Unit B - Inputs & Outputs

07. Digital Outputs, RGB LEDs & Buzzers

Drive LEDs safely, mix RGB colour, and play tones on a passive buzzer.

Estimated time 3 hours

Learning outcomes

Use pinMode OUTPUT and digitalWrite HIGH/LOW correctly
Limit LED current with a series resistor on each channel
Explain common-cathode versus common-anode RGB wiring and logic
Distinguish active and passive buzzers and use tone / noTone
Recognise loads that need a driver instead of a bare Uno pin

Parts and preparation

Uno, common-cathode RGB LED (or separate red/green/blue LEDs), 3 x 330 ohm resistors, passive piezo buzzer, breadboard and jumpers.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Digital Outputs, RGB LEDs & Buzzers instructional connection diagram

What a digital output does

After pinMode(pin, OUTPUT), digitalWrite chooses a logic level. On a 5 V Uno, LOW is about 0 V and HIGH is about 5 V.

That is enough for indicator LEDs (with resistors) and for signalling other logic chips. It is not a power supply for motors, heaters or big lamps.

Two cards showing LOW near 0 V and HIGH near 5 V with digitalWrite examples — OUTPUT pins switch between roughly 0 V and 5 V on a Uno.

Safe loads vs drivers

A classic Uno pin is rated for modest current (plan around 20 mA per pin, and stay well under absolute maxima). Always use a series resistor with an LED.

Motors, relays, solenoids and heaters need a transistor, MOSFET or driver module plus a suitable supply - and often a flyback diode. Lesson 12 covers drivers in more depth.

OK loads like LED with resistor versus loads that need a driver such as motors and relays — LEDs and small piezo loads: OK with care. High current: use a driver.

RGB LEDs: three dies, three resistors

An RGB LED packs red, green and blue emitters. Each colour needs its own series resistor - sharing one resistor for all three is incorrect and uneven.

Course wiring assumes common cathode: the shared leg goes to GND, and each colour leg goes through 330 ohm to a digital pin. HIGH turns that colour on.

RGB pins through resistors into a common-cathode LED plus a passive buzzer on D6 — Course pattern: RGB on D9/D10/D11 with resistors; passive buzzer on D6.

Signal	Course pin	Role
Red	D9	Red channel via 330 ohm
Green	D10	Green channel via 330 ohm
Blue	D11	Blue channel via 330 ohm
Buzzer	D6	Passive piezo + tone()

Common cathode vs common anode

Common cathode: shared pin to GND; drive colours HIGH to light. That is this lesson's default.

Common anode: shared pin to 5 V; drive colours LOW to light (active-low). If your RGB looks always-on or inverted, you may have the other type - check the datasheet and invert the logic.

Common cathode shares GND with HIGH on, versus common anode shares 5 V with LOW on — Same package shape does not guarantee the same pinout or logic.

Active vs passive buzzers

An active buzzer has a built-in oscillator: a steady HIGH makes a fixed beep. A passive piezo is closer to a speaker: it needs a square wave at an audible frequency.

Arduino tone(pin, frequency) generates that wave. An optional third argument is duration in milliseconds. noTone(pin) stops sound. This course uses a passive buzzer with tone().

Active buzzer driven by DC HIGH versus passive buzzer driven by tone frequency — Active: DC beep. Passive: tone() sets the pitch.

tone(buzzerPin, 440, 200);  // A4, 200 ms
delay(250);
noTone(buzzerPin);

What the worked sketch practises

The sketch cycles red, green and blue with a different tone for each step. Only one colour is HIGH at a time so you can confirm each channel and hear pitch changes.

Try mixing colours later by turning two pins HIGH together (yellow ≈ red+green).

Wiring and safe build sequence

Breadboard wiring for lesson 07: Digital Outputs, RGB LEDs & Buzzers — Breadboard layout for this lesson. Match colours and pins before powering the circuit. Click the image for a larger view.

RGB common cathode -> GND (confirm datasheet if unmarked)
Red leg -> 330 ohm -> D9
Green leg -> 330 ohm -> D10
Blue leg -> 330 ohm -> D11
Passive buzzer + -> D6; buzzer - -> GND
If colours are inverted or always on, you may have a common-anode LED - invert HIGH/LOW

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

const byte redPin = 9;
const byte greenPin = 10;
const byte bluePin = 11;
const byte buzzerPin = 6;

void allOff() {
  digitalWrite(redPin, LOW);
  digitalWrite(greenPin, LOW);
  digitalWrite(bluePin, LOW);
}

void setup() {
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);
  // tone() does not require pinMode, but OUTPUT is fine if you also digitalWrite
  pinMode(buzzerPin, OUTPUT);
  allOff();
}

void loop() {
  allOff();
  digitalWrite(redPin, HIGH);
  tone(buzzerPin, 440, 200);
  delay(500);

  allOff();
  digitalWrite(greenPin, HIGH);
  tone(buzzerPin, 660, 200);
  delay(500);

  allOff();
  digitalWrite(bluePin, HIGH);
  tone(buzzerPin, 880, 200);
  delay(500);
}

How the code works

allOff clears every colour before lighting the next - easier to see each channel.
tone duration is in milliseconds; delay should be a little longer if you want a gap.
Each RGB colour needs its own resistor even when only one colour is on.
For common-anode RGB, swap the sense of HIGH and LOW (or write a small helper).

Test and record evidence

Expected result: Red, then green, then blue light in turn. A higher pitch sounds with each step (440, 660, 880 Hz).

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Verify RGB pinout from the module datasheet - long leg is not always common.
A common-anode LED needs inverted HIGH/LOW logic compared with this sketch.
No sound: confirm a passive buzzer (not active-only) and polarity; try a known frequency like 1000.
Only one colour works: check that channel's resistor and pin number.
USB brown-out or reset when wiring: you may have shorted a pin - disconnect and recheck.

Extension challenge: Create a three-note status melody and a matching RGB colour sequence (for example red/error, green/ok, blue/busy). Put the melody in a function playStatus(byte code).

Check your understanding

Q1. Why does each RGB channel need a resistor?

Show answer

Each LED die needs its own current limiting.

Q2. What does tone control on a passive buzzer?

Show answer

The frequency of the square wave (the pitch).

Q3. How does common-anode drive differ from common-cathode?

Show answer

Shared leg to 5 V; colours turn on with LOW instead of HIGH.

Q4. Why not drive a DC motor from a digital pin directly?

Show answer

Current and inductive kick exceed what a logic pin should supply - use a driver.

Q5. What does allOff achieve in the sketch?

Show answer

It turns every colour channel LOW before the next colour so only one shows at a time.