Measure PWM duty and LED timing

Compare true DAC output with PWM duty-cycle output by measuring PWM HIGH time, period, and duty at a real simulated LED node.

Use the module's DAC and PWM output route to compare measured switching duty with the kind of output a physical load actually needs., your practice guide

Use the module's DAC and PWM output route to compare measured switching duty with the kind of output a physical load actually needs.
Predict the reading, then compare it with the measurement.

Wokwi ESP32

Third party Tool

Compare true DAC output with PWM duty-cycle output by measuring PWM HIGH time, period, and duty at a real simulated LED node.

Tier 1 · Web · No account

Version tested: Compiled and run in the public Wokwi ESP32 web editor on 2026-10-09; Wokwi analyzer VCD opened in browser Surfer. Date: 2026-10-09.

Open the ESP32 editor, paste diagram.json, then paste sketch.ino.

Open Wokwi to paste in the files (new tab)

Get the files

Use both prepared files. This is a paste-in setup; saving a project requires a Wokwi account.

diagram.json

794 bytes · Circuit connections

Download

sketch.ino

1,487 bytes · ESP32 program

Download

#include <Arduino.h>

constexpr int PWM_PIN = 2;
constexpr int SENSE_PIN = 4;
int frequencyHz = 1000;
int dutyCode = 0;
char stage = 'O';
unsigned long stageAt = 0;
bool pendingMeasurement = false;

void choose(char command) {
  if (command == 'A') { frequencyHz = 1000; dutyCode = 64; }
  else if (command == 'B') { frequencyHz = 1000; dutyCode = 191; }
  else if (command == 'C') { frequencyHz = 2000; dutyCode = 191; }
  else return;
  ledcChangeFrequency(PWM_PIN, frequencyHz, 8);
  ledcWrite(PWM_PIN, dutyCode);
  stage = command;
  stageAt = millis();
  pendingMeasurement = true;
  Serial.printf("STAGE=%c configured_hz=%d duty_code=%d\n", stage, frequencyHz, dutyCode);
}

void setup() {
  Serial.begin(115200);
  pinMode(SENSE_PIN, INPUT);
  ledcAttach(PWM_PIN, frequencyHz, 8);
  ledcWrite(PWM_PIN, 0);
  Serial.println("PWM TIMING LAB READY; LED off; D2 loopback D4 and analyzer D0");
  Serial.println("Commands: A=25% at 1kHz, B=75% at 1kHz, C=75% at 2kHz");
}

void loop() {
  if (Serial.available()) choose(Serial.read());
  if (pendingMeasurement && millis() - stageAt >= 200) {
    pendingMeasurement = false;
    unsigned long highUs = pulseIn(SENSE_PIN, HIGH, 10000);
    unsigned long lowUs = pulseIn(SENSE_PIN, LOW, 10000);
    unsigned long periodUs = highUs + lowUs;
    Serial.printf("MEASURE stage=%c high_us=%lu low_us=%lu period_us=%lu duty_pct=%.1f\n",
      stage, highUs, lowUs, periodUs,
      periodUs ? 100.0 * highUs / periodUs : 0.0);
  }
  delay(5);
}

Use the launch button above to open the ESP32 editor in Wokwi.
Select the editor’s diagram.json tab and replace all its text with the supplied diagram.json.
Select the sketch.ino tab, replace all its text with the supplied sketch.ino, then click Start Simulation.

Steps

Screens captured against Wokwi ESP32 Compiled and run in the public Wokwi ESP32 web editor on 2026-10-09; Wokwi analyzer VCD opened in browser Surfer on 2026-10-09; the tool may have moved on — the text steps are the contract.

