient is 5C or so cooler than the house, back off when the differential falls, say to 2C, simple, cheap and highly reliable op-amps will work. If you decide you want to data-log later, we can monitor the op amps with a Von-Neumann type computer board, or supplant them completely with one.
There is something elegant about avoiding such a computer in this application. In 50's Sci-Fi they may have refereed to this overall circuit as an example of a homeostat.
Actually it's kind of hard to justify analogue control systems at all any more. As soon as you need a mixture of logic, thresholds and time delays it's almost always cheaper and more relaible to use a micro. (I'm excluding "junk box" sourcing here). The nice thing about micros is that you can so easily adjust thresholds and have long time constants reliably at low cost. A (32 bit) micro with on chip ADC, timers etc etc will cost less than 50p in decent numbers - you can't get one reliable >5 minute time constant for that in analogue.
For the experimental one off the micro pays back because it's so easy to connect it to a PC or tablet for monitoring during development - and when you realise that actually you need an extra input or two and some more logic there's no trouble in adding it.
In about 1964 my father installed a central heating system with 100% mechanical "compensator" type controller - measured the outside temperature and adjusted the radiator water temperature accordingly - there was a copper bulb outside with a long flexible copper pipe to the controlelr/valve assembly near the boiler - elegant perhaps, with lot's of lovely machined metal parts, but not the way to do it now.
Actually it's kind of hard to justify analogue control systems at all any more. As soon as you need a mixture of logic, thresholds and time delays it's almost always cheaper and more relaible to use a micro. (I'm excluding "junk box" sourcing here). The nice thing about micros is that you can so easily adjust thresholds and have long time constants reliably at low cost. A (32 bit) micro with on chip ADC, timers etc etc will cost less than 50p in decent numbers - you can't get one reliable >5 minute time constant for that in analogue.
For the experimental one off the micro pays back because it's so easy to connect it to a PC or tablet for monitoring during development - and when you realise that actually you need an extra input or two and some more logic there's no trouble in adding it.
In about 1964 my father installed a central heating system with 100% mechanical "compensator" type controller - measured the outside temperature and adjusted the radiator water temperature accordingly - there was a copper bulb outside with a long flexible copper pipe to the controlelr/valve assembly near the boiler - elegant perhaps, with lot's of lovely machined metal parts, but not the way to do it now.
