At CoderDojo Athenry, the Hackers spent some time examining the components of a desktop PC and a Raspberry Pi 3+ and a Raspberry Pi Zero.

Even though the Pis are much smaller than a desktop PC, they are functionally equivalent – as we saw, you can plug the Pi into the keyboard, mouse and screen of the desktop PC and use it like one.

We identified the major components of a desktop PC, and saw where each of them appear on the Raspberry Pi also:

• CPU – the central processing unit that does all calculations and processing. All data in a PC gets represented as numbers, so all data processing ends up as calculations.
• GPU – a dedicated processing unit just for graphics, that specialises in multiplying and adding matrices (pixels on a screen are represented as a matrix). Not all PCs have one, but they are important for high-performance graphics.
• RAM – the short-term memory of the computer, used by the CPU to store data.
• Hard Drive – this might be a hard disk drive or a solid-state drive. This is for long-term storage. It holds much more than RAM and the data remains when the PC is powered off, but it is much slower for the CPU to get data from the hard drive than from RAM.
• DVD Drive – not all PCs have this. DVDs or CDs allow permanent storage that can be removed. Some are read-only and some allow reading and writing.
• Motherboard – the circuit board on which everything else is mounted.
• Power Supply – this is built into a desktop PC. For a Pi, this is a 5-volt supply such as a phone charger.
• Networking – ethernet for wired networks and/or wifi for wireless networks.
• Controller chips and connection ports (such USB and HDMI) for peripherals.
• Case – Pis don’t always have these.

We noted that the Pi has a single chip that has its CPU, a basic GPU and up to 1GB of RAM all stacked in layers on top of each other. While its CPU is lower power than a standard PC CPU, it benefits from having a really short distance that data has to travel from RAM to CPU. CPUs run so fast that having electrons travel a few centimetres is a significant delay!

PCs and the Pi also have connections for peripherals, which is anything that can be connected to it, using USB, Bluetooth, HDMI, or other connection types:

• Keyboard and mouse
• Screen

The Raspberry Pi Zero has micro-USB and micro-HDMI connectors to keep everything as small as possible, and it has wifi only, no ethernet port (though it is possible to get a micro-USB to ethernet adapter).

A couple of members of the group have built their own desktop PCs, which is an impressive feat!

In the past two weeks in the Hackers group at CoderDojo Athenry, we have started Python programming on the Raspberry Pi.

The Pi is about the same size as the Arduino that we used earlier, and the Pi Zero is about the size of the Arduino Nano, and both Pi and Arduino have input/output pins for physical computing. However, they have significant differences.

Unlike the Arduino which is a microcontroller (which means it is designed to run a single program that was uploaded onto it), the Raspberry Pi has a full computer operating system, so it is more like a PC to use. It can be programmed in many languages, but Python is a popular choice as it is clear to read and there are lots of libraries to make tasks easier. Because it’s a full computer, you can write and run your programs all on the Pi, without connecting it to a laptop.

The first step in programming is to figure out how to do loops, variables and decisions, as these are fundamental. Here is our first Python program to try out these:

# Python comments start with #

age = 14 # a variable holding an int
name = "Michael" # variable holding a string

# Output
print ("My name is", name, "and my age is ", age)

# Loop
for x in range (1, 5):
    print ("This is line ", x)

# Decision
if (age  17):
    print("Adult")
else:
    print("Teenager")

Next we moved on to using the GPIOZero libraries for controlling lights and buttons. We will continue to explore this in the coming weeks.

The documentation is here: https://gpiozero.readthedocs.io/en/stable/

This week in the Hackers group at CoderDojo Athenry, we built on last week’s work on blinky lights, in which we made a simple circuit involving LEDs and resistors connected to an Arduino, and wrote code to get the LEDs to blink.

An LED is an example of an output from our microcontroller. We would also like to have inputs. Examples of circuit inputs are:

• Switches
• Dials
• Sensors that measure something

We focused on dials, specifically a variable resistor or rheostat. This is the kind of knob or dial you find on dimmer switches, volume controls on old radios, electric guitars, and many others.

A variable resistor has 3 connectors: the two outer ones connect across a voltage source (e.g. 5V and ground pins on the Ardiuno) and the voltage at the middle pin can be adjusted from 0 to 5V by turning the knob.

We connected the middle pin of the variable resistor to Analog input 2 of the Arduino. The connections are shown above.

Then, the code to read its value is:

potValue = analogRead(potPin);

where potValue and potPin are ints that were defined already.

The value that you get is in the range 0-1024, and changes as you turn the dial.

Here is a full Arduino program to read a value and display it on the Serial Monitor window if you have a computer connected to your Arduino:

int potPin = 2;    // select the input pin for the potentiometer
int potValue = 0;  // value to read from potentiometer

void setup() {
  Serial.begin(9600);  // need for print commands later
}

void loop() {
  potValue = analogRead(potPin);    // read the value from the sensor
  Serial.println(potValue);
}

In the group, we used this as the basis to improve last week’s program. This time, the speed at which the LED blinks is controlled by turning the potentiometer dial.

