James wrote:
Many thanks for the formula, I just couldnt get my head round a way to shift
the values, but as you say it is a gain change which again gives me
something else to search for on google.

Is there a formula to calculate R and GR values? I assume the circuit is a
noninverting amplifer.

Thanks again


Yes- but I won't write a small book about everything you need to know.
YOU need to tell me:

1) range of exterior temperatures over which you want this thing to
work- and will this always be less than interior temperature;

2) power supplies you have available if any- line voltage;

3) means of controlling heater- is this a logic input to a PLC or a
relay that is to be controlled or what is it;

4) the distance to the temperature sensors- how connected?

5) basic allowable error- 10%, 5%, 1%?

- a brief description of what you're working with and what you envisage
for this little controller form factor/package size/cost- in other words.





That is not a "shift"- it is a gain change. Your equations are
Tint=G*(20oC-Text)+20oC where Tint is interior temperature cutoff and
Text is exterior temperature measurement. Then G=3 in the first case,
and G=1 in the second. Rewriting the equation as Tint=(G+1)*20oC-G*Text
suggests- not to be taken literally:

Please view in a fixed-width font such as Courier.



|\
REF V ----------|+\
20o | ----+---V (setpoint)
+-|-/ | T
| |/ | int
| |
V --/\/\--+---/\/\--+
T R GR
ext