Art,

I just plan to use it for ambient light levels. Actually I'm only
interested in if there is lights on (sun, lamps) or if its dark. I need
the threshold to be user settable as "dark" is a subjective thing. The
thought is that a node send out a message (VSCP) when the transition
occurs (with hysteresis of course). This message can be used by other
nodes to turn on/off lamps or do other things.

I found a Light to voltage sensor, TSL250 from TAOS, that was rather low
cost and I will probably use that instead of the LDR.

For those interested I why I need this:
VSCP, stands for Very Simple Control Protocol and is a very, *very*
simple higher level protocol over CAN/TCP/UDP and such. It is event
driven and all that a node does is that it sends out its event when it
occurs or at even intervals. Other nodes  can "do stuff" or react on
this event (light a lamp, log to a database) etc. It's really stupid but
makes a system that is very easy to configure. More information is
available at http://www.vscp.org

I have been working with this since 1984 (yes I know I'm slow...) when
the node was a 6800 and I think the manufacturing cost was about $250
and RS-232 was the only communication option. Now the node is based
around a 12F675 or 18F248/258 using CAN and the cost is very low. Still
this is may something that never will be finished but its a fun ride
soldering, thinking about it, trying it out...

/Ake

-----Ursprungligt meddelande-----
Fr=E5n: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]F=F6r Art
Skickat: den 18 april 2004 22:21
Till: PICLIST@MITVMA.MIT.EDU
=C4mne: Re: [EE:]Light detector


You didn't say what data rate you were trying to receive. LDR's have
really
poor frequency characteristics, don't expect much better than 5 to 20
Hz,
especially with a 500k series resistor.

When I first read you message, I thought you were just trying to detect
ambient light levels, but not sure.

Phototransistors are good for low data rates, up to 10Khz or a tad
higher
if you are using them in small signal mode. Otherwise, look at a
photodiode
for big bandwidth and a very large dynamic range.

Smile, it can always be worse:>:

Art

At 02:05 PM 4/18/04, you wrote:
>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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