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Q: Detecting sunlight ( Answered 5 out of 5 stars,   1 Comment )
Subject: Detecting sunlight
Category: Science > Instruments and Methods
Asked by: matseng-ga
List Price: $10.00
Posted: 24 Oct 2003 13:54 PDT
Expires: 23 Nov 2003 12:54 PST
Question ID: 269441
I'm interested in measuring the number of hours of sunlight each day.
With sunlight I mean only the time when the sun is visible and not
blocked by clouds.

I want to this electronically and not by the old method using a
"crystal ball" that focuses the sunlight onto a heat sensitive paper
beneath it.

What methods of doing this are both decently reliable and feasible for
an experienced electronics/computer tinkerer to implement/build?

Request for Question Clarification by mathtalk-ga on 25 Oct 2003 09:52 PDT
Hi, matseng-ga:

What would you think of a circuit that measures the total time when
sunlight is at some threshold of brightness?

I'm thinking of circuits that might use photovoltaic cells or
photoresistance cells to detect light.  With a photovoltaic cell the
response to increased illumination is a corresponding increase in
current.  Photoresistance cells, on the other hand, change their
electrical resistance with light and would require some external power
for operation.

A simple arrangement might be a self-contained (waterproof) unit,
probably battery powered, that could be left outside for a number of
days.  At the end of a trial, dividing the elapsed time recorded by
the unit by the number of days would give the average period of
daylight per day (over the trial period).

This would involve setting the threshold for light intensity so it's
attained during direct illumination by the sun but not when the sun is
obscured by clouds.

regards, mathtalk-ga

Clarification of Question by matseng-ga on 26 Oct 2003 05:49 PST
I suppose that could work for a limited period of the year.  But isn't
the light intensity quite different at noon in the summer compared to
an hour before sunset in late autumn? Thus making it neccessary to
adjust the threshold point from time to time.  The angle between the
sun and the light sensitive unit would affect the light intensity a
great deal.

How does the national weather institues measure the number of

Request for Question Clarification by mathtalk-ga on 26 Oct 2003 09:14 PST
There are electronic instruments rather than simply mechanical or
mechanical-electric ones.  I found a somewhat detailed description of
such a project, but I'm not sure whether it would be satisfactory for
a do-it-yourself hobbyist.

I think the sun's varying angles can be compensated for with
reflectors, if one is simply tracking intensity.  However the article
I found makes a point of defining sunlight (versus cloudiness) by the
presence of shadows, i.e. a contrasting source of illumination rather
than a diffuse one.

If this material is likely to be useful to you in line with your
expectations, I'd be happy to post it as an answer.

regards, mathtalk-ga

Clarification of Question by matseng-ga on 26 Oct 2003 12:42 PST

I've been thinking about measuring the light intensity of several
locations in the sky at the same time and then comparing the values
relative to each other.

The shadow measuring method that you have located sounds promising and
I would probably accept that as an acceptable answer.
Subject: Re: Detecting sunlight
Answered By: mathtalk-ga on 27 Oct 2003 07:34 PST
Rated:5 out of 5 stars
Hi, matseng-ga:

Here is the Web site I mentioned.  The first link is to a copy of a
1990 article from Elektor Electronics, and the second page is New
Zealander Jeff Northcott's discussion of his progress in building this

[Jeff's Home Weather Station - Sunshine Recorder]

"Like the original Campbell-[S]tokes recorder, the all-electronic
version discussed here has no moving parts. its measuring principle
does, however, require a computer for data recording purposes...

"The electronic sunshine recorder is based on four photodiodes Type
BPW21 which measure the ambient light intensity. Since meteorologists
hold that there is sunshine if there is a shadows the position of each
of the four sensors enables it to measure a different light intensity
when the sun shines. When the illumination is homogeneous, as is the
case with an overcast sky, all sensors measure the same light
intensity. When the sun shines, the sensor that is best aimed at the
sun receives the highest light intensity. The computer connected to
the sensor assembly runs a continuous calculation on the position of
the sun while accounting for the geographical co-ordinates, the season
and the local time. In this manner, the system always knows which
sensor faces the sun and thus receives more light than the others."

"I have now started to construct the sunshine recorder project as
described in the Elektor Electronics article on my web site. I will
add to this section as I progress with the project. I would be
interested in hearing from anyone else who has either got this project
going or is attempting construction. You can email me through my
contact page."

* * * * * * * * * * * * * * * * * * * * * * * *

The "classic" instrument for recording sunshine hours is the
Campbell-Stokes Sunshine Recorder, the "crystal ball" device you spoke
about in your original question.  An attractive picture of one (for
sale?) is shown here:

[Campbell-Stokes Sunshine Recorder, $1,950]

Another picture of an older version of this, from the National
Maritime Museum at Greenwich, England, is here:

[National Maritime Museum - Sunshine Recorder(1876)]

An updated "electrical" design uses a blackened bulb, upon which
direct sunlight causes the air within to expand.  See for example this

[Important Tools Used to Predict Changes in Weather]

"Sunshine Recorder"

"A sunshine recorder records the hours of direct sunlight in a day.
This is done with the help of a little black bulb. When sunlight
strikes the bulb, air inside the bulb warms up and expands. Then an
electrical switch closes and sends a message to a recording pen in the
weather station. The pen draws a line on a moving piece of paper.
Broken lines on the piece of paper show when clouds blocked the sun's

regards, mathtalk-ga

Search Strategy

Keywords: "sunshine recorder"
matseng-ga rated this answer:5 out of 5 stars
Great work!

I've got exactly what I asked for, and as a bonus some extra info of
the traditional way of measuring sunlight.

Thanks mathtalk-ga, and may be the sun be with you :-)


Subject: Re: Detecting sunlight
From: snsh-ga on 25 Oct 2003 16:57 PDT
solar powered cuckoo clock

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