Hi,
[EMAIL PROTECTED]:
> Why do all (that I've found) light-measuring circuits based on the DS2438
> use reverse-biased photodiodes, rather than, say, a photoresistor?
I've tried that. Turns out that the typical photoresistor has too large
a resistance range, so you have a choice between "no difference between
cloudy and night" and "no difference between cloudy and sunlight".
The former is bad for automagically turning the yard ligths on and off,
the latter is bad for automagically pulling the shades down, and two
photoresistors was too much work because the box I wanted to put it in
had *one* nicely pre-fabricated hole for the resistor. ;-)
I've solved the problem with a DS2450, setting up a minimal resistor
network:
+5V---+
|
PhotoRes
|
I/O A-+
|
1kOhm
|
I/O B-+
|
100kOhm
|
GND -+
Thus, when I pulldown pin B, pin A measures light, while when I don't,
it measures dark. ;-) (Or vice versa. I don't remember off-hand. The
resistor values may be somewhat different too; my notes are Somewhere
Else.)
The resulting curve doesn't look like there's a switch-over point at
all, other than a stupid spike that I have been too lazy ^W overworked
to chase down yet. Probably a buglet in the code.
--
Matthias Urlichs | {M:U} IT Design @ m-u-it.de | [EMAIL PROTECTED]
Disclaimer: The quote was selected randomly. Really. | http://smurf.noris.de
- -
A lifetime isn't nearly long enough to figure out what it's all about.
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