Confused By Tiara NeoPixel Voltages
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Confused By Tiara NeoPixel Voltages

by mcordingley on Mon Oct 07, 2019 8:55 pm

I've been reading the tiara guide and the NeoPixel pages and am confused by an apparent mismatch. The tiara guide mentions using a 6v battery pack and uses the Gemma's Vout pin to power the NeoPixels. But, the Gemma is a 3v device and so its data output to the NeoPixels would also be 3v. The NeoPixel guides assert that NeoPixels need a data input voltage of at least 70% of the power voltage. This is much higher than the data line. Shouldn't this cause issues with the NeoPixels?

Side question: the Uberguide mentions looking at the individual NeoPixel products for specific voltage tolerances. The "Smart NeoPixel v2" (https://www.adafruit.com/product/1260) explicitly calls out an acceptable range of 5v to 9v power input, but I can find no such clear-cut guidelines when looking at, e.g. the NeoPixel Jewel (https://www.adafruit.com/product/2859). It seems that, per the Uberguide, the NeoPixel strips are much more serious about the 5v limit and could be damaged by using 6v power, but I don't know when or why this is or isn't the case. Can you please help shed some light on this?

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Re: Confused By Tiara NeoPixel Voltages

by michaelmeissner on Tue Oct 08, 2019 12:34 am

Note, I haven't used a Gemma for a long time, but I have been using neopixels with 3.3v processors (mostly Teensy) for some time.

In general, the neopixels provided by Adafruit have changed slightly in terms of specs over time, and how well the neopixel works outside of the specs can also vary.

I have had neopixels in the past that DID NOT work if you fed the pixel 5v of power (from USB) and used a 3.3v data signal (due to 70% of 5v being 3.5v, and so it depends on sample variation). Some neopixels did work, some did not work. It can be a bummer if you work on a project and need to replace a part, and the new version of the part had slightly different variances and did not work, because I was out of spec.

In fact when I was using the V1 Gemma with neopixels, when I programmed it via USB, the neopixel ring I had at the time would not work after re-programming it. So I had to program it via USB, unplug the USB, and plug in a lipoly battery to test the pattern.

Regarding 6v, that can be dangerous. The datasheet for the SK6812 (listed as the chipset for modern neopixels since 2016) says the voltage should be 3.5 - 5.5v. The WS2812B data sheet used in older neopixels says the power range should be 3.5 - 5.3v. Will either work at 6v and not burn out? Hopefully, but it is out of spec. The Gemma M0 can be feed up to 6v, and the Gemma V2 can go to a higher voltage, but if you are using neopixels you probably want a lower voltage.

This is what the uberguide has to say:
Some (not all) NeoPixel products can work with slightly higher voltages. This depends on the additional support components around the chip, based on available space, cost and the most likely application. Refer to the specific product description page for guidance on acceptable voltage limits for each type. When in doubt, aim for 5 Volts.

So in general, if you want the simplest amount of work, feed it 3.7-4.5 volt (such as the lipoly batteries) or a 3 AA battery holder (https://www.adafruit.com/product/3287). Now unfortunately, the Gemma does not have a battery charger in it, so you would need to add a separate charger that charges the lipoly battery from USB or other source. Similarly with the AA batteries, if you are going to be using a bit, you probably don't want non-rechargeable alkaline or lithium batteries, but instead rechargeable NiMH's.

You want to use VOUT to attach to the rings, as this is whatever voltage is powering the Gemma (i.e. 3.7-4.5v) and not 3.3v, which is below the spec. Some neopixels will in fact run at 3.3v, but note the 3.3v voltage regulator on the Gemma isn't that powerful.

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