Why do I see +1.5V on Din of a WS2811-based LED strip?

I'm wiring up an LED array I recently purchased (specifically, this one), and trying to identify the correct connections. I'm thrown by an unexpected voltage appearing at [what I think is] Din.

There are three connectors; +5v and ground are obvious. The third is the data line. Although you can't see very clearly in the picture below, trying to figure out which end is Din and the Dout is pretty difficult (the LEDs, ws2811 and supporting circuits are encased in a solid polymer that is virtually impossible to read through).

So it's 50/50. And I've seen comments (like here) indicating there's no harm just trying both ends.

But before plunging in, I thought I'd at least check if there's any voltage on the data line:

• At one end (which from visual inspection and my best guess is probably Dout), I see 0V.
• At the other end (which I think is probably Din), I'm actually seeing +1.5V). Hmm.

I tried pumping a data signal in the "wrong end" (Dout/0V) and nothing happened. So far, as expected.

But I'm concerned about trying the reverse. It looks like I'm at risk of pumping reverse current into the Arduino PWM output pin I'd be using to provide the signal.

So I'm left with questions, and unfortunately I don't have another working strip to compare.

I'd appreciate any guidance on this:

• is +V on Din normal?
• perhaps it indicates a problem with the strip?
• any hints on safely exploring further?

The picture shows the voltage measurement of Din reference to Ground, with the strip powered by 5V (that's a 1000uF cap across the power lines). A bit fuzzy, sorry - third hand problem.

Edit: solved, thanks for the help!

I did identify DIN correctly, and it seems the voltage was just a floating level. Strip runs perfectly - all LEDs are correctly addressable with full RGB control - with 1k resistor in series at DIN, using an Arduino Uno and the FastLED library with WS2811 chipset selected i.e.

CRGB leds[NUM_LEDS];


Update: all sorted! I asked this some time ago, but I thought worth providing an update. Thanks to the help here, my LED strip project is cranking along nicely. If you're interested, you can see all the details and a demo at fretboard.tardate.com.

• Add a ~1K resistor between DIN and GND and see if you get the same voltage. – geometrikal Dec 6 '14 at 5:11
• If you are worried about the back current, add a current limiting resistor between the output of the arduino and the LED strip. e.g. in this case a 75 Ohm resistor would limit the current to 20mA if the strip really was driving 1.5V and the output pin was set to 0V. The input is probably CMOS so just use a 1K resistor. – geometrikal Dec 6 '14 at 5:13
• @geometrikal - thanks for the tip! With 1k from DIN to ground, voltage goes to 0V. Hmm, what does that mean? Was it just a floating level, and if I connect up in circuit it should all be OK? (if that's the answer, please submit so I can accept;-) – tardate Dec 6 '14 at 5:30
• Done :) Added some more info. BTW I just made a christmas light decoration using a WS2812B strip. Lots of fun. I think the adafruit neopixel library can be used to program the colours (?) – geometrikal Dec 6 '14 at 7:43