Hi,

>    has anyone got a neat way to interface photodiodes to a pic? Ideally I
> would like to hang four of them on a 16C84 and still have some pins left to
> display the results.
>
> So far the best I have come up with is to use the photodiodes as current
> sources to charge a capacitor. Use a low leakage FETs to discharge the
> capacitors and  comparators to signal when the voltage exceeds a preset level.
>
> This involves quite a lot of components (capacitors, quad comparator, FETS,
> voltage reference) and isn't particularly easy  to set up. Has anyone found
> a neater and more linear solution?

Texas Instruments make a light to frequency chip. (model TL????)

Burr-Brown make a light to voltage chip (very slik and high speed response
of about 50kHz over many decades of input) (model OPT202, 209, 301) I
have used these to detect the fast weak flash of light inside a camera
that could not be synchronised otherwise (aerial photo work).

Burr brown make a Photo current to Digital chip with 20 bit accuracy
and 15 kHz sampling rate (model DDC101) just add a photo diode of
your favourite size.

None of the above are real budget items though.

Cheers
--
Kalle Pihlajasaari     kalle@data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751