myke predko wrote:
>
> Sorry, I just sent this note to just Paul...
>
> >D. L. Davis wrote:
> [snip]
> >>
> >> Everything works great with one big exception.... the PIC is
 extraordinarily
> >> sensitive to light!
> [snip]
> >
> >This doesn't surprise me at all.  Silicon is photoconductive, regardless
> >of whether you are talking about junctions or MOS capacitors (i.e.,
> >gates).  The degree to which a given function is affected by light would
> >depend on several factors:
> [snip]
> >1. Metallization: the Al layer is opaque.
> >2. Impedance: lower impedance circuits are less affected by leakage
> >3. Location: circuits nearer the center of the die have a wider field of
> >view
> >
> >Whether the A/D is the charge balancing type, current sources, or
> >resistor ladders, it's bound to have some relatively high impedance
> >sections.
> [snip]
> >Paul Mathews, consulting engineer
> >AEngineering Co.
> >optoeng@whidbey.com
> >non-contact sensing and optoelectronics specialists
>
> Does this mean that the PIC (I presume 16C7x) could be used as a light
> meter?  ie put on a constant voltage (ie from a voltage divider), leave the
> window open and output what the ambient light is?

I try to avoid designing things that depend on unspecified parameters,
so I hesitate to get into this.  Since many of the PICLISTers seem to be
in love with integrated (i.e., expensive but easy to apply) solutions, I
would point them toward the Burr Brown and TI light metering chips with
pulse outputs.  Or, if you're really designing low cost products, a
reverse-biased photodiode charging a capacitor works well.  You can also
use an LED as a photodetector, so a clever designer could use an
indicator (turn it off, too briefly for human eye response, and measure
reverse current).


>
> What frequencies would this be best for?

Depending on junction depths, surface coatings, and so on, most Si
structures have peak responsivity at 950nm, but sensitivity extends from
250nm or so to 1100nm.

 I presume you would want to put in
> a ultra-violet filter (any ideas of what to use)?
>

Ordinary window glass blocks most UV. (Which is why your houseplants get
tall and spindly even next to a window: greenhouses use special glass.)

--

Paul Mathews, consulting engineer
AEngineering Co.
optoeng@whidbey.com
non-contact sensing and optoelectronics specialists