Lights and the first program

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.

• 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.

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.

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)