Re: Nice CdS light detector in col_pic

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
.

Relevant Pages

  • Re: Nice CdS light detector in col_pic
    ... Jim Thompson wrote: ... and use PIC internal pullup to provide the current for the LDR. ... CdS are cool, very sensitive. ... Very sensitive, in terms of change of resistance with exposure to light, ...
    (sci.electronics.design)
  • Re: Nice CdS light detector in col_pic
    ... on PIC comparator input, ... and use PIC internal pullup to provide the current for the LDR. ... CdS are cool, very sensitive. ... Very sensitive, in terms of change of resistance with exposure to light, ...
    (sci.electronics.design)
  • Re: Nice CdS light detector in col_pic
    ... Jim Thompson wrote: ... on PIC comparator input, ... and use PIC internal pullup to provide the current for the LDR. ... CdS are cool, very sensitive. ...
    (sci.electronics.design)
  • Re: Nice CdS light detector in col_pic
    ... on PIC comparator input, ... and use PIC internal pullup to provide the current for the LDR. ... CdS are cool, very sensitive. ... the curve is much better controlled than CdS's. ...
    (sci.electronics.design)
  • Re: Nice CdS light detector in col_pic
    ... on PIC comparator input, ... and use PIC internal pullup to provide the current for the LDR. ... CdS are cool, very sensitive. ... Photodiodes run almost-almost open circuit 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. ...
    (sci.electronics.design)