mirror of https://github.com/joan2937/pigpio
142 lines
5.0 KiB
Plaintext
142 lines
5.0 KiB
Plaintext
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<p>The following code shows a method of reading analogue sensors on
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the digital input only Pi. A Light Dependent Resistor (LDR)
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varies its resistance according to the incident light
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intensisty.</p>
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<h3>SETUP</h3>
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<img src="images/LDR-fritz.png" alt="fritzing diagram" style=
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"width: 200px; height: 300px;" align="left" hspace="10">The LDR
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used is a Cadmium Sulphide device with a 1MOhm dark resistance and
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2-4KOhm at 100 lux. The capacitor is a 104
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ceramic.<span itemprop="name"><br>
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<br>
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One end of the capacitor is connected to Pi ground.<br>
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<br>
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One end of the LDR is connected to Pi 3V3.<br>
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<br>
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The other ends of the capacitor and LDR are connected to a spare
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gpio.</span><br>
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<p>Here P1-1 is used for 3V3, P1-20 is used for ground, and gpio 18
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(P1-12) is used for the gpio.<br clear="all"></p>
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<p><img src="images/LDR-photo.jpg" style=
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"width: 500px; height: 667px;" alt="photo of set-up"></p>
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<h3>CODE</h3>
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<code>#include <stdio.h><br>
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<br>
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#include <pigpio.h><br>
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<br>
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/*
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-----------------------------------------------------------------------<br>
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<br>
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3V3 ----- Light Dependent Resistor --+-- Capacitor
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----- Ground<br>
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|<br>
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+-- gpio<br>
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<br>
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<br>
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cc -o LDR LDR.c -lpigpio -lpthread -lrt<br>
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sudo ./LDR<br>
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<br>
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*/<br>
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<br>
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#define LDR 18<br>
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<br>
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/* forward declaration */<br>
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<br>
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void alert(int pin, int level, uint32_t tick);<br>
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<br>
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int main (int argc, char *argv[])<br>
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{<br>
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if (gpioInitialise()<0) return 1;<br>
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<br>
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gpioSetAlertFunc(LDR, alert); /* call alert when LDR
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changes state */<br>
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<br>
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while (1)<br>
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{<br>
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gpioSetMode(LDR, PI_OUTPUT); /*
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drain capacitor */<br>
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<br>
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gpioWrite(LDR, PI_OFF);<br>
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<br>
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gpioDelay(200); /* 50 micros is
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enough, 200 is overkill */<br>
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<br>
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gpioSetMode(LDR, PI_INPUT); /* start
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capacitor recharge */<br>
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<br>
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gpioDelay(10000); /* nominal 100
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readings per second */<br>
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}<br>
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<br>
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gpioTerminate();<br>
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}<br>
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<br>
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void alert(int pin, int level, uint32_t tick)<br>
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{<br>
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static uint32_t inited = 0;<br>
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static uint32_t lastTick, firstTick;<br>
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<br>
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uint32_t diffTick;<br>
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<br>
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if (inited)<br>
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{<br>
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diffTick = tick - lastTick;<br>
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lastTick = tick;<br>
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<br>
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if (level == 1) printf("%u %d\ ",
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tick-firstTick, diffTick);<br>
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}<br>
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else<br>
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{<br>
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inited = 1;<br>
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firstTick = tick;<br>
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lastTick = firstTick;<br>
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}<br>
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}<br></code>
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<h3>BUILD</h3>
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<code>cc -o LDR LDR.c -lpigpio -lrt -lpthread<br></code>
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<h3>RUN</h3>
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<code>sudo ./LDR >LDR.dat &</code><br>
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<br>
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While the program is running you can capture the waveform using the
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notification feature built in to pigpio. Issue the following
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commands on the Pi.<br>
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<br>
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<code>pigs no<br>
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pig2vcd </dev/pigpio0 >LDR.vcd &<br>
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pigs nb 0 0x40000 # set bit for gpio 18<br></code>
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<p>Change the light falling on the LDR for a few seconds (e.g.
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shine a torch on it or shade it with your hands).<br></p>
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<code>pigs nc 0</code><br>
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<p>The file LDR.vcd will contain the captured waveform, which can
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be viewed using GTKWave.</p>
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<p>Overview</p>
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<img src="images/LDR-wave-1.png" style=
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"width: 600px; height: 100px;" alt="LDR waveform 1"><br>
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<p>Reading circa every 10ms<br></p>
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<img src="images/LDR-wave-2.png" style=
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"width: 600px; height: 100px;" alt="LDR waveform 2"><br>
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<p>One reading, circa 400us<br></p>
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<img src="images/LDR-wave-3.png" style=
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"width: 600px; height: 100px;" alt="LDR waveform 3"><br>
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<p>The file LDR.dat will contain pairs of timestamps and recharge
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time (in us). The following script will convert the
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timestamps into seconds.<span style=
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"font-style: italic;"><br></span></p>
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<p><code>awk '{print $1/1000000, $2}' LDR.dat
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>LDR-secs.dat</code></p>
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<p>Gnuplot is a useful tool to graph data.<br></p>
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plot [14:24] 'LDR-secs.dat' with lines title 'LDR'
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<p>Gnuplot readings 14-24 seconds<br></p>
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<p><img src="images/LDR-gnup-1.png" style=
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"width: 600px; height: 321px;" alt="gnuplot 1"></p>
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plot [18:21] 'LDR-secs.dat' with lines title 'LDR'<br>
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<br>
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Gnuplot readings 18-21 seconds
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<p><img src="images/LDR-gnup-2.png" style=
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"width: 600px; height: 321px;" alt="Gnuplot 2"></p>
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