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'Interfacing photodiodes'
1996\05\16@051153 by Keith Dowsett

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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?

Keith
==========================================================
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Snail mail:  MRC Clinical Sciences Centre, Cyclotron Unit.
                Hammersmith Hospital. London W12 0NN.

1996\05\16@074925 by Kalle Pihlajasaari

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

1996\05\17@122737 by eder Ferlemann

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Hi Keith,

> 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?
>
> Keith

how about using 4 capacitors, one 74HC4066 (4 fet switches),
(or even better a 1 of 8 analog switch - the 4xxx family has it)
, one discharge resistor tied to one side of the switches and a single
comparator behind that stuff?

Hope it helps

Frieder


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1996\05\17@140043 by Reginald Neale

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{Quote hidden}

The Texas Instruments light-to-frequency device is the TSL230. Seems to me
the price was about US$2.50 in quantity.

Reg

.....................Reg Neale.....................
"Ignorance is a renewable resource"   P.J. O'Rourke

1996\05\18@111945 by Bill Cornutt

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----------
{Quote hidden}

You can simplify your circuit some by using a pic input as
a comparator.  The pic input will compare the input signal to the
voltage it needs to switch to a one state.

pic control--photo diode--(x)--resister--pic sense/discharge--cap--ground


The pic sense/discharge line is set to input
the pic control line is made to output 5v
the cap charges and the pic counts until
the pic sense/discharge line goes positive
the count is the relative value of light hitting photo diode

the pic control is tristated, removing 5v
the pic sense/discharge is set to output ground
the cap discharges through the pic sense/discharge line

after some time, you are ready to do it again

**** you should maybe have some limiting resistence in the
**** pic sense/discharge line to keep the pic from
**** getting too much current

I would put the limiting resister between the pic and where the
cap and charging resister connect.

For more photo diodes, add more pic control---photo diodes
and hook them at the (x) point.  So each photo diode has
its own pic control and they share the resister, pic sense/discharge
and cap.

Back thirty years ago, the time it took a cap to fully (for all
intents and purposes) discharge through a resister was 5RC time.
This may still be true.  So multiply the cap (in farads)
by the limiting resister (in ohms) and get the RC time (in seconds).
Then multiply this by five for 5RC time, then double it just because.
This should give the cap enuff time to discharge through the
pic sense/discharge line when the pic sense/discharge line goes low.

I have never done this, but I have sat on a catcus.

Bill C.

1996\05\18@122635 by Todd Peterson

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At 07:23 AM 5/18/96 -0800, you wrote:

...

>I have never done this, but I have sat on a catcus.
>
>Bill C.

What a coincidence...

-Todd Peterson

1996\05\18@131619 by hoss karoly

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maybe i didn't quite catch what U need
if U tie a pd between +5 and the pin and add a pulldown resistor of
100K to GND it'll work
this means 2 parts
bye
charley

1996\05\26@143519 by Claus K|hnel

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{Quote hidden}

Keith,

to answer your question more info is needed. What light levels resp. photo
currents, what
temperature range and what timing is required.

Ciao, Claus
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1996\05\26@170125 by Mark K Sullivan

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>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?
>
>Keith

The TLS220, TLS230, and TLS235 from Texas Instruments, though pricey, are an
excellent way to measure light with high accuracy and *wide* dynamic range.
They are light to frequency converters and can easily be interfaced to the PIC
for either frequency or pulse-width measurements.  They work the way you
described your proposed solution but all elements (including the photodiode) are
on a chip.

- Mark Sullivan -

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