Photodiode Question
 

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
--
| 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 |

I love to cook with wine. Sometimes I even put it in the food.

Photodiode Question
 

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.

Lots -- I did something like this a few years ago and saw several milliamps.

But as you really should know, the real answer is "it depends". It
depends on the diode area, whether it's shaded, what the customer thinks
"bright" is, ect.

Order a few from Digikey, cobble up something with a 9V battery and an
ammeter, and go measure it! Then use the sensitivity on that data sheet
to extrapolate to what you'd expect from other similar diodes.

One thing that you will learn is that your eyeballs have an amazing
capability to accommodate changes in lighting, and that "bright" for a
room doesn't hold a candle to "bright" for a sunny day.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

Photodiode Question
 

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

As Tim said, "It depends". Tim mentioned the "obvious suspects" - how
big is the photodiode? (1 mm x 1 mm or 1 cm x 1 cm - it will make a
difference...), how bright is "bright"? (IIRC, the eye's photoresponse
is logarithmic, but a photodiode is linear over several orders of
magnitude in illumination), etc.

One thing Tim didn't mention is the frequency response of the
photodiode. Do you and your client only care about DC illumination, or
does the photodiode have to be able to deal with the illumination
switching on and off at several tens or a few hundred kHz. In general,
if only DC is of concern, the photodiode can be designed to have better
responsivity - the current out per unit optical power in.

Also, with the photodiode be reverse-biased or zero-biased? (Reverse
bias will improve the responsivity) but not applying an external bias
voltage across the PD will make things simpler.

Depending on what the client's needs are, you might want to consider
using a photoconductor instead of a photodiode.

Bob Pownall

Photodiode Question
 

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.

Photodiode could be not a very good idea, as they depend on everything.
A good part for luminance detection is TPS851 from Toshiba. See the
datasheet for details. If you want to isolate the light sensor from
being illuminated by your own backlight, you can time multiplex the
backlight and the sensor.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Photodiode Question
 

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.

Photodiode Question
 

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 net
http://electrooptical.net

Photodiode Question
 

On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs
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

Photodiode Question
 

On 3/22/2010 9:52 AM, John Larkin wrote:
On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs
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


Nah, it's just my rhetorical way of trying to get people to do a bit of
calculation (or even experiment) before giving advice. I know, I know,
I'm on Usenet, but we must live in hope. ;)

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

Photodiode Question
 

On Mon, 22 Mar 2010 11:51:00 +0530, "pimpom"
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

Photodiode Question
 

On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs
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

Photodiode Question
 

"Jim Thompson" wrote in
message ...
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.

Well, you didn't specify what kind of proportionality, so I guess I win
firsties.
http://webpages.charter.net/dawill/I...odiode_PWM.gif

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Photodiode Question
 

"Tim Williams" wrote in message
...

http://webpages.charter.net/dawill/I...odiode_PWM.gif

Tim

Geez, I haven't futzed with PUTs in ages. Does anybody still make them?

Photodiode Question
 

On Mon, 22 Mar 2010 10:07:56 -0500, "Tim Williams"
wrote:

"Jim Thompson" wrote in
message ...
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.

Well, you didn't specify what kind of proportionality, so I guess I win
firsties.
http://webpages.charter.net/dawill/I...odiode_PWM.gif

Tim


.... for certain definitions of "PWM."


But yeah, PUTs were fun.

John

Photodiode Question
 

"Oppie" wrote in message
...
http://webpages.charter.net/dawill/I...odiode_PWM.gif

Geez, I haven't futzed with PUTs in ages. Does anybody still make them?

PUTs aren't that hard to find. Maybe you're thinking of UJTs?
http://www.mouser.com/ProductDetail/...URy1Y7q4hnw%3d
Although I'll admit none of them are actually *in stock* right now.

Anyway, if they're on the expensive side ($0.25 in singles?), you can
Jeorgify your own from a 2N3904 and 2N3906 ($0.06/ea in singles)... the
equivalent circuit is as indicated (i.e., like an SCR, but take the second
junction).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Photodiode Question
 

On Mon, 22 Mar 2010 10:07:56 -0500, "Tim Williams"
wrote:

"Jim Thompson" wrote in
message ...
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.

Well, you didn't specify what kind of proportionality, so I guess I win
firsties.
http://webpages.charter.net/dawill/I...odiode_PWM.gif

Tim

Except that's current-to-frequency :-)

...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

Photodiode Question
 

"Jim Thompson" wrote in
message ...
http://webpages.charter.net/dawill/I...odiode_PWM.gif

Except that's current-to-frequency :-)

On-time is constant, so it's PWM. You didn't say if frequency had to be
constant. No specs, remember? ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Photodiode Question
 

On Mon, 22 Mar 2010 09:58:43 -0400, Phil Hobbs
wrote:

On 3/22/2010 9:52 AM, John Larkin wrote:
On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs
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


Nah, it's just my rhetorical way of trying to get people to do a bit of
calculation (or even experiment) before giving advice. I know, I know,
I'm on Usenet, but we must live in hope. ;)

Cheers

Phil Hobbs

I recall a recent lawsuit (maybe the Hynix/TI thing) over a design for
an ambient light sensor chip that used semi processing to get
human-similar wavelength response.

Scads of people offer ambient light sensor chips.

John

Photodiode Question
 

John Larkin wrote:

On Mon, 22 Mar 2010 08:01:35 -0400, Phil Hobbs
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! ;)

Photodiode Question
 

On 2010-03-22, Jim Thompson wrote:
On Mon, 22 Mar 2010 10:07:56 -0500, "Tim Williams"
wrote:

"Jim Thompson" wrote in
message ...
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.

Well, you didn't specify what kind of proportionality, so I guess I win
firsties.
http://webpages.charter.net/dawill/I...odiode_PWM.gif

Tim

Except that's current-to-frequency :-)

It doesn't have to be, feed a square wave clock through a small
capacitor to the gate if there's a particular frequency you want.



--- news://freenews.netfront.net/ - complaints: ---

Photodiode Question
 

"Tim Williams" wrote in message
...
"Oppie" wrote in message
...
http://webpages.charter.net/dawill/I...odiode_PWM.gif

Geez, I haven't futzed with PUTs in ages. Does anybody still make them?

PUTs aren't that hard to find. Maybe you're thinking of UJTs?
http://www.mouser.com/ProductDetail/...URy1Y7q4hnw%3d
Although I'll admit none of them are actually *in stock* right now.

Anyway, if they're on the expensive side ($0.25 in singles?), you can
Jeorgify your own from a 2N3904 and 2N3906 ($0.06/ea in singles)... the
equivalent circuit is as indicated (i.e., like an SCR, but take the second
junction).

Tim


Last time I worked with them was about 25 years ago. It was used as a
oscillator/ramp generator for a proportional heating control system in a
'Centrifichem' blood chemistry analyzer.
Oppie