ESP32 FastLED, and APA106 noise

Other Hardware LEDs and Multiplexing
1

Hi All,

I've been wrestling with this for some time, but I think I finally have a handle on it. With 16 LEDs, I get lots of flashing. I tried all of the suggested software fixes including delay(1) before fastLED.show(), yield(), etc., with no resolution.

The ESP32 is connected to the LEDs through a 74HC125. Today I realized that it could be a hardware issue, so I checked the data in line on a scope, and saw lots of noise. I added a 470pf cap on the data line between the ESP32 to the 74HC125 to ground, and the LEDs are now working perfectly. After doing further research, I see that a resistor is sometimes recommended in series with the microcontroller data out line.

I have to finalize a pc board, so I'm wondering if I should stay with the 470pf cap, or do further testing with a resistor between the ESP32 and 74HC125.

Advice is appreciated. Thanks.

2

Schematic please.

3

Easier to draw this, to isolate problem. I'm just guessing on the resistor value. The capacitor value is 470pf.

IMG-0564.jpg
IMG-0564.jpg1135×507 191 KB

IMG-0564.jpg
IMG-0564.jpg1135×507 191 KB

4

OK.

  • use 74hc14, or 74hct14, in preference to 74hc125. 74hc(t)14 has Schmitt Trigger inputs which will deal better with 3.3V signal from esp. use 2 x inverters in series, so signal not inverted
  • 150R should be between buffer chip and led strip, close to the first led. also a little low, change to 330~470R
  • 470pF should be 470uF and across power wires going to strip, close to first led, not on data wire
  • 0.1uF bypass cap needed for buffer chip, close to power pins of chip
  • your schematic does not show how buffer chip is powered, should be 5V not 3.3V
5

The 74HC125 is powered by 5v, and it does have a bypass cap on it. Sorry for not showing that. Also, there is a large electrolytic on the power supply. I will make all of the changes that you suggest. Thank you.