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From: Hal Murray on 22 Mar 2010 18:52
In article <4BA7797C.7020609(a)electrooptical.net>,
Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> writes:


> Night lights use CdS or CdSSe photoconductors, but they don't have to
>have any kind of stability or repeatability, and they don't. (CdSSe's
>resistance can vary 5X due entirely to previous illumination history.)

What is the mechanism for that history quirk?

What is the time scale? How long does it remember the history?

--
These are my opinions, not necessarily my employer's. I hate spam.

From: John Larkin on 22 Mar 2010 19:06
On Mon, 22 Mar 2010 10:06:52 -0400, Phil Hobbs
<pcdhSpamMeSenseless(a)electrooptical.net> wrote:

>On 3/22/2010 10:00 AM, Wimpie wrote:
>> On 22 mar, 13:01, Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net>
>> wrote:
>>> On 3/21/2010 11:10 PM, Jim Thompson wrote:
>>>
>>>> What current would one expect to see from a photodiode exposed to
>>>> typical bright room light (used to control a backlight proportional to
>>>> room light)?
>>>
>>>> As usual, my customer has no clue. My only concern, design wise, is
>>>> to build a micropower transconductance amplifier with sufficient
>>>> output drive to handle the maximum available photo current.
>>>
>>>> ...Jim Thompson
>>>
>>> Those guys are all nuts.
>>>
>>> For backlight control, you can use one of the ambient light sensors such
>>> as the Intersil ISL29000 or TAOS TSL2560, for instance. They look after
>>> all of that nonsense, and produce an analogue or digital output
>>> proportional to just the perceived brightness (i.e. luminous intensity),
>>> without being fooled by all the IR from incandescents, for instance.
>>>
>>> Full zenith sunlight is about 800W/m**2, so figuring 0.3 A/W average
>>> responsivity, a typical 2.3 mm square photodiode such as a BPW34 will
>>> never produce more than ~1.3 mA without optical concentration of some
>>> sort. Indoors it's a factor of 10**3 to 10**4 dimmer than that, so
>>> you're looking at a few microamps in bright room lights.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>
>> Hello Phil,
>>
>> Maybe they aren't nuts. A good ambient light sensor (with human eye
>> sensitivity and lambertian radiation pattern) is more expensive than
>> just a photo diode or photo transistor.
>>
>> Searching for "ambient light sensor" or "daylight sensor" will
>> probably give some afffordable components. I agree that just a photo
>> diode is not a good idea as their maximum sensitivity is mostly in the
>> IR range and radiation pattern will be too far from lambertian.
>>
>> Best regards,
>>
>> Wim
>> PA3DJS
>> www.tetech.nl
>> without abc, PM will reach me.
>
>
>There are certainly cheaper ways to measure light, at some level, but it
>depends what you want to do.
>
>The ALS parts are way under a buck in volume, and come in chip scale
>packages if you like. That gets you the right spectral response,
>built-in PGA and ADC, with an I2C interface. They're slow as molasses,
>of course, but that doesn't matter in that application.
>
> Night lights use CdS or CdSSe photoconductors, but they don't have to
>have any kind of stability or repeatability, and they don't. (CdSSe's
>resistance can vary 5X due entirely to previous illumination history.)
>Phototransistors have somewhat similar issues, due to the poorly
>specified beta.
>
>Cheers
>
>Phil Hobbs

The very concept of a photoresistor seems strange to me. I've read up
on the effect and don't understand it. I think their history effect is
worst at low currents.

People used to use photoresistors as choppers. HP made a voltmeter
that used CdSe cells as both the chopper and demodulator, illuminated
by an incandescent lamp with a motor-based chopper wheel.

