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I'm trying to control a few NeoPixels using a 3.3V Particle Photon.

I've got a 5V power source that I'm using to power the NeoPixels (as they require that voltage. That works fine.

Them I'm using a 74AHCT125 level shifter to increase the data signal from the Photon from 3.3 to 5.

But I'm seeing sporadic response from the NeoPixels and my hunch is there's something wrong with the power output from the 74AHCT125.

Is there a way I can measure the voltage from the data output of the 74AHCT125? I tried connecting the wire from the 74AHCT125 to my multimeter but I'm not getting any measurement from it.

  • Many ways. What equipment do you have at your disposal? An oscilloscope would be perfect but presumably you don't have one. Do you have a 5v Arduino, for example? Or an assortment of electronic components? – Mark Smith Nov 12 '16 at 19:41
  • @MarkSmith No oscilloscope, but I do indeed have a 5v Arduino (Uno). – Shpigford Nov 12 '16 at 19:43
  • Don't they work with 3.3v signals? – Gerben Nov 12 '16 at 19:49
  • @Gerben Negative: "NeoPixels powered by 5v require a 5V data signal. If using a 3.3V microcontroller you must use a logic level shifter such as a 74AHCT125 or 74HCT245." – Shpigford Nov 12 '16 at 19:55
  • Actually Vih should be at least 0.7*Vdd. So that would be 3.5V. I would be surprised if it would still work at 5V. – Gerben Nov 13 '16 at 9:38
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The reason your mulitmeter is not telling you much is probably because it reads some sort of averaged voltage, and your signal is flipping between low and high very rapidly.

If you had an oscilloscope you could easily have a look and see what the peak-to-peak voltage is, but since you don't you'll have to find another way.

If you have some basic electronic components at your disposal, you could knock up a simple peak voltage front-end for your multimeter - this looks like roughly the right thing. You'll need to calibrate it with a known 5v level (i.e. see what reading your meter tells you for a fixed 5v supply) because you'll drop some voltage across the diode.

Otherwise perhaps you could use an analogue input of a 5v Arduino: write a simple sketch to measure (very rapidly!) and tell you the maximum reading every second, say. I don't know off hand how fast the Uno will sample, or the clock rate of your Neopixels - you'll need the Arduino to sample at least as fast as your clock rate.

For the purpose of a quick hassle-free check, I'd probably plump for the first option.

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