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Q: Detecting sunlight ( Answered ,   1 Comment )
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 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 sun-hours?``` 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 ```Hi! 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.```
 ```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 instrument. [Jeff's Home Weather Station - Sunshine Recorder] http://weather.northcott.co.nz/instruments/sun.php "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." http://weather.northcott.co.nz/instruments/projects/ "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] http://www.gemmary.com/instcat/14/p20-188-14.html Another picture of an older version of this, from the National Maritime Museum at Greenwich, England, is here: [National Maritime Museum - Sunshine Recorder(1876)] http://www.nmm.ac.uk/site/request/setTemplate:singlecontent/contentTypeA/conWebDoc/contentId/647 An updated "electrical" design uses a blackened bulb, upon which direct sunlight causes the air within to expand. See for example this glossary: [Important Tools Used to Predict Changes in Weather] http://www.eng.iastate.edu/explorer/topics/weather/tools.htm "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 rays." regards, mathtalk-ga Search Strategy Keywords: "sunshine recorder" ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22sunshine+recorder%22&btnG=Google+Search```
 matseng-ga rated this answer: ```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 :-) /Mats```
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