Tom Handley <PICLIST@MITVMA.MIT.EDU> wrote:

> Andy, I understand your points but if a user has to adjust the
> display, I should think a pot would be more intuitive.

    Tom:

    If the user can see anything at all on the screen, you're
    right... But if the temperature's so high that the display is
    completely blank, he's more likely to assume the thing's broken
    than to go looking for a contrast adjustment.

    I tested verious contrast-setting methods on about 100 people,
    and ended up using the contrast-cycling-on-startup method
    because it was the ONLY one with a 100% success rate.

> You can scale the pot value with a resistor or two to ensure that
> there is a minimum display. Temperature compensation can easily be
> added for any method of adjustment.

    You're right, of course, but all those components cost money,
    board space, and power... This was a small, solar-powered
    consumer product for which those three quantities had to be
    absolutely minimized.

> For water-tight applications you are dealing with hermetically
> sealed components be they switches/keys or pots. If you already
> have the switches/keys and available program memory then I can see
> a software approach. It just seems like a lot of overhead for such
> a simple task.

    Since we were using a PIC16C74 (which has a hardware PWM
    generator), there was essentially no software overhead.

    The switches were just membranes, sealed water-tight behind a
    plastic faceplate.  To add a sealed pot would have been very
    costly.

    -Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499