LED indicator for PWM signal

Question

I want to make a PCB that has an ESP32 on it, and the ESP sends a PWM signal to the motor driver. I want to indicate if the PWM signal is being sent or not by LED, without interfering with the PWM signal.

Is there any particular sensor or way in which I can do this? Prior to this, I tried putting an SMD LED parallel to the PWM signal, but there was some problem the frequency of the PWM that doesn't work with LED.

I have a project that uses all the pins of esp, and the pwm trace goes from one pcb to another via header pins, so I need to make sure that pwm is coming through the headers. Sometimes there may be some mechanical error(in wiring) that might block PWM signals.

I am using sign-magnitude mode to drive the motor, so I would be using 0-100% PWM to drive the motor, so adding any element directly in the PWM wire should not drop the voltage, because it would reduce the resolution of my drive mode(lets say voltage drop is 1v, so i would get the speed resolution of 1-4volt only on pwm to control the speed of motor)

Answer 1

If you want to implement this without using the ESP, one solution would be to charge a small capacitor via a diode with the PWM and use an constant current driver to drive the LED with a constant current.
An easy to use CC would be a CRD (Current regulating diode), which is a simple 2-pin component you can place in series with your LED. One example for a constant current of 2mA would be the S-202T from Semitec.

^{simulate this circuit – Schematic created using CircuitLab}

The following simulation in LTspice shows a 20 kHz PWM with 7.4V charging the cap and an ideal constant current source driving the LED with 2mA. Duty cycle is only 1% in this example, to show that the current peaks through D1 might be critical. Choose R1 to achieve a high enough voltage on your cap for the lowest duty cycle you want to detect.

Answer 2

It sounds like you want an external way to ensure that a PWM signal is actually present, rather than trusting the ESP software. Sure, so you could route to the PWM signal to a second GPIO as well and set up an interrupt on that pin. Use a third GPIO to turn the LED on/off, and do something like setting the LED pin low if a certain amount of time passes without the interrupt triggering.