PWM Breathing LED

1. PWM Overview

1.1 What is PWM

PWM (Pulse Width Modulation) is simply a technique that "uses digital switches to simulate analog voltage".

In digital circuits, GPIO pins can only output two states: high (3.3V) or low (0V), no intermediate values. But PWM cleverly bypasses this limitation through "rapid switching"—the faster the switching, and with different ratios of on-time, the resulting "average effect" on the load differs.

Think of it this way: you can't make a water tap produce "half-thick" water column, but you can rapidly turn it on and off, 50% on / 50% off, and the bucket receives only half the water of a fully open tap. PWM is this concept.

1.2 Three Core Parameters of PWM

① Period: Time for one complete "on→off" cycle, in seconds. Period = 1 / frequency.

② Frequency: How many "on→off" cycles per second, in Hz. How high should frequency be?

• Too low (<50Hz): Human eye can detect LED flickering, motor vibrates

• Moderate (~1kHz): LED appears stable, ordinary motors run smoothly—this experiment is 1kHz

• Too high (>20kHz): Beyond human hearing range, suitable for quiet motor drive scenarios, but switching losses increase

③ Duty Cycle: In one cycle, the ratio of "high level time" to "entire cycle time".

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Duty cycle = (high level time / period) × 100%

Example frequency 1kHz (period = 1ms):
  25% duty cycle  = high 0.25ms + low 0.75ms  → average voltage ≈ 0.825V
  50% duty cycle  = high 0.50ms + low 0.50ms  → average voltage = 1.65V
  80% duty cycle  = high 0.80ms + low 0.20ms  → average voltage = 2.64V

PWM's magic is: You only change the "on/off" time ratio (duty cycle), but the load feels like voltage is continuously changing. This is the "area equivalence principle"—the average value of high-frequency square wave equals a constant analog voltage value.

1.3 What This Experiment Does

This experiment uses ESP32-S3's LEDC (LED PWM Controller) peripheral to generate PWM signals, driving an LED to display "breathing light" effect by dynamically changing duty cycle: gradually bright → gradually dim → off stay → repeat.

Application Scenarios:

2. Core Concepts

2.1 Duty Cycle and Resolution

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PWM signal (1kHz frequency, period = 1ms):

  0% duty cycle:   ──────────────────────────────  average voltage = 0V
                   (entire low level)

  25% duty cycle:  ┌──┐┌─────────────────────────  average voltage = 0.825V
                   └──┘└─────────────────────────

  50% duty cycle:  ┌─────┐┌──────────────────────  average voltage = 1.65V
                   └─────┘└──────────────────────

  100% duty cycle: ┌──────────────────────────────  average voltage = 3.3V
                   └──────────────────────────────

Duty cycle resolution: LEDC_TIMER_10_BIT means duty cycle divided into 0~1023 (1024 levels total). With 10-bit resolution:

2.2 Common-Anode LED PWM Control

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Common-anode connection:
  VDD (3.3V) ──→ LED anode ──→ LED cathode ──→ Current-limiting resistor ──→ GPIO44

  GPIO output HIGH (1023): LED both ends at same potential, no current → OFF
  GPIO output LOW  (0):   Current VDD→LED→GPIO→GND  → ON

This experiment's "reverse brightness" relationship:

In common-anode connection, duty decreases → low level ratio increases → LED gets brighter. This is opposite to the conventional understanding "duty increases = brighter", so code uses MAX_DUTY (1023) for OFF, VISIBLE_DUTY (200) for visible brightness.

2.3 Why VISIBLE_DUTY = 200 Instead of 0

If duty drops to 0 (100% low level = brightest), LED will instantly jump from "fully bright" to "fully off" (phase switch), visually discontinuous. 200 provides a "brightest but not fully bright" visible threshold, forming brightness contrast with MAX_DUTY, making breathing effect smooth.

2.4 LEDC Peripheral Architecture

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┌──────────┐    ┌──────────┐    ┌──────────┐
│ Timer 0  │───→│ Channel 0│───→│ GPIO44   │
│ Timer 0  │    │ Channel 0│    │ (LED)    │
│ 1kHz     │    │ duty=... │    │          │
│ 10-bit   │    │          │    │          │
└──────────┘    └──────────┘    └──────────┘

One timer can be shared by multiple channels (same frequency, different duty cycles),
one channel can be bound to different GPIO pins.

LEDC Resources:

3. Project Architecture and Flow

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app_main()
  ├─ led_pwm_init()              Initialize LEDC timer + channel
 ADC
RGB LED Effects