Chapter 2: Passive Buzzer (LEDC PWM Tone Control)

2.1 Key Points

• LEDC timer and channel configuration
• Dynamic PWM frequency changes for tone generation
• Duty cycle and sound output relationship
• Note frequency table and melody playback logic

2.2 Course Content

A passive buzzer does not contain an internal oscillator. It must be driven by an external square wave, and the frequency determines the pitch. ESP32-S3's LEDC peripheral can generate PWM at flexible frequencies, making it suitable for melody playback.

This chapter implements an ascending and descending C major scale.

2.3 Basic Learning

LEDC Structure

LEDC contains timers and channels:

• The timer defines clock source, resolution, and frequency.
• The channel binds a GPIO to a timer and sets duty cycle.

For the buzzer, OSRCORE uses LEDC_TIMER_1 with 10-bit resolution. A duty value of 512 produces a 50% duty square wave. Muting the buzzer is done by setting the duty to zero.

Note Frequencies

C4=262  D4=294  E4=330  F4=349
G4=392  A4=440  B4=494  C5=523

2.4 Program Study

Initialize the timer and channel:

ledc_timer_config_t timer = {
    .speed_mode      = LEDC_LOW_SPEED_MODE,
    .duty_resolution = LEDC_TIMER_10_BIT,
    .timer_num       = LEDC_TIMER_1,
    .freq_hz         = 1000,
    .clk_cfg         = LEDC_AUTO_CLK,
};
ledc_timer_config(&timer);

ledc_channel_config_t ch = {
    .gpio_num   = 42,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel    = LEDC_CHANNEL_2,
    .timer_sel  = LEDC_TIMER_1,
    .duty       = 0,
};
ledc_channel_config(&ch);

Play a single note:

static void buzzer_tone(uint32_t freq_hz, uint32_t duration_ms)
{
    ledc_set_freq(LEDC_LOW_SPEED_MODE, LEDC_TIMER_1, freq_hz);
    ledc_set_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_2, 512);
    ledc_update_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_2);
    vTaskDelay(pdMS_TO_TICKS(duration_ms));
    ledc_set_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_2, 0);
    ledc_update_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_2);
}

2.5 Summary

This chapter showed how to drive a passive buzzer with LEDC and how to change tone by changing PWM frequency. LEDC is also used later for ESC and servo PWM output.

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