Lights and the first program

Electricity flows in a loop

Electricity only flows round a complete loop, called a circuit. Break the loop anywhere and it stops.

On an Arduino, 5V is the + side. GND (ground) is the − side, at 0 volts.

The parts

• Arduino Uno: a small board with a microcontroller, a tiny computer on one chip. Wires join it at its numbered pins.
• Breadboard: a board for building circuits without solder. The long + and − rails carry power along its edges.
• LED (light-emitting diode): a small light that lets current through one way only.
• Resistor: limits the current. It works either way round.
• Pushbutton: a switch. Pressed, it closes the loop. Let go, it breaks the loop.

An LED's anode (+) must face the + side. Its cathode (−) faces the − side.

On a real LED the anode is the longer leg. In Tinkercad it is the bent leg.

Why an LED needs a resistor

With no resistor, too much current flows. The LED burns out, and the Arduino pin can be damaged.

Ohm's law links the three words: V = I × R. Turned round, it tells you the resistor you need.

1. Take the LED's share away from the supply: 5 − 2 = 3 V.
2. Divide by the current: 3 ÷ 0.02 = 150 Ω.
3. Use 150 Ω, or the next size up. Kits use 220 Ω.

Blink: the first program

Wire pin 13 to the resistor instead of the button. Now the Arduino flips the switch.

const int LED_PIN = 13;        // the LED is wired to pin 13

void setup() {
  pinMode(LED_PIN, OUTPUT);    // pin 13 will send electricity out
}

void loop() {
  digitalWrite(LED_PIN, HIGH); // switch ON: 5 volts, a 1
  delay(500);                  // wait 500 milliseconds
  digitalWrite(LED_PIN, LOW);  // switch OFF: 0 volts, a 0
  delay(500);                  // wait again, then loop() starts over
}

1. setup() runs once, when the board switches on. pinMode makes pin 13 an output.
2. loop() runs again and again, for ever.
3. digitalWrite(LED_PIN, HIGH) puts 5 V on the pin. That is a 1, so the LED lights.
4. delay(500) waits 500 milliseconds: half a second.
5. LOW puts 0 V on the pin. That is a 0, so the LED goes off.

With a micro:bit, the same idea in MicroPython uses the screen of lights:

from microbit import *

while True:
    display.show(Image.HEART)
    sleep(500)
    display.clear()
    sleep(500)