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PWM appears on an oscilloscope as a repeating rectangular waveform: the signal switches between low and high, and its duty cycle is the percentage of each period spent high. To see how it works, compare traces at different duty cycles, then measure each trace’s period and high-time. An ESP32 or Arduino can generate the signal, while a resistor-protected LED makes the effect visible.
What PWM looks like on an oscilloscope
Pulse-width modulation (PWM) is a digital signal that repeatedly switches on and off. On a scope, it looks like a train of rectangular pulses. The time between equivalent points on successive pulses is the period; the high portion of that period is the pulse width.
Duty cycle is calculated as high-time ÷ period × 100%. For example, a 20% duty cycle means the signal is high for one-fifth of every cycle. If the period is one second, the high-time is 200 milliseconds, an illustrative example given in Mastering STM32 (2018).
Frequency and duty cycle describe different things. Frequency determines how often cycles repeat; duty cycle determines how much of each cycle is high. At the same frequency, compare 20%, 50%, and 80% duty-cycle traces: their periods remain the same while their high portions grow wider.
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How to measure PWM duty cycle with a scope
- Connect the probe safely. Connect the probe tip to the PWM output and its ground lead to circuit ground. Use a probe and scope input suitable for the signal, and avoid shorting adjacent pins with the probe tip.
- Calibrate and set up the display. Calibrate the oscilloscope as appropriate for the probe and instrument. Choose a time base that displays several complete periods and trigger on the signal’s rising edge so the waveform remains stable. The Hackster demonstration specifically calls out scope calibration before observing the pulses.
- Measure the period and high-time. Use the scope’s automatic period and positive-pulse-width measurements if available; otherwise use the time cursors. Period is the interval between matching edges on successive cycles. High-time is the interval from a rising edge to the next falling edge.
- Calculate or read duty cycle. Divide the measured high-time by the measured period and multiply by 100. If the scope offers an automatic duty-cycle measurement, compare it with this calculation.
- Repeat at several settings. Record duty cycle, frequency, period, and high-time at low, middle, and high PWM settings. Confirm that changing duty cycle alone widens or narrows the high portion without changing the repetition rate.
Automatic measurements are useful, but the displayed waveform remains important: it can reveal unstable triggering, unexpected edge behavior, or a frequency different from the one intended by the program.
Make the waveform visible with an ESP32 and LED
JeremyCook’s Hackster.io article, “PWM Concepts Illustrated with an Oscilloscope”, presents R. Jordan Kreindler’s demonstration using an ESP32 development board, a red LED, breadboard jumpers, and an oscilloscope. The LED visibly responds as pulse width increases or decreases. The article says other ESP32 boards should work and mentions an Arduino Uno as another possible controller.
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- Gather an ESP32 development board, red LED, breadboard, jumper wires, a suitable current-limiting resistor, and an oscilloscope with probe.
- Wire the LED and resistor to a GPIO that supports PWM on the selected board. Follow the board’s pinout and electrical limits; include the resistor rather than connecting the LED directly to a GPIO. Connect scope ground to circuit ground.
- Configure PWM in the board’s supported software interface, then increase and decrease duty cycle gradually. Watch the scope trace and the LED together.
- Use the measurement procedure above to record the output at several settings. Treat brightness as a visible indication of the load’s response, not as a substitute for waveform measurements.
PWM changes the average energy delivered to a load by changing how long the output is active in each cycle; it does not make the GPIO pin produce a continuously varying analog voltage. The LED’s apparent brightness reflects its response to the repeated pulses.
Arduino and ESP32 PWM are not interchangeable
The PWM behavior depends on the board and software core. Check the documentation for the exact board, pin, frequency, resolution, and channel behavior rather than assuming one Arduino example applies everywhere.
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Arduino Uno and analogWrite()
Arduino’s classic analogWrite() interface accepts values from 0 (always off) to 255 (always on). Its reference says the pin generates a steady rectangular wave at the specified duty cycle until another call changes the output, or digitalRead() or digitalWrite() is called on that same pin. The Arduino reference lists common Uno PWM frequencies as 490 Hz, with 980 Hz on pins 5 and 6; rates differ on other boards. It also warns that Uno pins 5 and 6 may behave unexpectedly at low duty-cycle values because they share a timer with millis() and delay(). See the Arduino analogWrite() reference.
ESP32 LEDC
Espressif’s LEDC peripheral is intended primarily for LED intensity control and can generate PWM for other uses. Current documentation lists 16 LEDC channels on ESP32, eight on ESP32-S2 and ESP32-S3, and six on ESP32-C3, C5, C6, and H2. Frequency, duty, and resolution are configurable through the API; the specific usable combination depends on the chip and configuration. Consult Espressif’s LEDC documentation for the target chip and software environment.
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What to compare in your measurements
For a useful experiment, keep the controller, PWM frequency, probe setup, and load consistent while changing duty cycle. A record like this makes the distinction between the programmed setting and observed output clear:
| Setting | Duty-cycle target | Measured period | Measured high-time | LED observation |
|---|---|---|---|---|
| Low | 20% | Record from scope | Record from scope | Record observation |
| Middle | 50% | Record from scope | Record from scope | Record observation |
| High | 80% | Record from scope | Record from scope | Record observation |
The target percentages are example test settings, not guaranteed measured results. For each row, calculate duty cycle from the scope’s high-time and period; if the observed frequency or pulse width differs from expectation, check the board-specific PWM configuration, selected pin, timer or channel allocation, and scope trigger and time-base settings.
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