pi3 gpio

Electronics With The Raspberry Pi

If you have any questions or get stuck as you work through this in-class exercise, please ask the instructor for assistance. Enjoy!

Blink

We begin this activity with a classic example of GPIO (general purpose input or output) control. Connect an LED in series with a 220Ω resistor between GPIO pin 25 and ground.
Create a new Python file: sudo nano blink.py
Enter the following:
```
import RPi.GPIO as gpio
from time import sleep

pin = 25
gpio.setmode(gpio.BCM)
gpio.setup(pin, gpio.OUT)

while True:
    gpio.output(pin, gpio.HIGH)
    sleep(1)
    gpio.output(pin, gpio.LOW)
    sleep(1)
```
The pin numbering is set to the Broadcom CPU’s definitions with gpio.setmode(gpio.BCM) and pin 25 is initialized as an output with gpio.setup(pin, gpio.OUT)
GPIO pins can be in one of two states:
- 0V (ground): gpio.LOW
- 3.3V: gpio.HIGH
Save and exit by pressing “ctrl” and “X” then “Y”.
Run the program and watch the LED blink on and off: python3 blink.py
Press “ctrl” and “C” to exit the program.
- Note that exiting while the LED is on will cause it to stay on.

Fade

Despite GPIO pins being either on or off, the brightness of an LED or the speed of a motor can be controlled by switching the pin on and off very quickly. This technique is called Pulse Width Modulation (PWM).
Keep the wiring of the blink example.

Create a Python file named fade.py and enter the following:

 import RPi.GPIO as gpio
 from time import sleep

 pin = 25
 gpio.setmode(gpio.BCM)
 gpio.setup(pin, gpio.OUT)

 pwm = gpio.PWM(pin, 100)
 pwm.start(0)

 for dc in range(0, 100, 1):
     pwm.ChangeDutyCycle(dc)
     sleep(0.05)

 for dc in range(100, 0, -1):
     pwm.ChangeDutyCycle(dc)
     sleep(0.05)

 pwm.ChangeDutyCycle(0)

Pin 25 is assigned pulse width modulation and the duty cycle (time interval of the pulses) is increased then decreased incrementally.

Upon running this program, the LED will appear to get brighter then dimmer.

Push Button

Connect a momentary push button in series with a 10kΩ resistor between GPIO pin 25 and 3.3V power.

Create a Python file named button.py and enter the following:

import RPi.GPIO as gpio

pin = 25
gpio.setmode(gpio.BCM)
gpio.setup(pin, gpio.IN, pull_up_down=gpio.PUD_DOWN)

while True:
    if gpio.input(pin) == gpio.HIGH:
        print("Button Pushed")

The new parameter pull_up_down=gpio.PUD_DOWN will internally connect the pin to ground with a very high value resistor.

Run the program and then press the button. The terminal should be filled with “Button Pushed” due to the while loop repeating many times per second.
In some cases, a more useful approach to GPIO input is callbacks. These work by interrupting the program to call a function. In this example, buttonPressedCallback will interrupt a counting process:
```
import RPi.GPIO as gpio
from time import sleep

def buttonPressedCallback(channel):
    print("Pressed")

pin = 25
gpio.setmode(gpio.BCM)
gpio.setup(pin, gpio.IN, pull_up_down=gpio.PUD_DOWN)
gpio.add_event_detect(pin, gpio.RISING, callback=buttonPressedCallback)
i = 0
     
while True:
    print(i)
    i = i + 1
    sleep(1)
```
The parameter gpio.RISING specifies calling the function when the state changes from low to high. Alternatively, gpio.FALLING could have been used for high to low or gpio.BOTH for either case.

Analog Input

Once again we encounter the obstacle of binary state GPIO pins. However, analog to digital conversion is still possible by measuring the amount of time a capacitor takes to charge from a “LOW” to a “HIGH” voltage level.
List of Parts:
- 1x Potentiometer (Variable Resistor)
- 1x 0.22μF Capacitor
- 2x 1kΩ Resistor
- Wires
Connect the components as shown in this diagram:
Warning: This type of capacitor is known to explode if connected backwards. The side of the capacitor marked with a line can only be connected to ground or a voltage lower than the other side.

Create a Python file named analog.py and enter the following:

import RPi.GPIO as gpio
from time import sleep

pin1 = 23
pin2 = 24
gpio.setmode(gpio.BCM)

def discharge():
    gpio.setup(pin1, gpio.IN)
    gpio.setup(pin2, gpio.OUT)
    gpio.output(pin2, gpio.LOW)
    sleep(0.004)

def chargeTime():
    gpio.setup(pin1, gpio.OUT)
    gpio.setup(pin2, gpio.IN)
    gpio.output(pin1, gpio.HIGH)
    t = 0
    while not gpio.input(pin2):
        t = t + 1
    return t

def analogRead():
    discharge()
    return chargeTime()

while True:
    print(analogRead())
    sleep(1)

Run the program and watch the printed value change as you turn the knob of the potentiometer. The animation below helps to visualize what is happening many times per second.

GPIO Speaker

This final circuit will be a simple transistor amplifier to play audio using a hardware PWM pin. Keep in mind the audio quality of this circuit is going to be nowhere near excellent, and most models of the Raspberry Pi have a 3mm audio jack for connecting better quality speakers. The only practical use of this circuit would be playing audio from a Raspberry Pi Zero with no audio jack.
List of Parts:
- 1x Speaker
- 1x NPN BJT Type Transistor (The DSC has BC547s)
- 1x 0.1μF Capacitor
- 1x 1μF Capacitor
- 1x 10Ω Resistor
- 2x 1kΩ Resistor
- 1x 10kΩ Resistor
- Wires
Connect the components as shown in this diagram:

The capacitor pointing right in the diagram is 0.1μF and the capacitor pointing down is 1μF.
The 10Ω resistor between the speaker and 3.3V helps limit the current through the transistor and did not restrict the volume during testing. This circuit will not require another Python file.
Edit the boot config file: sudo nano /boot/config.txt
Add a new line dtoverlay=audremap,pins_12_13 to the end of the file then save and exit. This assigns 2 channel audio output to use the hardware PWM pins 12 and 13.
Test the audio output: speaker-test -c2
Wait until you hear static, then press “ctrl” and “C” to exit.
Open Chromium and browse for music on YouTube. You will notice that piano notes in particular are extremely distorted. I warned you about the audio quality!

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