Raspberry Pi and Basic Electronics

The purpose of this lab is to introduce you to some very basic electronics and how we can integrate the Raspberry Pi. Please view this as a taster of a much larger area. The assumption is that you have already completed Install Raspbian get connected and basic networking.

A Raspberry Pi 3 Pinout

Getting the packages

To get started, it is important that we have the Python packages that will allow us to manipulate the pins on the Raspberry Pi. For future-proofing, are standardizing all code on Python 3. So to complete this lab, you will need to install some packages:

sudo apt update

Then

sudo apt install python3-pip

After this:

sudo apt install python3-rpi.gpio

Turning on an LED

So in this first activity, I will show you how you can use Python code to turn on an LED in a Raspberry Pi. Start by watching the following video on breadboards: https://www.youtube.com/watch?v=fq6U5Y14oM4. First, we will do the wiring: carefully inspect the pinout of the Raspberry Pi below. This is the Pinout for the Raspberry Pi 3.

Pick out one of your LEDs. Note that one side is longer than then other. The longer side is positive (red) and the shorter side is negative (black).

Raspberry Pi Header: https://learn.sparkfun.com/tutorials/polarity/diode-and-led-polarity

Physical wiring

With this in mind, I want you to use a black, female to male, wire to connect from the ground Pin 6 on the Raspberry Pi, to the negative line attached to the shorter side of the LED.

Now I want you to get a resistor and run the resistor from the positive line of the LED to a separate line I then want you to connect the circuit by running a red, female to male connection between the Raspberry Pi's GPIO 18 pin to join with this resistor.

It may be better to try to copy the diagram below and then re-read the steps above for understanding.

LED wiring

The Python3 code

Now we will write the code to tun on the GPIO 18 Pin to switch on the light.

Open your favourite text editor on the raspberry pi:

nano led_light.py

Then insert the following code.

#!/usr/bin/python3

import RPi.GPIO as GPIO
import time

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

print("LED on")
GPIO.output(18,GPIO.HIGH)
time.sleep(2)
print("LED off")
GPIO.output(18,GPIO.LOW)

Traffic light system

Challenge

You should have a pack of LED lights and resistors. Can you write code to implement a basic traffic light system. Further task can you read about the breadboard's rails (see: https://learn.sparkfun.com/tutorials/how-to-use-a-breadboard/all) and then minimise the wiring to make this traffic light system work.

Save the code and then run it with:

python3 led_light.py

Make sure that your code loops infinitely through the traffic lights by using a while loop.

Inputs that cause outputs

In this next section, we will get use the existing lighting system but we rather than a simple traffic light, we will use lights to represent the varying temperatures of the CPU.

We would like the operation of the fan to turn on to cool the CPU on your mini computer. The following command will show you the current temperature of your CPU

/opt/vc/bin/vcgencmd measure_temp

For me this reports:

temp=32.0'C

But the value will vary. What we want to do is get this value 32 into your python code.

#!/usr/bin/python3

import RPi.GPIO as GPIO
import time
import os
import sys
import signal
import subprocess

GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
GPIO.setup(18,GPIO.OUT)
GPIO.setup(17,GPIO.OUT)

print("LED test")
GPIO.output(18,GPIO.HIGH)
GPIO.output(18,GPIO.HIGH)
time.sleep(1)
GPIO.output(17,GPIO.LOW)
GPIO.output(17,GPIO.LOW)
print("LED test done")

temp = os.popen('vcgencmd measure_temp').readline()
print(temp)
temp= temp.replace("temp=","").replace("'C\n","")
print(temp)

while True:
    if  float(temp) > 45:
        print("Temperature over 45C Warning LED on")
        GPIO.output(18,GPIO.HIGH)
        GPIO.output(18,GPIO.LOW)
        time.sleep(2)
    else:
        print("Temperature below 40C System Normal LED on")
        GPIO.output(17,GPIO.HIGH)
        GPIO.output(17,GPIO.LOW)
        time.sleep(2)

Run the code, in a terminal with:

python temperature_monitoring.py

Generating CPU load

We want to generate load on the CPU; paste the following code into a new text file.

n = 100000
p = 2

for p in range(2, n+1):
    for i in range(2, p):
        if p % i == 0:
            break
    else:
        print p,
print 'Done'

Same as before save it. You can call this primes.py as it will calculate all the prime numbers between 2 and 100,000. As before ensure it has execute permissions then run it with:

python primes.py

You should run it in a separate window. To do this you could use tmux or a second ssh session. You should be able to watch the temperature of the CPU increasing. You can stop the program with Ctrl+c, and the temperature should drop.

Challenge

Modify the code and the wiring so that there are three levels and three different colors that will be displayed, so work out the code and the circuit for low, medium and high temperatures.

Using the Fan

Trying to power through GPIO

Lets start by plugging your fan straight into your Raspberry Pi, Joining the black wire to the ground (pin 9) shown in diagram Xyz and the red wire to GPIO 17 as shown in the same diagram. Refer te the picture of my cabling as well.

Pins in use to power fan through GPIO

File:Attempt to power through GPIO.png

To power fan through GPIO

Then, use the code below to try to run the fan.

 #!/usr/bin/python3

 import RPi.GPIO as GPIO
 import time
 GPIO.setmode(GPIO.BCM)
 GPIO.setwarnings(False)
 GPIO.setup(17,GPIO.OUT)
 print "Fan on"
 GPIO.output(17,GPIO.HIGH)
 time.sleep(1)
 print "Fan off"
 GPIO.output(17,GPIO.LOW)

This should not work. Why? Because the maximum power draw from a GPIO pin is 16 mA.

Trying to power through the 5v

We can, however, power straight through the 5V Pin. Plug your fan straight into the 5V and Gnd as shown in Xyz and the picture

Pins in use to power fan through 5v

File:Attempt to power through 5v.png

Powering fan through 5v

So note that this works, and we didn't even need any code. The problem is that we have no capacity to control this. This 5V pin in continuous, it is the power that is coming over the USB into the Raspberry Pi, but we cannot switch it on and off. Most electronics that have a reasonable power draw we would like the capacity to turn them on and off.

Using a MOSFET

MOSFETS provide us with the capacity to switch on and off a power source. In this activity, we will draw the power from the 5v Raspberry power supply that we just used but for practical purposes, we could have been drawing the power from 12 or 48 volts or more and just using the GPIOs as a switch.