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Per advice in this post I am using a 74AHCT2G125DC (datasheet) to drive a Neopixel/WS2812b strip's data pin from a 3.3v MCU. The MCU is driving A1, and the addressable LEDs are chained from Y1. VCC on the high side is 5V and the ground is shared.

I'm seeing numerous communication errors and flickers on the LEDs although there is no voltage sag on the 5V rail. Oscilloscope probing the input side of the 74AHCT2G125DC buffer shows a clean signal between 0-3.3v. Probing the output side shows a much noisier signal ranging from 1.2-5.0V. It never reaches the ground which I suspect is causing some of the issues.

What is the best explanation and how do I fix this? My current theory is that somehow driving down the LED chip's pin to GND must sink more than the current limit of the driver (8mA, if I read it correctly.) Or am I misunderstanding that a driver of this sort can feed a digital signal out to a device like a WS2812b? Is there any explanation for why on a low signal, the output doesn't fall to zero?

Note: The reason for using a tri-state driver is so I can shift the digital out to a high-impedance state during circuit shutdown preventing an observed current backflow to the MCU when the strip is unpowered.

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SamGibson
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Jeremy Gilbert
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  • Explanation is somehow messy. Can you show whole circuit? There're no LEDs on current one. – Anonymous May 16 '17 at 18:21
  • I'm more concerned about your wiring, how are you powering the LED strip and how do you have the grounding set up. – Trevor_G May 16 '17 at 18:38
  • Updated schematic should answer both questions. Thanks, I appreciate the help. – Jeremy Gilbert May 16 '17 at 19:15
  • Are you sure you do not confuse connecting WS2812B's DO instead of DI to the output of 74AHCT2G125DC? HCT outputs up to Vcc, LED's high level is 0.7, low level is 0.3, thus levels must match. LED's input current is 1uA, thus it should not affect output of the gate - unless you wire something wrongly. – Anonymous May 16 '17 at 20:00
  • @anonymous, not seeing how I could have done this but I will try with different strips. If the notched edge is on the upper left, i believe the DIN is the lower left side. I copied the adafruit neopixel strip design from Eagle. – Jeremy Gilbert May 16 '17 at 20:30
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    Agree, DIN seems to be where you say... You can also just disconnect LED strip at all and see what is going on the 125's output without load. – Anonymous May 16 '17 at 20:54

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