John


From: Jamie on 22 Mar 2010 20:32
John Larkin wrote:

> On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs
> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>
>
>>On 3/21/2010 11:10 PM, Jim Thompson wrote:
>>
>>>What current would one expect to see from a photodiode exposed to
>>>typical bright room light (used to control a backlight proportional to
>>>room light)?
>>>
>>>As usual, my customer has no clue. My only concern, design wise, is
>>>to build a micropower transconductance amplifier with sufficient
>>>output drive to handle the maximum available photo current.
>>>
>>> ...Jim Thompson
>>
>>Those guys are all nuts.
>
>
> Then I'm sure glad I didn't make any suggestions.
>
> John
>

I couldn't agree any more. Your decision is well understood! ;)



From: Phil Hobbs on 22 Mar 2010 20:27
On 3/22/2010 7:06 PM, John Larkin wrote:
> On Mon, 22 Mar 2010 10:06:52 -0400, Phil Hobbs
> <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
>
>> On 3/22/2010 10:00 AM, Wimpie wrote:
>>> On 22 mar, 13:01, Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net>
>>> wrote:
>>>> On 3/21/2010 11:10 PM, Jim Thompson wrote:
>>>>
>>>>> What current would one expect to see from a photodiode exposed to
>>>>> typical bright room light (used to control a backlight proportional to
>>>>> room light)?
>>>>
>>>>> As usual, my customer has no clue. My only concern, design wise, is
>>>>> to build a micropower transconductance amplifier with sufficient
>>>>> output drive to handle the maximum available photo current.
>>>>
>>>>> ...Jim Thompson
>>>>
>>>> Those guys are all nuts.
>>>>
>>>> For backlight control, you can use one of the ambient light sensors such
>>>> as the Intersil ISL29000 or TAOS TSL2560, for instance. They look after
>>>> all of that nonsense, and produce an analogue or digital output
>>>> proportional to just the perceived brightness (i.e. luminous intensity),
>>>> without being fooled by all the IR from incandescents, for instance.
>>>>
>>>> Full zenith sunlight is about 800W/m**2, so figuring 0.3 A/W average
>>>> responsivity, a typical 2.3 mm square photodiode such as a BPW34 will
>>>> never produce more than ~1.3 mA without optical concentration of some
>>>> sort. Indoors it's a factor of 10**3 to 10**4 dimmer than that, so
>>>> you're looking at a few microamps in bright room lights.
>>>>
>>>> Cheers
>>>>
>>>> Phil Hobbs
>>
>>> Hello Phil,
>>>
>>> Maybe they aren't nuts. A good ambient light sensor (with human eye
>>> sensitivity and lambertian radiation pattern) is more expensive than
>>> just a photo diode or photo transistor.
>>>
>>> Searching for "ambient light sensor" or "daylight sensor" will
>>> probably give some afffordable components. I agree that just a photo
>>> diode is not a good idea as their maximum sensitivity is mostly in the
>>> IR range and radiation pattern will be too far from lambertian.
>>>
>>> Best regards,
>>>
>>> Wim
>>> PA3DJS
>>> www.tetech.nl
>>> without abc, PM will reach me.
>>
>>
>> There are certainly cheaper ways to measure light, at some level, but it
>> depends what you want to do.
>>
>> The ALS parts are way under a buck in volume, and come in chip scale
>> packages if you like. That gets you the right spectral response,
>> built-in PGA and ADC, with an I2C interface. They're slow as molasses,
>> of course, but that doesn't matter in that application.
>>
>> Night lights use CdS or CdSSe photoconductors, but they don't have to
>> have any kind of stability or repeatability, and they don't. (CdSSe's
>> resistance can vary 5X due entirely to previous illumination history.)
>> Phototransistors have somewhat similar issues, due to the poorly
>> specified beta.
>>
>> Cheers
>>
>> Phil Hobbs
>
> The very concept of a photoresistor seems strange to me. I've read up
> on the effect and don't understand it. I think their history effect is
> worst at low currents.
>
> People used to use photoresistors as choppers. HP made a voltmeter
> that used CdSe cells as both the chopper and demodulator, illuminated
> by an incandescent lamp with a motor-based chopper wheel.
>
> John
>
>

The old HP Nixie tube frequency counters actually use CdS photoresistors
illuminated by neon bulbs to drive the Nixie segments! (I have a few in
my basement.)

Photoconductors exhibit gain equal to the carrier lifetime divided by
the transit time--essentially you get to re-use the carriers many times.
That's also the origin of the speed/gain tradeoff.

Unfortunately both the photogeneration and recombination are stochastic,
so they have twice the shot noise of a photodiode. This is sometimes
called 'generation-recombination noise', but that causes confusion with
normal thermionic G-R noise in IR photoconductors.

