17 Visual and Audio Actuators: Displays and Buzzers

17.1 Start With the Decision

This section: Start With the Decision — An OLED is a screen made of tiny lights.

An OLED is a screen made of tiny lights. It can show rich detail but uses memory and a shared bus, a set of wires that several devices use. I2C is the two-wire bus used here. A buzzer makes a sound cue that works when no one is looking at the screen.

17.2 Route Overview

This section: Route Overview — This is part 2 of 2.

This is part 2 of 2. Review Visual and Audio Actuators: LED Feedback for the preceding evidence.

17.3 Learning Objectives

This section: Learning Objectives

• Estimate OLED memory, power, and I2C needs.
• Set buzzer tone and pattern for distinct alerts.

17.4 Chapter Roadmap

This section: Chapter Roadmap

• OLED Displays
• Try It: OLED Display Memory and Power Estimator
• Checkpoint: Displays
• Buzzers and Tone Generation
• Musical Note Frequency Reference
• Try It: Buzzer Alert Pattern Designer
• Display Selection Guide
• Bedside Feedback Actuators
• Checkpoint: Alert Selection
• Knowledge Check
• Key Takeaway
• For Kids: Meet the Actuator Crew!
• Knowledge Check
• Quiz: Visual and Audio Actuators
• Match: Visual and Audio Actuator Concepts
• Order: Steps to Control LED Brightness with PWM on ESP32
• Label the Diagram
• Code Challenge
• Deep Dive: Feedback Loads, Perception, and Timed LEDs
• Summary
• Common Pitfalls
• 1. LED Without Current Limiting Resistor
• 2. Passive Buzzer Driven by DC Instead of Oscillating Signal
• 3. Driving High-Current LED Strips Directly from MCU GPIO
• 4. Active Buzzer and Passive Buzzer Confusion
• What’s Next?

17.5 OLED Displays

This section: OLED Displays — OLED displays offer high contrast and graphical control in a compact panel.

OLED displays offer high contrast and graphical control in a compact panel. Inspect Figure 17.2 to connect that visual flexibility to the module’s power and I2C wiring before choosing it over a character display.

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In Figure 17.2, start with the lit pixel area, then follow the few wires back to the controller board. The SSD1306 code below owns pixel placement rather than character cells, increasing information density while also making refresh strategy, readability, and display-on energy part of the interface decision.

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

Adafruit_SSD1306 display(128, 64, &Wire, -1);

void setup() {
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  display.setTextColor(SSD1306_WHITE);
}

void loop() {
  // Display sensor readings
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0, 0);
  display.print("IoT Data");

  display.setTextSize(1);
  display.setCursor(0, 20);
  display.print("Temp: 25.4 C");
  display.setCursor(0, 35);
  display.print("Humid: 65.2 %");
  display.display();
  delay(2000);

  // Draw a progress bar
  for (int pct = 0; pct <= 100; pct += 2) {
    display.clearDisplay();
    display.setCursor(0, 10);
    display.print("Loading...");
    display.drawRect(10, 30, 108, 10, SSD1306_WHITE);
    display.fillRect(12, 32, pct, 6, SSD1306_WHITE);
    display.display();
    delay(40);
  }
}

+Checkpoint: Displays

You now know:

• Character LCDs are layout-constrained: a 16x2 display only gives 16 columns per row before text is truncated.
• OLED choices involve resolution, frame-buffer bytes, lit-pixel current, and interface speed.
• A display is information-dense, but it only helps when someone is close enough and looking at it.

Visual feedback can show state quietly. The next section covers the output that interrupts attention.

17.6 Buzzers and Tone Generation

This section: Buzzers and Tone Generation

#define BUZZER_PIN 25

// Musical notes (frequencies in Hz)
#define NOTE_C4 262
#define NOTE_D4 294
#define NOTE_E4 330
#define NOTE_F4 349
#define NOTE_G4 392
#define NOTE_A4 440
#define NOTE_B4 494
#define NOTE_C5 523

