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67 lines
2.8 KiB
67 lines
2.8 KiB
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\documentclass[11pt]{article}
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%Gummi|065|=)
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\usepackage{graphicx}
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\usepackage{caption}
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\title{\textbf{Flammable Gas Sensor}}
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\author{Steak Electronics}
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\date{}
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\begin{document}
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\maketitle
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\tableofcontents
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\section{Overview}
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Shop needs a flammable gas sensor, for safety.
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\section{Chip Hunting}
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I'm looking at the following:
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\begin{itemize}
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\item SGAS711
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\item 200K fixed resistor
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\item 1M potentiometer
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\item Arduino Nano (for speed)
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\item Ample Power Supply They are recommending 7 Volts for the heater. So, one rated for 1A.
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\item led notifiers
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\item
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\end{itemize}
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\subsection{Gas Sensor Tuning}
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The flammable gas sensor has different sensitivities for different gases (see data sheet, Figure 8). I'm going to need to test for something specific, i.e. if the shop needs to watch out for acetone, I should test acetone. Of course, if enough of a flammable gas is in the air, it will set it off no matter what, but I should focus on what the danger is for calibrating.
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Based on the resistance chart, I'm going to use a fixed 200K and a 1M pot. Pots are to be avoided, but here we need to calibrate over time. For the response of the v divider, the sensor is not linear, but closer (though not quite) logarithmic. So what I will do, is have to use some math on the micro, and use the formula they give in the Datasheet, to get a logarithmic output that appears linear (figure 5). For my needs, it is good enough.
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\includegraphics[scale=0.8]{../pics/resistances.png}
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\captionof{figure}{Application Note resistance table}
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\subsubsection{Fixed resistors}
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If these sensors are consistent enough, possibly I could use fixed resistors later.
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\subsubsection{Gas Sensor Power Usage}
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Rated at 900mW for 7V, so about 150mA (128mA). I know from prior experience these things heat up, so we need plenty of power.
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\subsubsection{PCB Layout}
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The gas sensor must be sideways, as there is excessive dust in the shop so, the holes will be on the side. I will do a 90 degree edge mount pcb.
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\subsection{Switcher}
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STS1024S6V5 Seems like a fair option for now. Output is 6.5 volts which is enough. Will use a module. Need to make a footprint.
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\subsection{Enclosure}
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We need a box that is tall enough to be a cube. Also want square, not rectangular. I plan to have the leds light from the back of the pcb. The pcb will be the top cover / front. The top cover / front will be the box, put on its side, so dust doesn't collect on the leds. Need a cube.
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\section{CAD Layout}
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I found that Kicad step up in Freecad is helpful for making sure your board will fit the enclosure. A very helpful addition to an arsenal. Although I didn't test it until after rev 1.
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\begin{center}
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\includegraphics[scale=0.5]{../pics/kicadstepup1.jpg}
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\captionof{figure}{Freecad has the ability to pull in boards from Kicad. Even without step file dependencies you can see how the PCB will fit a case}
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\end{center}
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\end{document}
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