Thanks Neil,

Doing mostly software work for the last decade... Why does the ADC input
see the two resistances in the divider in parallel as its input
impedance? Why isn't it just the LDR that set this? Sorry if this is a
very dumb question...

I plan to use the 12F675 and you are right there is a 10K recommended
impedance of the analog voltage source. I', glad I asked the question.
Nothing is ever as simple as it first look is it!

/Ake

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Fr=E5n: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]F=F6r PicDude
Skickat: den 18 april 2004 20:31
Till: PICLIST@MITVMA.MIT.EDU
=C4mne: Re: [EE:]Light detector


There is a 10k maximum input impedance requirement for the 16F PIC's,
which
you will exceed with this setup.   IIRC, it's about 1/4 of that for the
18F
series.

You should be able to do this successfully with a lower-resistance LDR.
I
picked one up from digikey ages ago with a range of 4k - 11k ohms, which
when
combined with the fixed resistor in the voltage divider, will let the
PIC
input see an impedance in it's required range.  (The PIC input sees an
impedance of the parallel combination of both resistors in the divider).

Alternatively, if you have an LDR with a slightly higher resistance than
will
meet the PIC's input impedance specs, you could try paralleling the LDR
with
another resistor, but you'll lose some linearity (which should not
really
matter if all you need is a threshold).

Cheers,
-Neil.



On Sunday 18 April 2004 01:05 pm, Ake Hedman scribbled:
> I am in need for a low cost light detector that has a use settable
> trigger point for the light/dark threshold. My first thought has been
to
> just set up a voltage divider in the following way.
>
>         +5V
>
>          | | 500K
>           |
>           |--------- PIC ADC
>           |
>          | | LDR
>
>          ---
>
> The LDR has >500K resistance in darkness and 3K-20K at 10 Lux.
>
> One could use a PIC output pin to feed the divider instead of the +5V
> and just activate this when a measurement is needed.
>
> Is there any drawbacks with such a scenario? Is there a better way to
do
> this in a low cost way?
>
> /Ake
>
>  ---
> Ake Hedman (YAP - Yet Another Programmer)
> eurosource, Brattbergav=E4gen 17, 820 50 LOS, Sweden
> Phone: 46 657 413430 Cellular: 46 730 533146
> Company home: http://www.eurosource.se      Kryddor/Te/Kaffe:
> http://www.brattberg.com
> Personal homepage: http://www.eurosource.se/akhe

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