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

void loop() {
  playMelody();
  delay(2000);

  playAlarm();
  delay(2000);

  playNotification();
  delay(5000);
}

void playMelody() {
  int melody[] = {NOTE_C4, NOTE_E4, NOTE_G4, NOTE_C5};
  int durations[] = {250, 250, 250, 500};

  for (int i = 0; i < 4; i++) {
    tone(BUZZER_PIN, melody[i], durations[i]);
    delay(durations[i] * 1.3);  // Pause between notes
  }

  noTone(BUZZER_PIN);
}

void playAlarm() {
  for (int i = 0; i < 5; i++) {
    tone(BUZZER_PIN, 1000, 200);
    delay(250);
    tone(BUZZER_PIN, 500, 200);
    delay(250);
  }

  noTone(BUZZER_PIN);
}

void playNotification() {
  tone(BUZZER_PIN, NOTE_A4, 100);
  delay(150);
  tone(BUZZER_PIN, NOTE_C5, 200);
  delay(250);
  noTone(BUZZER_PIN);
}

17.6.2 Active Buzzer (Simple On/Off)

Active buzzers have built-in oscillators, so the controller supplies an on/off command rather than an audio-frequency waveform. Inspect Figure 17.3 to identify the simple two-lead load before comparing it with the passive tone() example.

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In Figure 17.3, trace the two leads into the enclosed sounder and note that no external resonant drive circuit is visible. Applying rated power starts its internal oscillator and fixed tone; firmware controls timing and pattern, while a passive buzzer is required when pitch itself must carry information.

#define BUZZER_PIN 25

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

void beep(int count, int onTime, int offTime) {
  for (int i = 0; i < count; i++) {
    digitalWrite(BUZZER_PIN, HIGH);
    delay(onTime);
    digitalWrite(BUZZER_PIN, LOW);
    delay(offTime);
  }
}

void loop() {
  // Single beep
  beep(1, 100, 0);
  delay(2000);

  // Double beep
  beep(2, 100, 100);
  delay(2000);

  // Alarm pattern
  beep(5, 50, 50);
  delay(2000);
}

Try It: Buzzer Alert Pattern Designer

Buzzers add urgency, but they also add fatigue. The selection guide that follows treats sound as one channel in a larger feedback design, not as the default answer for every event.

17.7 Display Selection Guide

This section: Display Selection Guide — Choose a display by tracing the application’s dominant constraint across the table.

Choose a display by tracing the application’s dominant constraint across the table. Start with update rate and information density, then compare contrast in the actual lighting environment, and finally calculate energy over the real duty cycle rather than using active current alone. LCD or OLED fits frequent updates, e-paper fits long static intervals, and TFT earns its higher power only when colour graphics and rapid refresh materially serve the interface.

+Checkpoint: Alert Selection

You now know:

• Passive buzzers need an audio-frequency waveform; active buzzers make their own fixed tone when powered.
• Alert tones usually target 1000-3000 Hz, while lower tones around 500 Hz can be useful where hearing protection changes perception.
• Critical states should use more than one channel when people may not be watching the device or may not hear it reliably.

17.8 Knowledge Check

This section: Knowledge Check

17.9 Knowledge Check

This section: Knowledge Check

17.10 Deep Dive: Feedback Loads, Perception, and Timed LEDs

This section: Deep Dive: Feedback Loads, Perception, and Timed LEDs — Inspect Figure 17.1 before treating a status light as a purely visual choice.

Inspect Figure 17.1 before treating a status light as a purely visual choice. The panel encodes several user-facing states, while the companion circuit exposes the GPIO, resistor, LED, and return path that make each indication electrically safe.

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Read Figure 17.1 from the named user states to the GPIO circuit. The panel decides what a person can distinguish; the resistor and return path decide whether the controller can drive the chosen LED safely. That pairing connects perception, timing, and electrical load rather than treating feedback as decoration.

LEDs, displays, and buzzers are simple only at the user interface. Electrically, each one still has a load path and a control signal. A small status LED shows the pattern: if a 3.3 V GPIO drives a red LED with about 2.0 V forward drop at 10 mA, the resistor must drop 1.3 V, so R = (3.3 - 2.0) / 0.010 = 130 ohms. Choosing 150 ohms gives I = 1.3 / 150 = 0.0087 A, or 8.7 mA. The resistor dissipates 1.3 V x 0.0087 A = 0.011 W, so a small signal resistor has margin. At 25% PWM duty cycle, the average LED current is about 8.7 mA x 0.25 = 2.2 mA.

