Why would you want to know where the dial was set after a power
failure ! :-)
You wanna start a fire?
For very short power outages it is perhaps OK to carry such settings
through but after any protracted period (seconds?, minutes?) the
failsafe thing to do is to shut it down. Users may complain but ... .
Of course, this is what a mechanical dial does.

I have a product which has a similar speed setting device (here a
faster / slower self centering toggle switch) and I save the speed to
eeprom after each change. Not a problem over the design life in my
application. If the oven was to be adjusted with a frequency that
would overtax the EPROM's storage life if the setting was stored
after every change you could wait a few seconds after the last change
and then save it. This gives you 27 temperature adjustments per day
for 10 years with 100,000 writes to 1 eerom location. Using 10
locations would handle most tasks.




Russell McMahon

From: Tom Handley <thandley@TELEPORT.COM>

>   Harold, thanks. As far as a quadrature encoder, I wanted to know
the
>absolute position. For example, what if there was a power failure
when the
>oven was set to 300F. How would I know where the dial was set? I
could use
>battery backup but I want to keep this simple and fairly `bullet
proof'. I
>like Lawrence's approach using a Pot.
>
>   I also like his PID method which should be easy to implement with
some
>tuning. The proof is in the pudding and it sounds like his results
are
>pretty tasty ;-)
>
>   - Tom