I have gotten microcontroller evaluation kit from Texas instruments for $4. Software is free.
$10 of bits and pieces from discount electronic stores....
We all have to buy the probe, this is about the controller.
I can also use left over IO for almost anything.
Internal clock I can auto shut off the pot in case I'm forgetful.
Oh the options are endless for a tinkerer.
I was thinking about the controller as well, not the I/O's.
(controlling temp, pressure, flow etc, is basic stuff. Only the external parameters change)
If the load to be controlled cannot be handled by the output on the controller, yes, you are right
an external relay capable of switching the load (whatever that may be)
is required.
If the input temperature needs to be 1000F, again, no problem, choose a sensor with
a different temp. range.
I will not go into other things that may or may not be needed like an enclosure, fuses, terminals, switches etc,
To make things easy for the average person, using an off-the-shelf product, now that you mentioned the parameters,
have a look at the following:
http://www.automationdirect.com/adc...es_(with_Lead_Wire_Transition)/THMK-T06L06-02
http://www.automationdirect.com/adc...n-Style,_40A_(AD-PR40_Series)/AD-PR40-1C-120A
And, yes, you are right, a tinkerer can do all kinds of wild and wonderful things, I love tinkering myself, but
most of the time I'm building equipment for industrial customers. They have to be built to code and field/shop approved
by ESSA or whatever standard the client requires.
Most people would want something easy to get, assemble and use. Google/used/scrap yard/discount electronics
is the answer if you want cheaper stuff and
know what to look for.
BTW, if you tinker with that stuff, you would LOVE my fun room, it's packed to the rafters with that kind of crap.
Cheers