I don't know the origin of the photoresponse hysteresis of CdS and
CdSSe, but a SWAG would be that there are long-lived trap states that
get saturated in very bright light, and that this effect makes the
carrier lifetime longer.

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
From: Michael A. Terrell on 22 Mar 2010 21:43

Phil Hobbs wrote:
>
> On 3/22/2010 7:06 PM, John Larkin wrote:
> > On Mon, 22 Mar 2010 10:06:52 -0400, Phil Hobbs
> > <pcdhSpamMeSenseless(a)electrooptical.net> wrote:
> >
> >> On 3/22/2010 10:00 AM, Wimpie wrote:
> >>> On 22 mar, 13:01, Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net>
> >>> wrote:
> >>>> On 3/21/2010 11:10 PM, Jim Thompson wrote:
> >>>>
> >>>>> What current would one expect to see from a photodiode exposed to
> >>>>> typical bright room light (used to control a backlight proportional to
> >>>>> room light)?
> >>>>
> >>>>> As usual, my customer has no clue. My only concern, design wise, is
> >>>>> to build a micropower transconductance amplifier with sufficient
> >>>>> output drive to handle the maximum available photo current.
> >>>>
> >>>>> ...Jim Thompson
> >>>>
> >>>> Those guys are all nuts.
> >>>>
> >>>> For backlight control, you can use one of the ambient light sensors such
> >>>> as the Intersil ISL29000 or TAOS TSL2560, for instance. They look after
> >>>> all of that nonsense, and produce an analogue or digital output
> >>>> proportional to just the perceived brightness (i.e. luminous intensity),
> >>>> without being fooled by all the IR from incandescents, for instance.
> >>>>
> >>>> Full zenith sunlight is about 800W/m**2, so figuring 0.3 A/W average
> >>>> responsivity, a typical 2.3 mm square photodiode such as a BPW34 will
> >>>> never produce more than ~1.3 mA without optical concentration of some
> >>>> sort. Indoors it's a factor of 10**3 to 10**4 dimmer than that, so
> >>>> you're looking at a few microamps in bright room lights.
> >>>>
> >>>> Cheers
> >>>>
> >>>> Phil Hobbs
> >>
> >>> Hello Phil,
> >>>
> >>> Maybe they aren't nuts. A good ambient light sensor (with human eye
> >>> sensitivity and lambertian radiation pattern) is more expensive than
> >>> just a photo diode or photo transistor.
> >>>
> >>> Searching for "ambient light sensor" or "daylight sensor" will
> >>> probably give some afffordable components. I agree that just a photo
> >>> diode is not a good idea as their maximum sensitivity is mostly in the
> >>> IR range and radiation pattern will be too far from lambertian.
> >>>
> >>> Best regards,
> >>>
> >>> Wim
> >>> PA3DJS
> >>> www.tetech.nl
> >>> without abc, PM will reach me.
> >>
> >>
> >> There are certainly cheaper ways to measure light, at some level, but it
> >> depends what you want to do.
> >>
> >> The ALS parts are way under a buck in volume, and come in chip scale
> >> packages if you like. That gets you the right spectral response,
> >> built-in PGA and ADC, with an I2C interface. They're slow as molasses,
> >> of course, but that doesn't matter in that application.
> >>
> >> Night lights use CdS or CdSSe photoconductors, but they don't have to
> >> have any kind of stability or repeatability, and they don't. (CdSSe's
> >> resistance can vary 5X due entirely to previous illumination history.)
> >> Phototransistors have somewhat similar issues, due to the poorly
> >> specified beta.
> >>
> >> Cheers
> >>
> >> Phil Hobbs
> >
> > The very concept of a photoresistor seems strange to me. I've read up
> > on the effect and don't understand it. I think their history effect is
> > worst at low currents.
> >
> > People used to use photoresistors as choppers. HP made a voltmeter
> > that used CdSe cells as both the chopper and demodulator, illuminated
> > by an incandescent lamp with a motor-based chopper wheel.
> >
> > John
> >
> >
>
> The old HP Nixie tube frequency counters actually use CdS photoresistors
> illuminated by neon bulbs to drive the Nixie segments! (I have a few in
> my basement.)

5245, 5248?

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'
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