Edit - Arduino Stack Exchange

You are not logged in. Your edit will be placed in a queue until it is peer reviewed.

We welcome edits that make the post easier to understand and more valuable for readers. Because community members review edits, please try to make the post substantially better than how you found it, for example, by fixing grammar or adding additional resources and hyperlinks.

Required fields*

Rev

Required fields*

ESP32 LED PWM fade not working?

Not sure if this belongs more in an electronics SE, but I'll give this a try. I'm trying to use an ESP32-C3 and an NPN transistor (2N5551) to PWM drive an LED (values below). I'm trying a simple fade, however the LED is not fading, but rather going in quite discreet steps.

I'm using analogWrite (snipped below), but I'm fairly sure the code is correct. It could very well be an issue with my soldering, but thought I'd ask here in case I'm missing something obvious?

I measured the LED at 100mA with a 2.7V drop, so I put it in series with a 22ohm resistor. Diagram below:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

 #include <Arduino.h>
 unsigned long fadeStartTime;
 void setup() {
 Serial.begin(115200);
 while(!Serial); // Wait for serial to be available
 Serial.println("Starting");
 setCpuFrequencyMhz(80);
 LOG_INFO(MODULE_NAME, "Finished setup.");
 }
 void loop() {
 unsigned long progress = millis() - fadeStartTime;
 long brightness;
 if (progress <= 3000) brightness = map(progress, 0, 3000, 0, 255);
 else if (progress <= 6000) brightness = map(6000 - progress, 0, 3000, 0, 255);
 else fadeStartTime = millis(); // restart fade again
 analogWrite(STEMS_DATA_PIN, brightness * brightness / 255);
 }

Answer*

Full credit to @6v6gt and the others in the comments for highlighting the issues here:

 **1. The transistor setup is wrong for a NPN transistor**

Due to the properties of NPN transistors, if they're used in "emitter follower" configuration, they only output the voltage at the base. In this case, the voltage across `R1+D1` would only be the 3.3v from the ESP GPIO. This leads to less current then expected across the led. Below is the correct setup. [See this answer for more details.][1]

 **2. The fading code was actually incorrect**

ESP's `analogWrite` function works by using PWM. This is a square wave with alternating high and low pulses, with the ratio of them being the duty-ratio which determines the average current going through the led. If this is called too quickly, the signal might not have gone through a full high-low pulse cycle, leading to essentially a continuous high signal. This is fixed by adding a small `delay(50)`, which is longer than a few cycles of pwm (esp has a default frequency of 1kHz).

In my case, the code could not be blocking even for that little, as it needed to pool various server as well as write data to other gpio lines for some WS2812 leds. The easy alternative is to check if the value we're writing has changed. This changes quickly, but still slowly enough that the pulses don't interfere with each other.


----------


[![Wrongly called PWM][2]][2]

 #include <Arduino.h>

 #define DATA_PIN 0
 
 unsigned long fadeStartTime;
 unsigned long lastValue;
 
 void setup() {
 Serial.begin(115200);
 while(!Serial); // Wait for serial to be available
 Serial.println("Starting");
 setCpuFrequencyMhz(80);
 LOG_INFO(MODULE_NAME, "Finished setup.");
 }
 
 void loop() {
 unsigned long progress = millis() - fadeStartTime;
 long brightness;
 if (progress <= 3000) brightness = map(progress, 0, 3000, 0, 255);
 else if (progress <= 6000) brightness = map(progress, 3000, 6000, 255, 0);
 else fadeStartTime = millis(); // restart fade again
 if(brightness != lastValue) {
 analogWrite(DATA_PIN, brightness);
 lastValue = brightness;
 };
 }

 [1]: https://electronics.stackexchange.com/questions/57845/why-would-one-drive-leds-with-a-common-emitter
 [2]: https://i.sstatic.net/fMiN9H6t.png



![schematic](https://i.sstatic.net/e86yDWvI.png)

Draft saved

Draft discarded

Edit Summary*

Cancel

Interesting observation of esp32 analogWrite() behaviour. In that configuration you need a series base resistor on the transistor (say 680 ohms to 1k). With the previous emitter follower configuration a base resistor was unnecessary. Also, if 22 ohms for the led current limiting resistor was correct in the previous configuration, it will surely be too low in the current configuration where 5v instead of about 2.7v is available. It depends on the led's forward voltage and maximum current.

6v6gt
– 6v6gt

2025年04月21日 00:10:06 +00:00
Commented Apr 21 at 0:10
@6v6gt I mean, this is my only reasonable guess as why it would work with the delay honestly. As for the resistor, values for the led are in the diagram, the 22ohms was calculated for the original 5v (with a 0.3v drop from the transistor). I also did add a base resistor of 150ohm, kind of winged it as not too expert and all the calculators online seemed to give different results. Edited the diagram to add it.

Lorenzo
– Lorenzo

2025年04月21日 00:22:49 +00:00
Commented Apr 21 at 0:22
That's indeed an "interesting" behavior! On AVR, you can analogWrite() as often as you want, and that doesn't break the PWM behavior.

Edgar Bonet
– Edgar Bonet

2025年04月21日 09:22:56 +00:00
Commented Apr 21 at 9:22
Out of curiosity, I have just tried the OP's original code (with the minimal necessary changes) both on a Uno and an Espressif ESP32_C3 devkitC-02 (Arduino ESP32 Core 3.0.4) and cannot duplicate the problem. The ESP32 analogWrite() appears to have undergone a number of changes. I've looked briefly at the ESP32 analogWrite issue history on github (there are currently 56 entries) so I could imagine that the OP has got an unlucky combination of board and core release level. Having said that, hammering away at analogWrite() at loop speed does not seem a good idea.

6v6gt
– 6v6gt

2025年04月21日 11:18:01 +00:00
Commented Apr 21 at 11:18
@6v6gt I think it might've also been some weird combination of other problems too. I didn't mention it, but my led was a high current (higher than a small TH led) and was wired with a 30cm thin awg30 wire. Some capacitance / inductance / whatever big electronic word might've played a part ahaha

Lorenzo
– Lorenzo

2025年04月21日 11:33:55 +00:00
Commented Apr 21 at 11:33

Add a comment |

How to Edit

Correct minor typos or mistakes
Clarify meaning without changing it
Add related resources or links
Always respect the author’s intent
Don’t use edits to reply to the author

How to Format

create code fences with backticks ` or tildes ~
```
like so
```
add language identifier to highlight code
```python
def function(foo):
print(foo)
```
put returns between paragraphs
for linebreak add 2 spaces at end
_italic_ or **bold**
indent code by 4 spaces
backtick escapes `like _so_`
quote by placing > at start of line
to make links (use https whenever possible)

<https://example.com>

[example](https://example.com)

<a href="https://example.com">example</a>

formatting help »
answering help »

How to Tag

A tag is a keyword or label that categorizes your question with other, similar questions. Choose one or more (up to 5) tags that will help answerers to find and interpret your question.

complete the sentence: my question is about...
use tags that describe things or concepts that are essential, not incidental to your question
favor using existing popular tags
read the descriptions that appear below the tag

If your question is primarily about a topic for which you can't find a tag:

combine multiple words into single-words with hyphens (e.g. arduino-uno), up to a maximum of 35 characters
creating new tags is a privilege; if you can't yet create a tag you need, then post this question without it, then ask the community to create it for you

popular tags »