The design decision is not only electrical. Feedback actuators differ in noticeability, power, and information density. A single green LED is excellent for a healthy status, but poor for a fault that must be noticed across a room. A display can show numbers and labels, but only helps when someone is looking at it. A buzzer interrupts attention, but repeated audio becomes annoying and may be masked by the environment. Strong products combine channels: quiet visual status for normal operation, stronger visual patterns for warnings, and audio only when the state needs immediate attention.

Displays need the same budget discipline. If a small OLED draws 20 mA from a 3.3 V rail while lit, the display load is 3.3 V x 0.020 A = 0.066 W. Left on for 24 hours, that is 0.066 W x 24 h = 1.58 Wh before regulator losses. On a small battery device, that can dominate the energy budget. If the value changes only occasionally, e-paper is often a better human interface because it can keep an image visible between refreshes without continuous display power.

Buzzers split into two kinds that are often confused. An active buzzer has an internal oscillator and produces one fixed tone when powered. A passive piezo or magnetic buzzer needs the controller to supply an audio-frequency waveform. A 2 kHz passive-buzzer alert has a period of 1 / 2000 = 0.0005 s, or 500 microseconds; a 50% square wave is high for about 250 microseconds and low for about 250 microseconds. A lower 500 Hz warning tone has a period of 1 / 500 = 0.002 s, or 2 ms. The controller changes pitch by changing the period, not by changing the supply voltage.

Addressable LEDs such as WS2812/NeoPixel parts add a different kind of timing constraint. Each pixel contains a driver chip, reads the first 24 bits as its green, red, and blue values, and forwards the rest downstream. There is no clock line; each bit is encoded by pulse width and lasts roughly 1.25 microseconds. A 30-pixel strip therefore needs 30 pixels x 24 bits = 720 bits, or 720 x 1.25 = 900 microseconds, plus the reset interval. An interrupt that pauses the GPIO waveform halfway through a bit can corrupt the data seen by every later pixel.

That timing issue is separate from the power issue. The same 30-pixel strip at full white can draw 30 x 60 mA = 1.8 A, which is 5 V x 1.8 A = 9 W. A microcontroller pin can generate the data waveform, but the 5 V supply and ground return must carry the power. Random flicker can come from data timing or from supply droop, so debug in order: limit brightness, confirm common ground, check the 5 V rail under load, then inspect whether the data driver can keep sub-microsecond timing without interruption.

17.11 Summary

This section: Summary — Visual and audio actuators transform digital data into human-perceivable feedback.

Visual and audio actuators transform digital data into human-perceivable feedback. Key concepts:

LED Control:

• PWM dimming varies duty cycle (not voltage) to control perceived brightness
• 8-bit PWM provides 256 brightness levels (0-255)
• Power consumption scales linearly with duty cycle
• RGB LEDs require three independent PWM channels

Addressable LED Strips:

• WS2812B (NeoPixel) LEDs contain integrated driver chips
• Single data wire controls hundreds of individually addressable pixels
• Full white draws ~60mA per LED; single colors ~20mA
• High current draws require external power supplies (not USB)

Displays:

• LCD (16x2): Simple text, 20-100mA, ideal for mains-powered devices
• OLED: High contrast, 10-50mA, battery-friendly with frequent updates
• E-paper: Ultra-low power (0.001mA standby), slow refresh, best for infrequent updates
• TFT LCD: Rich graphics, 100-300mA, requires mains power

Buzzers:

• Passive: Requires frequency input, can play any tone/melody
• Active: Built-in oscillator, simple ON/OFF control, fixed frequency
• Alert tones: 1000-3000 Hz for maximum audibility
• Low frequencies (~500 Hz) penetrate hearing protection better

Design Considerations:

• Power budget is critical for battery-powered devices
• Multi-modal feedback (visual + audio) improves critical alert perception
• Distinct patterns prevent alarm fatigue in safety-critical applications
• Standard resistor values (E12/E24 series) for current limiting

17.12 What’s Next?

This section: What’s Next? — Now that you understand visual and audio actuators, explore related actuator types and deeper control techniques.

Now that you understand visual and audio actuators, explore related actuator types and deeper control techniques.

17.13 Continue Your Route

This section: Continue Your Route — This is the last part of this chapter.

This is the last part of this chapter. Return to Visual and Audio Actuators: LED Feedback or continue from the actuators module index.