The previous code to control how long the LED blinks for was:

delay(one_second);

We changed this to:

delay(potValue);

Of course, we also had to add the code to read the potentiometer value at the top of the loop() function.

Today we built our first circuits on a breadboard.

A breadboard is used to try out electronic circuits and figure out what works.  Once we are happy with a circuit we can make a permanent version on something like strip-board.

Breadboards have blue and red lines across the top and bottom.  A wire strip runs across the board under each line.  If we connect a power source to any point on the lines, all of the points (holes) on the line receive the power.

The middle area of the board has wire strips running vertically.  A wire strip runs down the board under each line of holes.  If we connect a power source to any point on the lines, all of the points (holes) above or below receive power.

The first circuit was one to power a single LED:

We used the Arduino here as a simple 5V battery.  Current flows from the 5V pin on the Arduino, through the resistor, then the LED, and on to the GND pin on the Arduino.  We should always used a resistor with an LED to protect it.  It doesn’t take much current to damage an LED.

This has the LED on permanently – not very exciting, but good to get started with.  It’s a good idea to start with a minimal circuit and build up to the more complicated circuit we actually want.  That way, we can test and verify as we go, so that we don’t have too much testing and backtracking to do if something goes wrong.

It took a while to get everyone’s circuit working.  Some of the LEDs had legs that were the same length, and there was no obvious way to see which one was the anode or cathode, so we had to guess and test.
Some of the resistors we took at random from the box were too high:  200 kΩ, for example, so the LEDs wouldn’t light up.  We found some 220 Ω (red-red-brown) and 330Ω (orange-orange-brown) resistors to get us going.
Some of the LEDs were very dim.  180 Ω resistors (brown-grey-brown) helped there.
We did a quick calculation to see what value of resistor we really needed:

According to the spec sheet, our LEDs had a forward voltage of 2.2 V and a forward current of 20 mA. In other words, the voltage drop across the LED is 2.2 V, and they can draw up to 0.02 A without burning out.
If our supply is 5 V, then the voltage across the resistor is 5 – 2.2 = 2.8 V.  This is Kirchoff’s Voltage Law.
Applying Ohm’s Law (V = RI), 2.8 = R*0.02, so R = 2.8/0.02 = 140 Ω.
A 140 Ω resistor will make our LED as bright as possible. Less than that will burn out the LED. More than that will produce a dimmer light. So, 180, 220, and 330 Ω are all fine.
Different LEDs have different forward voltages and currents, so it’s important to keep and consult the specification sheets before using them.

The next step in our circuit building was to connect the Arduino’s ground pin to the ground rail on the breadboard.  Then we connected the negative (cathode) pin of the LED to the ground rail.  This is an important step in circuit building:  using a common ground for all our circuits on one board.  It allows us to only have one wire running back to the Arduino ground, instead of one for each circuit.  We would run out of space on the Arduino very quickly otherwise.
Next we did the same on the positive side. This isn’t necessarily as useful as the common ground as we’ll see later.

Once we were happy that we all had a working circuit, we moved on to use the Arduino to control the LED.
We did this by taking the wire that went to the 5V pin on the Arduino and plugging it into digital pin 12 instead.  We went back to our Arduino sketches and defined a new integer variable, ledPin, and gave it a value of 12 to match where we plugged it in.
Then we changed every occurrence of LED_BUILTIN in our sketches from last week to ledPin instead.  The Find (Replace with) function on the Edit menu of the IDE was helpful here.

Once we compiled the sketch and uploaded it to the Arduino, we had the LED on
the breadboard blinking our messages, just like the internal LED was last week.

Next, we added a second LED and resistor to the board, and wired it up the same as the first.  The positive side of the resistor was connected to the positive power rail, and from there on to the Arduino. Now both LEDs blinked together.
To get them to blink separately, we took the wire from our second LED going to the positive rail and plugged it into digital pin 8 on the Arduino.

Then we created a new variable in our sketch, led2Pin, with a value of 8.
We added setup code for pin 8.  Finally, we added two more lines to the loop, to have the output to LED 2 go low after LED 1 was set high, and vice versa.
The outcome was LED 2 was on when LED 1 was off and LED 2 was off when LED 1 was on.
Some demonstration code is shown below.

// Blink 2 LEDs: one on, one off
// Demonstration for CoderDojo Athenry

int led1Pin = 12;
int led2Pin = 8;

int one_second = 1000;  // delay() function expects milliseconds

void setup() {
  pinMode(led1Pin, OUTPUT);
  pinMode(led1Pin, OUTPUT);
}

void loop() {
  digitalWrite(led1Pin, HIGH);  // on
  digitalWrite(led2Pin, LOW);
  delay(one_second);
  digitalWrite(led1Pin, LOW);  // off
  digitalWrite(led2Pin, HIGH);
  delay(one_second);
}