source code of /projects/light-sensor/index.html

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        <title>Bloody light sensor</title>

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    <h1 id="title">Analog light sensor</h1>

    <p>This is a report on an assignment for a course in process measurement

        technology at Novia UAS. The task is to design, simulate, construct,

        test and calibrate an analog illuminance sensor based on an LDR.</p>

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    <ul>
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        <li><a href="light-sensor-part-1-design.pdf"

            >Report part 1: Design</a></li>

        <li><a href="light-sensor-part-2-testing.lowres.pdf"

            >Report part 2: Construction, testing, and calibration</a>

            <a download="" href="light-sensor-part-2-testing.pdf"

            >(Download high quality version)</a></li>

        <li><a href="https://gitlab.com/osksko-novia/light-sensor"

            >Browse the project files on GitLab</a> or

            <a href="/archive/light-sensor/light-sensor.tar.xz" download=""

            >download entire project as compressed tape archive</a></li>

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    (Downloads are throttled, go make a cup of coffee)

    <h2>Sneak peek at the schematic</h2>

    <img src="schematic.png" alt="(Picture of schematic)"/>

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    <h2>LDR characteristic curve</h2>
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    <p>It's surprisingly difficult to find any information about

        resistance/illuminance characteristics for photoresistors/LDRs.

        At best you get a straight line on a log-log graph which looks

        too good to be true considering that datasheets specify at which

        illuminance the gamma value is measured.</p>

    <p><var>R = R<sub>10</sub>*(10/E)<sup>gamma</sup></var></p>

    <p><var>R</var> is the resistance, <var>R<sub>10</sub></var> is the

        resistance at 10 lux, <var>E</var> is the illuminance and gamma is a

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        "constant"</p>
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    <p>Datasheets tend to define
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        <var>gamma = log<sub>10</sub>(R<sub>10</sub>/R<sub>100</sub>)</var></p>

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    <p>
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        Adafruit is the only source I've found with <a class="print"

href="https://cdn-learn.adafruit.com/downloads/pdf/photocells.pdf#page=6"

        >more detailed information about typical characteristics of LDRs:</a>

        slightly curvy traces on a log-log plot.  I meant to add this as a

        reference in part 2, but unfortunately I forgot about and I don't feel

        like making any more late ammendments.</p>

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    <h3>Own "research"</h3>
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    <p>The sensor is designed to translate conductance (the inverse of

        resistance) directly into a voltage: <var>U = k/R</var></p>

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    <p>Approximate model:
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       <var>log<sub>10</sub>(U) = a*log<sub>10</sub>(E)<sup>2</sup>

                                + b*log<sub>10</sub>(E) + c</var></p>

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    <img src="curve.svg" alt=""/>
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    <p>A lot more details are in the second PDF</p>

    <p>The inverse formula is a bit more funny:</p>

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    <img src="formula.svg"
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        alt="Sprechen Sie LaTeX?
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        $$ E = 10^(\frac{0.789-\sqrt{0.488-0.124 log_{10}(U)}}{0.0621}) $$

        E is the illuminance in lux, U is the voltage in volts. $E \pm 9.4\%$"

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    />
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    <p>I like to call this "unevenly engineered". If it was over-engineered

        it wouldn't use a sucky LDR and have a sucky 10% accuracy.</p>

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    <h2>Copyright</h2>
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    <p>I won't bother cluttering the reports any further with explicit

    permissions unless someone actually wants permission to use them.

    If you do please send me a message and I'll do something about it.</p>

    <p>I also may have to do something about one of the images, I'd rather

    make an equivalent myself than trusting fair use if I were to open source

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    this project.</p>
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