Step 1
Do
In the Wokwi editor, paste the supplied diagram and sketch and inspect the LED, loopback input, and logic analyzer before starting.
You will see
Input: loaded the supplied ESP32 diagram and sketch in the public editor. Observed: GPIO2 drives the green LED through a 220 Ω resistor. Observed: the same GPIO2 node connects to GPIO4 and analyzer channel D0. Observed: the stopped Simulation panel showed the LED off and the analyzer blank.
Why it matters
The GPIO4 loopback measures the digital PWM node, not a calculated ideal value. The analyzer independently records transitions at that same node. The resistor limits LED current in the simulator circuit. The stopped circuit provides a baseline before any duty command.
Step 2
Do
In Serial Monitor, start simulation, send A for 25% at 1 kHz, and read the GPIO4 loopback measurement.
You will see
Input: sent `A`; firmware configured 1,000 Hz with 8-bit duty code 64. Observed: `MEASURE stage=A high_us=254 low_us=748 period_us=1002 duty_pct=25.3`. Observed: the LED lit during PWM pulses and analyzer D0 accumulated samples. Observed VCD at 2 s: one HIGH interval was 250.00 µs in a 1,000.00 µs period.
Why it matters
The GPIO4 reading measures a short HIGH interval relative to LOW. The Wokwi analyzer independently records the same switching node. The VCD and loopback differ by a few microseconds due to read timing. Neither measurement is an analog LED voltage or light-output measurement.
Step 3
Do
In Serial Monitor, send B for 75% duty at the same 1 kHz and compare the measured HIGH interval.
You will see
Input: sent `B`; firmware kept 1,000 Hz and changed 8-bit duty code to 191. Observed: `MEASURE stage=B high_us=749 low_us=260 period_us=1009 duty_pct=74.2`. Observed: analyzer D0 sample count rose while the LED continued switching. Observed VCD at 4 s: HIGH was 746.09 µs and period was 1,000.00 µs.
Why it matters
The measured HIGH interval grew from roughly 250 to 750 µs. The period remained near 1 ms in both stages. Changing duty at fixed frequency changes time on, rather than pulse count per second. Wokwi's LED appearance is illustrative; no luminance was measured.
Step 4
Do
In Serial Monitor, send A again and compare the two same-frequency measurements in the visible log.
You will see
Input: restored `A` without changing the nominal 1,000 Hz frequency. Observed: the second A line was `high_us=254 low_us=748 period_us=1002 duty_pct=25.3`. Observed: the preceding B line was `high_us=749 low_us=260 period_us=1009 duty_pct=74.2`. Observed: both measured periods stayed near 1,000 µs while HIGH duration changed.
Why it matters
The A-B-A sequence repeats the low-duty result in this run. The measured period remained approximately constant. The loopback readings and VCD give separate timing evidence. This is a digital switching comparison, not a true DAC output test.
Step 5
Do
In Serial Monitor, send C to keep about 75% duty but double frequency to 2 kHz; inspect the new period.
You will see
Input: sent `C`; firmware configured 2,000 Hz and retained duty code 191. Observed: `MEASURE stage=C high_us=371 low_us=123 period_us=494 duty_pct=75.1`. Observed: the Wokwi logic analyzer continued accumulating D0 transitions. Observed VCD at 9 s: HIGH was 373.05 µs and period was 500.00 µs.
Why it matters
The period halved from about 1 ms to about 0.5 ms. The measured duty remained near three quarters. The high-time also halved as expected for fixed duty at double frequency. The result describes simulator digital timing, not physical LED efficiency.
Step 6
Do
In the Wokwi Simulation panel, inspect the LED, analyzer sample count, and final Serial Monitor line, then stop to export the VCD.
You will see
Input: left stage C active, then stopped the real Wokwi simulation. Observed final line: `MEASURE stage=C high_us=371 low_us=123 period_us=494 duty_pct=75.1`. Observed: the analyzer display showed 29.4K samples in the running capture. Observed: Wokwi downloaded `pwm-duty-led.vcd`, opened in the real browser Surfer viewer for 25%, 75%, and 2 kHz waveform inspection.
Why it matters
The final screenshot ties the circuit, analyzer, and serial reading to one running editor. The exported VCD preserves the digital transitions for independent inspection. A waveform viewer shows shorter and longer HIGH intervals directly. This does not establish actual analog voltage ripple, LED luminance, or physical ESP32 timing tolerance.

Chapter checks

These questions refer to the chapter’s examples. Use the return links to review their answers.

A plant lamp receives a PWM command. What should the designer examine to predict its physical output?
Return to the chapter’s knowledge check
A design smooths PWM pulses into an analog-like control signal. What choice does the chapter ask the learner to evaluate?
Return to the chapter’s knowledge check

Return to DAC and PWM Output · Browse Labs

Wokwi ESP32

Get the files

diagram.json

sketch.ino

Steps

1 Step 1

2 Step 2

3 Step 3

4 Step 4

5 Step 5

6 Step 6

Chapter checks

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6