(r2) SessionOnSensorAndActuatorAccess < AFNOG

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---+ Exercises on Sensor and Actuator access
---++ Introduction

Once we are comfortable with Python basics we can try to access the sensors and actuators connected to our Raspberry Pi. In case of this tutorial interfacing is exceedingly simple: All devices are connected through simple GPIO (General Purpose IO) lines, which we can program to be either input or output and to/from which we can read/write logic levels.
<p align="center"><font face="Liberation Sans, sans-serif"><font size="6"> *Exercise Session 2* </font></font></p>
---+ Hardware access
---+++ 1. First try on the LED

Plug the KY-016 LED into the bread board. Connect the GND pin to ground and connect the r(Red) pin to 3-3 V. The LED should light up in red. Try the same thing with the g(Green) and b(Blue) pins.
---+++ 2. A first program to light the LED

Connect the r,g,b pins to the GPIO pins 18,17,22 respectively. Does the LED light up?

Write a python program that switches the red LED on and another one to switch it off and finally a third program to blink it with a frequency of 1 Hz. In order to accomplish this you must:
   * <p>make sure that the pigpiod daemon is running ( use <i>ps ax | grep pigpiod </i>to check)</p>
   * <p>In your program you must import the pigpio module and create a pi object:<br /> <font face="Courier New, monospace">import pigpio</font><br /> <font face="Courier New, monospace">pi = pigpio.pi()<br /> </font><font face="Liberation Serif, serif">This establishes the link of your program with the daemon</font></p>
   * <p><font face="Liberation Serif, serif">set the gpio mode to </font><font face="Liberation Serif, serif"><i>output:<br /> </i></font><font face="Courier New, monospace">_RED = 18 # the red pin is connected to GPIO 18</font><font face="Liberation Serif, serif"><i><br /> </i></font><font face="Courier New, monospace">pi.set_mode(_RED,pigpio.OUTPUT)</font></p>
   * <p><font face="Liberation Serif, serif">and finally write a value to the GPIO pin:<br /> <font face="Courier New, monospace">pi.write(_RED,pigpio.HIGH) # or pigpio.LOW</font></font></p>
   * <p><font face="Liberation Serif, serif">if you also import the module <i>time </i>then you can use its method sleep, to wait for 0.5 s<br /> <font face="Courier New, monospace">time.sleep(0.5)</font></font></p>
---+++ 3. Try all basic colors

Loop over a 3 bit number (0..7) to show all basic colors. Bit 0 of the number corresponds to the red, bit 1 to the green and bit 2 to the blue component. Display each color for 1 s.
---+++ 4. PWM

For each basic color (r,g,b) run through color intensities from 0 .. 255 with PWM:

<font face="Courier New, monospace">pi.setPWM_ductycycle </font>varies the color intensity. Display each color for 50 ms.
---+++ 
---+++ 5. Reading the HC-SR04

The HC-SR04 ultrasonic distance sensor uses 5 pins which must be connected as shown in this table:

<img alt="hc-sr04.png" height="132" src="%ATTACHURL%/hc-sr04.png" title="hc-sr04.png" width="206" />

<table cellpadding="4" cellspacing="0" style="width: 156px;"><col width="69"></col> <col width="69"></col> <tbody> <tr valign="top"> <td width="69">

Vcc
</td> <td width="69">

5V
</td> </tr> <tr valign="top"> <td width="69">

Trig
</td> <td width="69">

5
</td> </tr> <tr valign="top"> <td width="69">

Echo
</td> <td width="69">

6
</td> </tr> <tr valign="top"> <td width="69">

GND
</td> <td width="69">

GND</td> </tr> </tbody> </table> To make a measurement we must trigger the device with a 10 <font face="Liberation Serif, serif">µ</font><font face="Liberation Serif, serif">s pulse on the Trig pin. This will provoke a 8 cycle sonic burst at 40 MHz which will travel to an obstacle and from there back to the sensor. The sensor generates a TTL signal whose length corresponds to the travel time of the ultrasonic wave.</font>

<img alt="hc-sr04Timing.png" height="282" src="%ATTACHURL%/hc-sr04Timing.png" title="hc-sr04Timing.png" width="663" />

The simplest way to read the sensor is by setting the Trig signal high (<font face="Courier New, monospace">pi.write(_TRIG,pigpio.HIGH)</font>) waiting 10 <font face="Liberation Serif, serif">µ</font><font face="Liberation Serif, serif">s </font><font face="Liberation Serif, serif">(</font><font face="Courier New, monospace">time.sleep(0.00001</font><font face="Liberation Serif, serif">) </font><font face="Liberation Serif, serif">before setting it low again.</font>

<font face="Liberation Serif, serif">Then we must read the echo signal, set startTime to the current time when the signal goes high and setting stopTime to the current time when the echo signal goes low again:</font>

<font face="Courier New, monospace">while pi.read(_ECHO)==0:<br /> startTime = time.time()</font>

<font face="Courier New, monospace">while pi.read(_ECHO)==1:<br /> stopTime = time.time()</font>

<font face="Liberation Serif, serif">The travelTime is then stopTime – startTime and the distance traveled:</font>

<font face="Courier New, monospace">soundSpeed = 34300 # speed of sound in air [cm / s]</font>

<font face="Courier New, monospace">distance = travelTime * soundSpeed / 2 <br /> # the sound travels to the obstacle and back</font>

<font face="Liberation Serif, serif">Write the readout program for the HC-SR04 and test it. You may run into a problem that the program blocks from time to time. Any idea why? [[#sdfootnote1sym][<sup>1</sup>]]</font>
---+++ 6. Callbacks

Re-write the program using callbacks. Implement it as a class with a read method that triggers the measurements and returns the result.

<div id="sdfootnote1">

[[#sdfootnote1anc][1]] Raspbian is a multi-tasking program and treats regular interrupts (time, SD card, keyboard …) It may happen that an interrupt arrives when the echo signal changes state and this is then not seen by our program. Either we include a timeout (restart the measurement cycle when we have to wait too long for the transition to occur) or, better, use callbacks to observe the transitions
</div>

-- %USERSIG{UliRaich - 2018-04-25}%

---++ Comments

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Attachments
Topic attachments
I	Attachment	History	Action	Size	Date	Who
txt	fullColors.py.txt	r1	manage	0.7 K	2018-04-25 - 11:58	UliRaich
png	hc-sr04.png	r1	manage	24.1 K	2018-04-25 - 11:41	UliRaich
png	hc-sr04Timing.png	r1	manage	92.0 K	2018-04-25 - 11:45	UliRaich
txt	rgbColorIntensities.py.txt	r1	manage	1.0 K	2018-04-25 - 11:58	UliRaich
txt	rgbLED.py.txt	r1	manage	0.7 K	2018-04-25 - 11:58	UliRaich
gz	rgbLED.tar.gz	r1	manage	0.7 K	2018-04-25 - 11:58	UliRaich
Topic revision: r2 - 2018-05-07 - TWikiAdminUser