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

Last modified	2026-06-27
Lines	134

Parent directory Download CGIread sitemap Main page

Quick links: contact content footer navigation title

```
<!DOCTYPE html>
```

<html lang="en" xmlns="http://www.w3.org/1999/xhtml">

```
    <head>
```
```
        <meta charset="utf-8"/>
```

        <meta name="viewport" content="width=device-width, initial-scale=1"/>

        <link rel="stylesheet" href="https://oskog97.com/style.css" type="text/css"/>

        <link rel="icon" type="image/png" href="/favicon.png"/>

        <link rel="canonical" href="https://oskog97.com/projects/light-sensor/"/>

```

```

        <title>Bloody light sensor</title>

        <meta name="description" content="Analog light sensor"/>

```
    </head>
```
```
    <body>
```
```

```

<nav><div id="navigation"><div id="nav_inner">

<p><a href="#content" class="textonly">Skip navigation</a></p>

```
<p class="row">
```

<span class="textonly" translate="no">[</span><a class="head" href="/">Home</a><span class="textonly" translate="no">]</span>

```
&gt;&gt;
```

<span class="textonly" translate="no">[</span><a class="sub" href="/projects/anonymine/">Anonymine</a><span class="textonly" translate="no">]</span>

<span class="textonly" translate="no">]</span><span class="sub active">Analog light sensor</span><span class="textonly" translate="no">[</span>

<span class="textonly" translate="no">[</span><a class="sub" href="/projects/PLLM-M702A/">Reverse-engineered schematics for PLLM-M702A</a><span class="textonly" translate="no">]</span>

<span class="textonly" translate="no">[</span><a class="sub" href="/small-scripts/">Small scripts</a><span class="textonly" translate="no">]</span>

```
</p>
```
```
<p class="row">
```

<span class="textonly" translate="no">[</span><a class="sub" href="/sitemap.py">Sitemap</a><span class="textonly" translate="no">]</span>

```
</p>
```
```
<hr class="textonly"/>
```
```
</div></div></nav>
```
```

```
```
<main><div id="content">
```

    <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>

```
    <ul>
```

        <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>

```
    </ul>
```

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

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

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

```
    <h2>LDR characteristic curve</h2>
```

    <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

```
        "constant"</p>
```
```
    <p>Datasheets tend to define
```

        <var>gamma = log<sub>10</sub>(R<sub>10</sub>/R<sub>100</sub>)</var></p>

```
    <p>
```

        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>

```
    <h3>Own "research"</h3>
```

    <p>The sensor is designed to translate conductance (the inverse of

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

```
    <p>Approximate model:
```

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

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

```
    <img src="curve.svg" alt=""/>
```

    <p>A lot more details are in the second PDF</p>

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

```
    <img src="formula.svg"
```
```
        alt="Sprechen Sie LaTeX?
```

        $$ E = 10^(\frac{0.789-\sqrt{0.488-0.124 log_{10}(U)}}{0.0621}) $$

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

```
    />
```

    <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>

```
    <h2>Copyright</h2>
```

    <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

```
    this project.</p>
```
```
</div></main>
```
```

```
```
<footer><div id="footer">
```
```
    <hr class="textonly"/>
```
```
    <p>
```

        Website content released under the <a

        href="https://creativecommons.org/licenses/by/4.0/" rel="license noopener"

        target="_blank">Creative Commons Attribution (CC-BY 4.0)</a> license

        and my software usually under the <span class="a"><a target="_blank"

```
        rel="noopener"
```

        href="https://opensource.org/license/BSD-2-Clause">FreeBSD license

```
        (2-clause)</a>.</span>
```
```
        <br/>
```

        Images may be from other sites, I should have cited useful sources

```
        somewhere on the page.
```

        <span class="notprint">Contact me if I haven't.</span>

```
    </p>
```
```
    <p id="contact" class="notprint">
```

        You can contact me at: <a href="mailto:oskar@oskog97.com"

        rel="noopener" target="_blank">oskar@oskog97.com</a>

        <span class="a">(<a href="/pgp-pub/oskar.asc"

                            >PGP public key</a>)</span>

```
    </p>
```

    <p> <a class="notprint" href="https://oskog97.com/read/?path=/style.css">

```
            CSS Stylesheet
```
```
        </a>
```
```
    </p>
```
```
</div></footer>
```
```

```
```
    </body>
```
```
</html>
```