John, thanks for the feedback.

re: Rotary encoder with absolute binary output.
>good idea (simpler is better.)  Make a big lighted switch which functions
>as a on/off button -- you dont want Mom to try turn off the oven by
>setting the dial *almost* off!

   Saftey is a big concern but I want to simulate, even use, the same dial
and markings on the stove that she is use to. I plan on adding a mechanical
thermal switch if the temperature gets too high. The rotary switch will have
detents with a range from Off, 150 to 500 degrees F, and Broil.

re: PWM
>I'd suggest ditching PWM.  As I remember, heating/cooling elements (oven,
>air conditioning, fan) are more efficient in on/off mode.  IE, for your
>oven, turn on until temp probe reads somewhat above your set point.  Then,
>wait for it to get somwhat cooler than that point, at which you'll turn it
>on again.

   My thought was to get up to the selected temperature as quickly as
possible which is the way it works now. Once I get within range, then I
start to adjust the duty-cycle. I use to design industrial energy management
systems so I have some experience here. These are standard resistance
heating elements with the top element used for Broil as you mentioned. The
elements have a significant thermal mass so it's possible to save energy by
adjusting the duty-cycle. This will have to be `tuned' in the PID loop.

>hope it goes well!

   Thanks, but this is all Lawrence's fault ;-). I've had this on the `back
burner' but his recent discussion of PID control and thermistors brought me
out of the `closet'.

   - Tom

At 04:02 PM 5/5/99 -0400, John Mitchell wrote:
>On Wed, 5 May 1999, Tom Handley wrote:
>
>
>>    The practical side of the user-interface has to deal with my mother...
>> I've selected a rotary encoder with an absolute binary output. This would
>> provide a range from 150F to 500F in 25F increments with 0x00 = Off...
>
>good idea (simpler is better.)  Make a big lighted switch which functions
>as a on/off button -- you dont want Mom to try turn off the oven by
>setting the dial *almost* off!
>
>>    To control the heating element, I was considering simple On/Off as
>> opposed to PWM. I want to use standard logic/AC solid-state relays.
>> [...] I'm still interested in using PWM.
>
>I'd suggest ditching PWM.  As I remember, heating/cooling elements (oven,
>air conditioning, fan) are more efficient in on/off mode.  IE, for your
>oven, turn on until temp probe reads somewhat above your set point.  Then,
>wait for it to get somwhat cooler than that point, at which you'll turn it
>on again.
>
>My oven has two elements, one at the top and bottom of the stove.  The top
>one only comes on in Preheating and Broiling mode, just keeping things hot
>is the duty of the lower one.  Dunno about your stove, ask your mom.
>
>
>hope it goes well!
>
>- j
>
>
>John Mitchell         "Beer is living proof that God loves us
>Magnet Interactive    and wants to see us happy"
>johnm@magnet.com      - Benjamin Franklin