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The goal

I want to isolate a 3.3 V MCU (ESP32 C3) from a WS2815-Strip but I had no luck so far. I already read these threads, because I think they're dealing with a similar problem: MOSFET - 800 kHz digital signal, ESP32 - 6N137, but had no luck so far.

This is my current schematic from there(slightly different), which works fine when I'm transmitting a DMX512-signal, but can't get it working with these pixel-LEDs. enter image desc]

What I already tried

  1. Changed R12 to 390 Ω, 240 Ω and 133 Ω - doesn't work.
  2. Changed R13 to 1kΩ - doesn't work.
  3. Tried other LEDs (WS2815, WS2812) - doesn't work.
  4. Wiring LEDs directly to DMX_TX -> works, but not isolated.

my thoughts and possible solutions

  1. What is the problem with my schematic? Other people seem to get this to work
  2. Might the circuit work with a small MOSFET to invert the signal like below?
  3. Does the gate need a small pull-down to GND or can I leave it floating only with R15?

enter image description here

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  • \$\begingroup\$ Welcome! Please show oscillograms. Have you tried to simulate your circuit? \$\endgroup\$
    – winny
    Commented Apr 5, 2023 at 15:12
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    \$\begingroup\$ You talk about 6N137 but have drawn a 6N136. They are completely different isolators. Which one is it? \$\endgroup\$
    – Justme
    Commented Apr 5, 2023 at 15:19
  • \$\begingroup\$ Assuming it's a 6N136, not a 6N137: you've left pin7 open. I have found that for phototransistors, it can help with the frequency response to pull the base down with, say, a 100k resistor. It helps them turn off quicker. \$\endgroup\$
    – Peter Balch
    Commented Apr 5, 2023 at 15:25

1 Answer 1

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Many many thanks to your quick answers! With a few changes in my circuit it workes just fine:

  • Resistor R3 = 390Ω
  • Pull-down on pin7 (R15) = 100k
  • Pull-up on pin6 (R4) = 1k
  • U10 has to be a 6N136

working circuit:

enter image description here

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  • \$\begingroup\$ It still can't be a 6N137 then, as that would disable 6N137 output. \$\endgroup\$
    – Justme
    Commented Apr 5, 2023 at 18:11
  • \$\begingroup\$ okay, then I'll use a 6N136 instead in the future. But right now, my circuit still works good with a "6N137S" and R15 as Pull-down. How is this possible? \$\endgroup\$
    – kokospalme
    Commented Apr 6, 2023 at 7:32
  • \$\begingroup\$ The 100k seems be too high a value to have any effect on 6N137. But like I said, 6N136 and 6N137 are not interchangeable and don't work in the same circuit, they are different and require different circuit. If you now have 6N137 working and change to 6N136, you need to redesign the output pull-up and the base pull-down. \$\endgroup\$
    – Justme
    Commented Apr 6, 2023 at 8:22
  • \$\begingroup\$ okay, I'll have that in mind! thank you for your expertise :) \$\endgroup\$
    – kokospalme
    Commented Apr 7, 2023 at 15:40

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