Nice CdS light detector in col_pic

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

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

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

...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  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

On a sunny day (Mon, 06 Apr 2009 13:05:55 -0400) it happened Phil Hobbs wrote in :

I am not sure about that Phil. I had an exposure meter for photography, in the times we still used film, it was very slow, but also very accurate, had a CdS cell in it, and a bunch of trimpots, and a meter plus battery, not much more IIRC. I never noticed any hysteresis, and I do not see hysteresis in this one either. Software is written now, this thing is even quite fast (it is a small one), If I set Vref at 2.5 V, and it shorts the 200 uA pullup to ground, I can move my hand above it and it will flash as my fingers pass over it :-) I know these get slower when there is less light.

Yes, but lower output voltage...

Indeed

But AFAIK the bureaucrats in Brussels have outlawed them. I can already picture it: Someone from the guvmint over there reads this, sends the goons out and we won't hear from Jan for the next couple of years ;-)

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http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

s.

ch

ither.

),

:-)

Hello Jan,

There are photocells that have reasonable eye sensitivity curve, but show (some) hysteresis (light history effect). For me no supprise that they used LDRs in a light meter. you may visit

=3DPhotocells. They have good literature on LDR.

I considered to use an LDR as an analog isolated modulator for LF signals....

Best regards,

Wim PA3DJS

without abc you can use the PM.

On a sunny day (Mon, 6 Apr 2009 12:21:27 -0700 (PDT)) it happened snipped-for-privacy@tetech.nl wrote in :

Hi Wim, you are from .nl, did you know that in the old TV days (about to 1970) the potentiometer in one of sound mixers was a LDR driven by a normal light bulb? The amount of light of the light bulb did set the volume. LDR makes a great audio fader :-) But as light bulbs age things change.

Kind of like Sloman.

--
And another motherboard bites the dust!

Tell them he got them from Bill and that he has half a million more, in his basement.

--
And another motherboard bites the dust!

s.

ch

ither.

),

:-)

I guess it depends on the age of the instrument, but my Gossen lightmeter uses a photodiode, or as they call it, silicon blue cell. It goes down to 0.5 footcandles.

On a sunny day (Mon, 6 Apr 2009 16:55:09 -0700 (PDT)) it happened " snipped-for-privacy@sushi.com" wrote in :

You may be interested to know that I now invented the 'darkness meter'. I wanted to also display the current light level, and as I only had a comparator, but could program 32 steps of it reference voltage, I added some lines of code to step it from 31 down to zero, and display the value where the comparator flips over. As the CdS call shorts to ground, '0' is absolute bright light, and 31 is darkness. So it shows the amount of darkness: 0 is no darkness, 31 is max darkness. Next is the black light lamp :-)

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

Leaving a CdS cell out in the sun will make a permanent (or very nearly so) change in cell resistance. The interior of a light meter isn't that bright even when used in sunlight.

A really great CdS light meter, e.g. a Gossen LumaPro, is good to about

1/4 to 1/3 stop--i.e. a fractional error of 18 to 25%. Not my idea of a good measurement, but more than adequate for photography. (When I was into film photography (back in the late 1970s), I really wanted a LumiSix III, the LunaPro's predecessor. I'd still like one--horses for courses.)

The output voltage of an open-circuited photodiode changes by 250 microvolts for every percent change in light intensity. For 1/4 stop accuracy, you have to be able to sense a 5 mV change. Not too hard. Of course you also have to temperature-compensate it carefully, because 2-3 mV per degree is pretty horrible at that level, but you have to use a second PD anyway to get an accurate Vf->Iphoto conversion. Running a second PD in the dark with, say, 1 uA applied current, and subtracting the two voltages, would probably work fine. You do have to watch out for the electrical conductance of black paint! (BTW solder mask also leaks like a sieve--I found that out the hard way building Footprints.)

I usually use this trick with quad cells, to make beam pointing indicators whose readings are insensitive to beam power. Works great, and you get the calibration and temperature compensation free.

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 ;-)

...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  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

On a sunny day (Tue, 07 Apr 2009 10:56:22 -0400) it happened Phil Hobbs wrote in :

I am so glad my Canon A470 CCD digital camera does it through the lens automatically, and a great focus and white balance too. Photograhy has become a lot easier.... If they only had the common sense to make the viewfinder so it could flip up, so you could look down at it.....

External light meters make it easier to set up photo shoots for realistic rendering. TTL metering is the bee's knees for snapshots, though, I agree. I want a digicam with a real TTL viewfinder, focusing screen, and spot metering--not a crappy LCD display. :(

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

On a sunny day (Tue, 07 Apr 2009 11:21:49 -0400) it happened Phil Hobbs wrote in :

I do not believe in TTL optical viewfinders, as these days most will be displayed on LCD screens, or flat paper, anyways, the LCD is still the most WYSIWYG. Manual focus is nice, all other controls should available in manual mode too. The lens is the eye... so even for setup of a photo shoot with say backlight lamps, there used to be a good free introduction to lighting scenes here:

but now unfortunately the guy is asking 50$ for it..... Guess his expertise did not make him enough, so he had to charge for his website. So best use your own judgement perhaps. Was a good tutorial, should have grabbed that site.

Quad detectors are found useful in so many ways. I really appreciated their use in CD players (and I assume in most optical disc readers) where astigmatism used with 45-degree rotated quad detectors make staying in focus a beauty to behold.

Jon

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I'm going to plead ignorance here since I haven't designed a light meter, but the PD puts out a current, which you then feed to a I-to-V. Why would you take an approach to measure the open circuit voltage over the typical I-to-V approach?

The dark current cancellation scheme you mention is used on some photo arrays. Of course, you get half the array density.

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

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And another motherboard bites the dust!