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diff --git a/DOC/src/html/ex_LDR.html b/DOC/src/html/ex_LDR.html new file mode 100644 index 0000000..3c54c68 --- /dev/null +++ b/DOC/src/html/ex_LDR.html @@ -0,0 +1,150 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> +<head> +<meta name="generator" content="Bluefish 2.2.10" > +<meta content="text/html; charset=utf-8" http-equiv="Content-Type"> +<title>LDR example</title> +</head> +<body> +<p>The following code shows a method of reading analogue sensors on +the digital input only Pi. A Light Dependent Resistor (LDR) +varies its resistance according to the incident light +intensisty.</p> +<h3>SETUP</h3> +<img src="images/LDR-fritz.png" alt="fritzing diagram" style= +"width: 200px; height: 300px;" align="left" hspace="10">The LDR +used is a Cadmium Sulphide device with a 1MOhm dark resistance and +2-4KOhm at 100 lux. The capacitor is a 104 +ceramic.<span itemprop="name"><br> +<br> +One end of the capacitor is connected to Pi ground.<br> +<br> +One end of the LDR is connected to Pi 3V3.<br> +<br> +The other ends of the capacitor and LDR are connected to a spare +gpio.</span><br> +<p>Here P1-1 is used for 3V3, P1-20 is used for ground, and gpio 18 +(P1-12) is used for the gpio.<br clear="all"></p> +<p><img src="images/LDR-photo.jpg" style= +"width: 500px; height: 667px;" alt="photo of set-up"></p> +<h3>CODE</h3> +<code>#include <stdio.h><br> +<br> +#include <pigpio.h><br> +<br> +/* +-----------------------------------------------------------------------<br> + +<br> + 3V3 ----- Light Dependent Resistor --+-- Capacitor +----- Ground<br> + +|<br> + ++-- gpio<br> +<br> +<br> + cc -o LDR LDR.c -lpigpio -lpthread -lrt<br> + sudo ./LDR<br> +<br> +*/<br> +<br> +#define LDR 18<br> +<br> +/* forward declaration */<br> +<br> +void alert(int pin, int level, uint32_t tick);<br> +<br> +int main (int argc, char *argv[])<br> +{<br> + if (gpioInitialise()<0) return 1;<br> +<br> + gpioSetAlertFunc(LDR, alert); /* call alert when LDR +changes state */<br> + <br> + while (1)<br> + {<br> + gpioSetMode(LDR, PI_OUTPUT); /* +drain capacitor */<br> +<br> + gpioWrite(LDR, PI_OFF);<br> +<br> + gpioDelay(200); /* 50 micros is +enough, 200 is overkill */<br> +<br> + gpioSetMode(LDR, PI_INPUT); /* start +capacitor recharge */<br> +<br> + gpioDelay(10000); /* nominal 100 +readings per second */<br> + }<br> +<br> + gpioTerminate();<br> +}<br> +<br> +void alert(int pin, int level, uint32_t tick)<br> +{<br> + static uint32_t inited = 0;<br> + static uint32_t lastTick, firstTick;<br> +<br> + uint32_t diffTick;<br> +<br> + if (inited)<br> + {<br> + diffTick = tick - lastTick;<br> + lastTick = tick;<br> +<br> + if (level == 1) printf("%u %d\ ", +tick-firstTick, diffTick);<br> + }<br> + else<br> + {<br> + inited = 1;<br> + firstTick = tick;<br> + lastTick = firstTick;<br> + }<br> +}<br></code> +<h3>BUILD</h3> +<code>cc -o LDR LDR.c -lpigpio -lrt -lpthread<br></code> +<h3>RUN</h3> +<code>sudo ./LDR >LDR.dat &</code><br> +<br> +While the program is running you can capture the waveform using the +notification feature built in to pigpio. Issue the following +commands on the Pi.<br> +<br> +<code>pigs no<br> +pig2vcd </dev/pigpio0 >LDR.vcd &<br> +pigs nb 0 0x40000 # set bit for gpio 18<br></code> +<p>Change the light falling on the LDR for a few seconds (e.g. +shine a torch on it or shade it with your hands).<br></p> +<code>pigs nc 0</code><br> +<p>The file LDR.vcd will contain the captured waveform, which can +be viewed using GTKWave.</p> +<p>Overview</p> +<img src="images/LDR-wave-1.png" style= +"width: 600px; height: 100px;" alt="LDR waveform 1"><br> +<p>Reading circa every 10ms<br></p> +<img src="images/LDR-wave-2.png" style= +"width: 600px; height: 100px;" alt="LDR waveform 2"><br> +<p>One reading, circa 400us<br></p> +<img src="images/LDR-wave-3.png" style= +"width: 600px; height: 100px;" alt="LDR waveform 3"><br> +<p>The file LDR.dat will contain pairs of timestamps and recharge +time (in us). The following script will convert the +timestamps into seconds.<span style= +"font-style: italic;"><br></span></p> +<p><code>awk '{print $1/1000000, $2}' LDR.dat +>LDR-secs.dat</code></p> +<p>Gnuplot is a useful tool to graph data.<br></p> +plot [14:24] 'LDR-secs.dat' with lines title 'LDR' +<p>Gnuplot readings 14-24 seconds<br></p> +<p><img src="images/LDR-gnup-1.png" style= +"width: 600px; height: 321px;" alt="gnuplot 1"></p> +plot [18:21] 'LDR-secs.dat' with lines title 'LDR'<br> +<br> +Gnuplot readings 18-21 seconds +<p><img src="images/LDR-gnup-2.png" style= +"width: 600px; height: 321px;" alt="Gnuplot 2"></p> +</body> +</html> |