Re: Nice CdS light detector in col_pic

On Tue, 07 Apr 2009 21:10:05 -0400, "Michael A. Terrell"
<mike.terrell@xxxxxxxxxxxxx> wrote:


Jim Thompson wrote:

On Tue, 07 Apr 2009 10:42:06 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx> wrote:

Jim Thompson wrote:
On Mon, 06 Apr 2009 13:05:55 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx> wrote:

Jan Panteltje wrote:
Found some LDR (CdS light dependent resistor) .. why not
connect it to my col_pic project?
http://panteltje.com/panteltje/pic/col_pic/index.html
Now I do believe in minimum parts electronics...
so.. on PIC comparator input,
other input comparator on internal programmable Vref
- so software control; still have to write that part -,
and use PIC internal pullup to provide the current for the LDR.
But how? comp input has no pullup... So configured other input with
pullup and put inputs parallel :-)
No other parts.
Works... 200uA (measured) weak pullup is just right for those CdS cells.
So now, with this feature, it only does light when dark.
Need to program that software override..
CdS are cool, very sensitive.

Very sensitive, in terms of change of resistance with exposure to light,
but insensitive in terms of how much light you need to get an
unambiguous result--you can easily have 5:1 resistance hysteresis due to
previous history. Photodiodes run almost-almost open circuit (e.g. with
a FET buffer amp and a 10**11 ohm shunt) give a pretty nice logarithmic
curve over a wide range, and even when they start to roll over due to
high level injection, the curve is much better controlled than CdS's.

For night lights, CdS is unbeatable.

Cheers

Phil Hobbs

CdS is slo-o-o-o-ow, so you can't pass any significant frequency
component thru them.

...Jim Thompson

It's slow with respect to light changes--as we discussed here awhile
back, photoconductors have gain equal to t_recomb/t_transit, i.e.
there's a linear tradeoff between gain and speed, much like avalanche
photodiodes and voltage-feedback op amps. Carrier lifetime in CdS is
very very long, so they're slow optically.

I haven't checked to see whether they're faster with respect to applied
voltage--AFAIK they're just like any other resistor that way. Certainly
they were commonly used in 1970s audio companders, so they have to
respond electrically in tens of microseconds if not faster.

Cheers

Phil Hobbs

In the '60's I designed a chip utilizing CdS as the sensor, for
headlight dimming, for Guide Lamp Division of GM (*).

CdS responds _fast_ to light _application_, _slow_ to light removal,
which is just fine for headlight dimming (and compressing audio).

(*) Anderson, Indiana, garden spot of the mid-west... liveliest place
in town, fire pit at the Holiday Inn ;-)


Just be gld that it wasn't Batesville, Indiana.

As in Bates Motel ?:-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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"The American Republic will endure until the day Congress
discovers that it can bribe the public with the public's money."

- Alexis de Tocqueville
.

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