I'm trying to whip up a little LEDness this evening for a Halloween weekend

(Marc Miller) #1

I’m trying to whip up a little LEDness this evening for a Halloween weekend night hike. Running the current FastLED, a 32 pixel LPD8806 strip, and a Teensy 3.0.
If the Teensy is plugged into/powered by:
a) my computer, everything is normal. Reading volts across Teensy GND and Vin pins shows 4.9V.

b) a XTG Technology battery (5V 1A 1500mAh), everything is normal. Volts across Teensy GND and Vin pins reads 4.9V.

c) an Anker Power Bank battery (5V 1A 4500mAh), the displayed pixels are incorrect and animations don’t move. Reading volts across Teensy GND and Vin pins shows 5.2V. If I unplug/plug power in about 1 out of 10 times the pixels are correct for about 2 seconds and then they are drawn incorrectly and animations don’t move.

So… I’m wondering what could be the difference between these batteries and why might the larger mAh one not work with the setup? 4.9V vs 5.2V doesn’t seem like it would be the issue as Teensy should be able to run on 3.7-5.5V range.

Surprisingly this is actually the first time I’ve ever tried to run on batteries so it’s new to me. I’d love to hear what sort of similar simple battery packs you all have had success with and like to use.

(Daniel Garcia) #2

The problem is probably very likely the 5.2v. This was a major issue with the WS281x strips - if you fed them 5.2v of power, they were unable to read data signals from the teensy. The teensy’s pins use 3.3v as “high” - so my guess is if the VIN of the leds is too high, it’s a crapshoot whether or not they’ll pick up a 3.3v high. The solution is to either drag the voltage down somehow or use level shifters.

(Daniel Garcia) #3

(And yes, clock/data based signals should be a bit more robust on this front, but I’ve still seen it manage to be a problem).

(Marc Miller) #4

I was /just/ wondering if it might be the LPD8806 strip that wasn’t liking the voltage. Thank you for confirming what is most likely the issue.
Good thing there’s room on that scrap of perf board I grabbed! I shall try adding a 74HCT245 and let you know!

(Dushyant Ahuja) #5

A simple diode between battery and VIn to drop the voltage should work.

(Daniel Garcia) #6

You don’t want to drop the voltage on the teensy, you want to drop the voltage on the leds. I’m not sure you want to be pumping an amp or two through your average diode! (It might be ok for a small number of leds, but it’s the wrong solution)

(Juergen Bruegl) #7

Simply try an LM7805 to check whether it is an voltage issue. As you only want to get rid of 0.2 V you can get 1.5A out of it.
2 small caps and you are ready to go

The datasheet of the LPD8806 recommends a signal level of -0.4 … 0.4V of the supply voltage. https://solarbotics.com/download.php?file=1889

(Daniel Garcia) #8

@Juergen_Bruegl I think you’re reading that bit of text wrong - it’s saying the range is from -0.4V to (Vout + 0.4V) — but that doesn’t give any indication as to what voltage is necessary to register reading a high (on either clock or data).

(Look at where the ~ is used. They use ~ to indicate range, so 4.5V ~ 5.5V for a range of 4.5V to 5.5V, etc… in what you’re referencing it is -0.4V ~ Vout+0.4V)

Of course, the data sheet in that chart gives a range of 0-2Mhz for the dat/clk frequency, even as the datasheet earlier on claims a 20Mhz clock.

One thing I’ve learned dealing with these datasheets over the last 5 years is that they often have inaccurate information. Especially when a chip is a refinement of an earlier chip - then there’s a lot of copy/paste of old data, and not everything gets updated to the new info :slight_smile:

(Marc Miller) #9

I added a 74HCT245 an everything works fine now. And now I have a little test rig that I can use with Teensy 3.2 and APA102 in the future. Huzzah.

So in general, are these sort of little power bank battery packs good/safe for using with a smallish number of pixels?
What about this one that’s 3x times more mAh then the larger one I currently have? http://www.amazon.com/gp/product/B00K64WR9C

(Scottie D369) #10

Those Solar Power Banks are JUNK!!! I own 3 of them by now and the solar cells don’t recharge!

(Robert Atkins) #11

I like my Limefuel L130x packs—just don’t tie the positive rails together from multiple packs.

(Marc Miller) #12

Thanks @Robert_Atkins