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'[PIC]: How to connect photocell to pic a-d'
2003\04\15@222635 by Brian L

picon face
Hello,

I am somewhat new to pics and would appreciate some help with using a
photocell on a pic A-D port. The range of light on the photocell will not
cover the full resistance range of the photocell (whichever one I use), so I
don't know how to interface this with a pic to provide maximum range.
Assuming that the resistance varies from R1 (lowest ohms) to R2 (highest
ohms), I can set this up in a voltage divider with the fixed resistance
being R0 with +5V to R0 to photocell to Gnd, and Vout coming from the point
where R0 connects with the photocell.  Now, V1 = 5V x (R1/(R1+R0)) and V2 =
5V x (R2/(R2+R0)).  For some test values I've calculated, this leaves me
with V2-V1 being around 1V-2V. Is there something other than a voltage
divider I should use for this? I would prefer to keep the vrefs at +5V and
ground since I will probably use other things on other A-D channels.

TIA
Brian



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2003\04\16@020216 by Jinx

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> 5V x (R2/(R2+R0)).  For some test values I've calculated, this
> leaves me with V2-V1 being around 1V-2V. Is there something
> other than a voltage divider I should use for this? I would prefer
> to keep the vrefs at +5V and ground since I will probably use
> other things on other A-D channels.

An A2D pin has a maximum recommended signal source impedance
of 10k. You don't mention which photocell you're using but if it is more
than 10k at any time then you'd need to buffer it

Perhaps this way, using a dual op-amp. One 1/2 to produce a DC
offset voltage and the other 1/2 as a summer amp to add and amplify
the DC offset and the photocell output. By making the DC offset and
gain adjustable you can make the photocell's output span between 0
and 5V at the A2D input

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2003\04\16@083233 by Olin Lathrop

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> I am somewhat new to pics and would appreciate some help with using a
> photocell on a pic A-D port. The range of light on the photocell will
> not cover the full resistance range of the photocell (whichever one I
> use),

Photocells convert light to electrical energy, and don't operate on
varying resistance.  Your best bet for measuring light intensity is to
measure the photocell's short circuit current.  This can be done with an
opamp and feedback resistor.


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2003\04\16@091117 by Dennis J. Murray

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You're right about photocells not operating on resistance unless he's using
Cadmium Sulfide cells, Olin (Heaven forbid)! ;-)


{Original Message removed}

2003\04\16@093641 by Dale Botkin

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On Wed, 16 Apr 2003, Dennis J. Murray wrote:

> You're right about photocells not operating on resistance unless he's using
> Cadmium Sulfide cells, Olin (Heaven forbid)! ;-)

What's wrong with CdS cells?  I've been using one for a while as the
daylight detector for my yard light timer, works fine.  I've seen them run
those dumb dusk-to-dawn outdoor lights, the ones that look liks gas
lights, for many years.

Dale
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2003\04\16@113521 by Dennis J. Murray
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Personally, I've had a higher failure rate in extreme conditions with CdS
cells than I've had with semiconducter types.  Logic says that shouldn't be,
since it doesn't get any simpler than a CdS cell.

May have been coincidence, but it tends to color my judgement.

You want to use Cds?  Have at it - no offense meant.

Dennis

{Original Message removed}

2003\04\16@121651 by Dale Botkin

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On Wed, 16 Apr 2003, Dennis J. Murray wrote:

> You want to use Cds?  Have at it - no offense meant.

None taken, I was just wondering.  It's the first time I've used one, but
they have been around for a long time and I've seen 'em forever.  So far
it's working fine under normal outdoor conditions, but of course it's out
of direct rain & sunlight.

Dale
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2003\04\16@125911 by Larry Bradley

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I've used a CDS photocell from radio Shaft as a "dawn and dusk" detector
for the anchor light on my sailboat for 12 years now. It is inside a
window, not exposed to the elements. Works like a charm.

Larry

At 11:16 AM 4/16/2003 -0500, you wrote:
{Quote hidden}

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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2003\04\16@142234 by Herbert Graf

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> On Wed, 16 Apr 2003, Dennis J. Murray wrote:
>
> > You want to use Cds?  Have at it - no offense meant.
>
> None taken, I was just wondering.  It's the first time I've used one, but
> they have been around for a long time and I've seen 'em forever.  So far
> it's working fine under normal outdoor conditions, but of course it's out
> of direct rain & sunlight.

       I think the biggest problem with Cds cells is part to part variation. The
stability of a single part is quite good, but the variability between parts
can be quite bad. Especially if you get parts from different production
runs. TTYL

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2003\04\16@143216 by Dwayne Reid

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At 07:48 AM 4/16/03 -0400, Olin Lathrop wrote:

>Photocells convert light to electrical energy, and don't operate on
>varying resistance.

The problem is that the term "photocell" refers to at least 3 different
sensors, each with radically different characteristics.  These are: Light
Dependent Resistors (LDRs), solar cells, vacuum tube photocell.

(side note: The last time I saw one of those vacuum tube photocells when I
was a teenager - I do remember that it wasn't a photo-multiplier - the tube
had 2 elements within: curved plate and a vertical rod up the center.  It
was so long ago that I can't remember if the curved plate was anode or
cathode.)

Anyways, what Olin is referring to could be either photo-diodes or solar
cells.

However, other types of photocell (CdS) *do* change resistance with changes
in light level.  Also note that a photo-transistor can give the illusion of
a resistance changing value in response to changes in illumination.

dwayne

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