> D. L. Davis wrote:
>
> I built an altimeter with a PIC14000 which I have in bench test now.  I am
> using the 16 bit A/D of the PIC14000 to measure voltage from an air pressure
> sensor.  It is used to control an autopilot, and the PIC runs the autopilot
> servo too.
>
> Everything works great with one big exception.... the PIC is extraordinarily
> sensitive to light!  The logic all functions fine and is unaffected by
> light, but the A/D measurement is strongly influenced by ambient light on
> the chip.  I'm using a JW part now, with tape over the window.  But even
> with tape over the window, I can effect the A/D count by an order of
> magnitude just by shining a flashlight on the PIC from across the room.  I
> have replaced the little red sticker tape that came with the part from
> Microchip with double layers of black tape, which improves the A/D
> stability, but only to about 6 or 7 bits, nowhere close to the 16 bit
> resolution.  If I control ambient light carefully, it still moves around
> randomly in the least significant 9 or 10 bits.
>
> (rest deleted for brevity)

Keep in mind that if you are using a silicon based pressure sensor,
it will also be sensitive to light. I have seen several altimiters
built for model rockets which had to use light sheilding on the
pressure sensor.

Just one more thing to watch out for!

Jerry