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From: Wimpie on 22 Mar 2010 10:00
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
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
> hobbs at electrooptical dot nethttp://electrooptical.net

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.
From: Phil Hobbs on 22 Mar 2010 10:06
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

--
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: Jim Thompson on 22 Mar 2010 10:24
On Mon, 22 Mar 2010 11:51:00 +0530, "pimpom" <pimpom(a)invalid.invalid>
wrote:

>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
>
>For simple detection of ambient light level with no requirement
>of high linearity, I find LDRs more sensitive and easier to work
>with. They correspond fairly well with human spectral response
>too.
>

That could be made to work as well.

...Jim Thompson
--
| James E.Thompson, CTO | 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 |

The only thing bipartisan in this country is hypocrisy
From: Jim Thompson on 22 Mar 2010 10:27
On Mon, 22 Mar 2010 14:44:34 GMT, zekfrivo(a)zekfrivolous.com (GregS)
wrote:

>In article <a8e2d54d-cd89-4910-a6e5-2091bab0ce85(a)f13g2000pra.googlegroups.com>, MooseFET <kensmith(a)rahul.net> wrote:
>>On Mar 21, 8:10=A0pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My-
>>Web-Site.com> 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)?
>>
>>I would expect a little under 0.5A per watt of light power hitting
>>the diode.
>>
>>How big is the diode?
>>
>>> As usual, my customer has no clue. =A0My only concern, design wise, is
>>> to build a micropower transconductance amplifier with sufficient
>>> output drive to handle the maximum available photo current.
>>
>>Since you are working at low frequencies, you can run the diode at
>>zero bias or some other value the circuit likes. This makes it a
>>lot easier to do the amplifier design.
>
>I would just MEASURE it with some diode. I just measured
>2 mv p-p into a scope 1M. Directly pointing at the light is
>14 mv p-p. Thats 2 micro volta to 14 micro volts.
>I don't know what the diode is, but I have been using it
>to measure light noise.
>
>greg

Current. Transconductance. Not VOLTS.

...Jim Thompson
--
| James E.Thompson, CTO | 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 |

The only thing bipartisan in this country is hypocrisy
From: Jim Thompson on 22 Mar 2010 10:31
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.
>
>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

My task is to produce a PWM output proportional to a photodiode
current.

I guess I need to find a typical one and measure it myself.

...Jim Thompson
--
| James E.Thompson, CTO | 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 |

The only thing bipartisan in this country is hypocrisy
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