# LED light meter

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Guest

Tue Dec 15, 2009 6:51 am

Connect white LED to DVM, point the LED at a light source,
and read light intensity as voltage -- pointing the LED at
the noonday sun produces 1.4 v. Very cool. Are there
any problems with this design? Is the output linear -- if
I read 0.7 v from a given light source, is that light source
one half as bright as the sun at high noon?

Jasen Betts
Guest

Tue Dec 15, 2009 11:06 am

Quote:
Connect white LED to DVM, point the LED at a light source,
and read light intensity as voltage -- pointing the LED at
the noonday sun produces 1.4 v. Very cool. Are there
any problems with this design?

white LEDs cost more than coloured LEDs and are less efficient in this
application. try with a RED led in an uncoloured case.

if you measure microamperes instead of volts it's approximately linear

Quote:
I read 0.7 v from a given light source, is that light source
one half as bright as the sun at high noon?

the LED only responds to frequencies above some limit.

brightness of light sources is not being measured with that arrangement
of optics.

George Herold
Guest

Tue Dec 15, 2009 6:53 pm

Quote:
Connect white LED to DVM, point the LED at a light source,
and read light intensity as voltage -- pointing the LED at
the noonday sun produces 1.4 v.  Very cool.  Are there
any problems with this design?  Is the output linear -- if
I read 0.7 v from a given light source, is that light source
one half as bright as the sun at high noon?

Forrest Mims uses LED's as spectrally selective photo detectors.

Cool stuff.

George H.

Guest

Wed Dec 16, 2009 3:30 am

On Dec 15, 11:53 am, George Herold <ggher...@gmail.com> wrote:
Quote:

Connect white LED to DVM, point the LED at a light source,
and read light intensity as voltage -- pointing the LED at
the noonday sun produces 1.4 v.  Very cool.  Are there
any problems with this design?  Is the output linear -- if
I read 0.7 v from a given light source, is that light source
one half as bright as the sun at high noon?

Forrest Mims uses LED's as spectrally selective photo detectors.

Mims is the writer who showed me that LEDs could be used
as sensors.

Quote:

Cool stuff.

George H.

George Herold
Guest

Wed Dec 16, 2009 4:06 pm

Quote:
On Dec 15, 11:53 am, George Herold <ggher...@gmail.com> wrote:

Connect white LED to DVM, point the LED at a light source,
and read light intensity as voltage -- pointing the LED at
the noonday sun produces 1.4 v.  Very cool.  Are there
any problems with this design?  Is the output linear -- if
I read 0.7 v from a given light source, is that light source
one half as bright as the sun at high noon?

Forrest Mims uses LED's as spectrally selective photo detectors.

Mims is the writer who showed me that LEDs could be used
as sensors.

Cool stuff.

George H.- Hide quoted text -

- Show quoted text -

Oh, Well If you want to use the LED as a 'linear' detector, then I
would try measuring the current and not the voltage. But read the
Mims articles.. he's much more of an expert than I am. I might expect
white LED's to be messier than red, orange or yellow ones (Doesn't the
'white' come from the phosphour around a blue LED?). Do you have
some project in mind or are you just fooling around?

George H.

Guest

Thu Dec 17, 2009 5:28 am

On Dec 16, 9:06 am, George Herold <ggher...@gmail.com> wrote:
Quote:

On Dec 15, 11:53 am, George Herold <ggher...@gmail.com> wrote:

Connect white LED to DVM, point the LED at a light source,
and read light intensity as voltage -- pointing the LED at
the noonday sun produces 1.4 v.  Very cool.  Are there
any problems with this design?  Is the output linear -- if
I read 0.7 v from a given light source, is that light source
one half as bright as the sun at high noon?

Forrest Mims uses LED's as spectrally selective photo detectors.

Mims is the writer who showed me that LEDs could be used
as sensors.

Cool stuff.

George H.- Hide quoted text -

- Show quoted text -

Oh,  Well If you want to use the LED as a 'linear' detector, then I
would try measuring the current and not the voltage.  But read the
Mims articles.. he's much more of an expert than I am.  I might expect
white LED's to be messier than red, orange or yellow ones (Doesn't the
'white' come from the phosphour around a blue LED?).   Do you have
some project in mind or are you just fooling around?

Meter for testing plant lights. Don't need extreme precision,
+/- 5% is good.

George Herold
Guest

Thu Dec 17, 2009 11:42 pm

Quote:
On Dec 16, 9:06 am, George Herold <ggher...@gmail.com> wrote:

On Dec 15, 11:53 am, George Herold <ggher...@gmail.com> wrote:

Connect white LED to DVM, point the LED at a light source,
and read light intensity as voltage -- pointing the LED at
the noonday sun produces 1.4 v.  Very cool.  Are there
any problems with this design?  Is the output linear -- if
I read 0.7 v from a given light source, is that light source
one half as bright as the sun at high noon?

Forrest Mims uses LED's as spectrally selective photo detectors.

Mims is the writer who showed me that LEDs could be used
as sensors.

Cool stuff.

George H.- Hide quoted text -

- Show quoted text -

Oh,  Well If you want to use the LED as a 'linear' detector, then I
would try measuring the current and not the voltage.  But read the
Mims articles.. he's much more of an expert than I am.  I might expect
white LED's to be messier than red, orange or yellow ones (Doesn't the
'white' come from the phosphour around a blue LED?).   Do you have
some project in mind or are you just fooling around?

Meter for testing plant lights.  Don't need extreme precision,
+/- 5% is good.- Hide quoted text -

- Show quoted text -

Hmm, are you interested in the spectrum from the lamp or just the
total intensity? The LED's will give you some spectral information...
but I've never used them and you'd have to think about how to
calibrate them. Again Mims is the source for that type of info. For
an intensity measument you could try a photodiode or if pirce is
important, a solar cell. (And agian it will be best to measure the
current from the diode.)

George h.

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