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I was wondering how voltage drop works when using PWM from a Raspberry Pi to generate the data signal for a WS2812 LED strip.

I already have experienced votlage drop when dealing with the 5 V, 300 LED strip, I only manage to get about 15-20 LEDs at pure full white so my current solution is to wire the source (5 V, 20 A switching power supply) in parallel and have it run along the strip powering it at multiple locations.

Right now I am trying to plan forward and I am wondering if I will run into any issues when trying to run more LED's. My goal is to get two 5 m strips, which would be about 600 LEDs. If I connect them in series but with independent sources, will the data line continue to travel at the 5 V needed.

I can easily measure the voltage drop in the strip from one end to the other using my multimeter, but I don't know how to measure the PWM 5 V data signal that is going into the LED strip to see if it is indeed dropping. My main concern of having 600 LED's is the possible fact that the data line might not be strong enough to make it through all 600 of them.

I am currently using a 3.3 V PWM from the RPI, then using a 3.3 up to 5 V level shifter to ensure that the signal entering the LED strip is accurate.

Is there anyway to measure the data line PWM voltage in a LED strip? And is it possible for the 5 V PWM data signal from the Pi to travel 10 meters with timing accuracy?

I originally tried to power the level shifter with the 5 V pin from the Pi but that caused many issues where the data signal wasn't strong enough to send the value along the LED strip.

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At least for the original WS2812 there won't be any significant voltage drop of the data line as long as the 5V is stable. Each LED is driving its own output line, there is no direct connection between DIN and DOUT.

See following statement of the datasheet (page 4):

Note: The data of D1 is send by MCU, and D2, D3, D4 through pixel internal reshaping amplification to transmit.

So in your case you only have to ensure that the input signal to the first LED of the strip is strong enough.

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