Lab 2

Intro to Electronics
GPIO basics
Controlling an LED
Controlling the brightness of an LED using pulse-width modulation (PWM)
Making sound using a piezo buzzer
Building user interfaces to control your Pi apps

Intro to Electronics

http://www.instructables.com/id/Basic-Electronics/*** (steps 1 through 17)
https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law***

GPIO basics and installing RPi.GPIO

Go through recipes 9.1 and 9.2.

Note that the Raspberry Pi 3 model B we are using has 40 pins and its pin layout is:

Connecting an LED

Do recipe 10.1. Start by creating the circuit shown in Figure 10-1. Note that the LED must be placed so that it's longer pin is on the high side (3.3V) of the circuit (rather than ground or 0V).

WARNING: Make sure your are using a series resistor in the circuit containing the LED.

With a voltage of 3.3V and a 1kOhm resistor, Ohm's Law says that the amount of current I through the LED satisfies the equation 3.3 volts = I x 1000 ohms so the amount of current going through the LED is no more than 3.3/1000 = .0033 amps or 3.3 milliamps.

To test your set up, you can move the wire going from pin GPIO18 to a 3.3V pin; if you do so, the LED should light up. Once you have made the check, replace the wire back on pin GPIO18.

The code on page 241 that gets the LED blinking is:

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

while (True):
    GPIO.output(18, True)
    time.sleep(0.5)
    GPIO.output(18, False)
    time.sleep(0.5)

Functions from module RPi.GPIO:

setmode() : There are two ways of numbering the I/O pins on a Raspberry Pi within RPi.GPIO. We will be using the numbering system BCM defined by BCM numbers (defined by the chip manufacturer Broadcom). The BCM numbers are those following the word GPIO in the above figure.
setup(): Any pin you are using must be set up as an input (IN) or output (OUT) pin; in this recipe, we are setting the pin as an output pin.
output(): sets the state of an output pin.

For more detailed info, see the documentation for the RPi.GPIO module at

http://sourceforge.net/p/raspberry-gpio-python/wiki/BasicUsage/ ***

Leaving the GPIO pins in a safe state

Do recipe 10.2.

Raspberry Pi Cookbook

Controlling the brightness of an LED using pulse-width modulation (PWM)

Raspberry Pi's GPIO pins do digital I/O rather than analog I/O. In other words, they can output or sense (for input pins) either 0V or 3.3V but nothing in between.

Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. Digital control is used to create a square wave, a signal switched between on and off. The number of times the signal is on during 1 second is the PWM frequency; it is expressed in Hertz or Hz.
This on-off pattern can simulate voltages in between full on (3.3 Volts) and off (0 Volts) by changing the portion of the time the signal spends on versus the time that the signal spends off. The duration of "on time" is called the pulse width and the percentage of "on time" is the duty cycle. To get varying analog values, you change, or modulate, that pulse width (or duty cycle). If you repeat this on-off pattern fast enough with an LED for example, the result is as if the signal is a fixed, steady voltage between 0 and 3.3v controlling the brightness of the LED. (From http://arduino.cc/en/Tutorial/PWM.)

Do recipe 10.3 which illustrates how to use PWM on a Pi. You will run program led_brightness.py which you can do as described in the book or from within IDLE3 as explained above:

import RPi.GPIO as GPIO

led_pin = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(led_pin, GPIO.OUT)

pwm_led = GPIO.PWM(led_pin, 500)
pwm_led.start(100)

while True:
        duty_s = raw_input("Enter Brightness (0 to 100):")
        duty = int(duty_s)
        pwm_led.ChangeDutyCycle(duty)

Functions from module RPi.GPIO:

PWM(): constructor that creates a PWM instance; the frequency of the square wave is specified here.
start(): starts PWM instance; takes the initial duty cycle as an input argument.
ChangeDutyCycle(): changes the duty cycle.

For more detailed info, see

http://sourceforge.net/p/raspberry-gpio-python/wiki/PWM/ ***

Making a sound using a piezo buzzer

Do recipe 10.4 to see how to control electronics other than LEDs. The code you will execute is:

import RPi.GPIO as GPIOimport timebuzzer_pin = 18GPIO.setmode(GPIO.BCM)GPIO.setup(18, GPIO.OUT)def buzz(pitch, duration): period = 1.0 / pitch delay = period / 2 cycles = int(duration * pitch) for i in range(cycles): GPIO.output(buzzer_pin, True) time.sleep(delay) GPIO.output(buzzer_pin, False) time.sleep(delay) while True: pitch_s = raw_input("Enter Pitch (200 to 2000): ") pitch = float(pitch_s) duration_s = raw_input("Enter Duration (seconds): ") duration = float(duration_s) buzz(pitch, duration)

Building user interfaces to control your Pi apps

You may want to control your Pi applications using GUIs rather than the IDLE interpreter.

Do recipes 10.8, 10.9, and 10.10 to see examples of this. Note that recipe 10.10 uses a RGB LED rather than a plain RED LED.

For a refresher on developping GUI apps with tkinter, see Chapter 9 of your CSC 241/242/243 textbook or https://docs.python.org/3/library/tkinter.html.

Charlieplexing

Do recipe 10.11.

This recipe illustrates how electricity will flow when several output and input pins are tied in a circuit. The flow will be from a high output pin to a low output pin. Input pins can detect voltage but no electricity flows through them.

Homework

Re-read the recipes in the textbook we have covered in Lab 2. Also go through the starred (***) online tutorials and online documentation covered in Lab 2.