Help Using WS2811 to drive 12v RGB LED strip

Question

Greetings all,

I am creating a lighting project that I want to have a wall display with large (15" x 20") panels and each panel acts as its own "pixel" (controlled by one WS2811).

I had purchased 12v RGB LED strips with 30 LEDs per meter. What I want do is control a section of the RGB strip (about 15 LEDs). I have been scouring the web for possible solutions. One avenue I was looking at was using MOSFETs to drive the strip from the PWM outputs from the WS2811. There is an example in the WS2811 datasheet that shows using 3 LEDs per color with 12v. I do know that the WS2811 is a constant current source device. My thought was to use a logic level N-channel MOSFET to drive the LED strip. This is just based upon the research that I have done. What I have not found was any schematic to go from. I am looking at possible 3 MOSFETs per WS2811.

Any help and schematics would greatly help!

Thanks in advance!

Don

Former Member · Answer

I was able to make this work by using a simple NOT gate between the WS2811 chip and the mosfet. I believe this works because it is using TTL (transistor based logic). The WS2811 requires some kind of load to create a PWM signal just like s habaz said and connecting it right to a Mosfet gate is like plugging it into a black hole. The NOT gate (consisting of two transistors and two resistors) completes the circuit and inverts the signal.

dmilton2004 · Answer

Thanks Steve. I have not tried to trigger the MOSFETs yet, but that was my next plan. I should be able to do that by applying +5v to the Gate and connecting the Source to GND. I should see voltage flow between the Drain and the Source using a resistor or LED as a load. The data sheet indicated that the switching speed should allow me to use this for PWM. I am looking for just a WS2811 so I can post what I have. Thanks in advance, Don

michaelkellett · Answer

You can't put much resistance in the gate circuit if you want the MOSFET to switch fast - which you will if you need a high efficiency switching circuit. Lots of MOSFET drivers offer currents in the 1-5 A range and you couldn't do this though a 1k resistor. I Here's a little demo. It's a nice crude boost switcher. Try varying R2 up to 1k and watch the current in L1. (simulate with LTspice). The operating frequency is only 100kHz but it is a nice low on resistance (so high gate capacity) MOSFET. MK

Robert Peter Oakes · Answer

Hi Steve

Totally agree if the intention is to use the WS2811 chips built into an LED (The ones Ada Fruit called Neo pixels) but in this case the user is not doing that and is instead using the chip standalone and basically hacking it to increase its current and voltage capacity beyond its design specifications, and that’s great an all for education purposes but in a larger array with driving strips, not individual LEDs per chip, there are better alternatives

Like the TI Chips 16channel LED controller TLC5940

The other issue is the wiring between boards, with either approach you will need a decent communications bus or it will all fail, this brings us back to something more than just the 2811 or the TLC5940

Which means you may well have a uController there with say a sub Ghz radio and therefor as the real IO is very low pin count could be handled directly by the uController rather than a separate chip

Peter

D_Hersey · Answer

The ZUNL110A is enhancement-mode, practically all mosfets are. --Pardon the jaggedness, way way too much 425 today-- This is how you make a mosfet switch quickly. That resistor business doesn't make a lot of sense, a capacitor opposes delta V. Its only like 100pF to 1nF, so its not like we need an inrush limiter or something. I stole this from here: http://www.amazon.com/Practical-Switching-Supply-Motorola-Electronics/dp/0121370305 use it if you are PWM-ing, it works great and it's cheap. --MC, HNY to you as well-- Or to get better coverage: Season's greetings to all!!

dragonstyne · Answer

The WS2811 is a specialized RGB driver. If you want individual controllable RGB pixels in a multi circuit array, no matter what size (limited by the current capability), this is the device for you.

If you look at the data sheet on Adafruit's site, you see this is not a just a constant current LED driver. You don't feed it with PWM. You feed it with data and it provides the PWM to the LED's to give you the color and intensity you command.

If you wanted to build your own jumbotron TV, albeit expensive, this would be a good choice. I use devices such as this in my lighting for holidays.

I do agree with Peter and Shabaz, there are more efficient and easier ways to drive LED's if that's what you are trying to do.

-Steve

dmilton2004 · Answer

Hi Peter, One reason is that I already have the chips. I am going to drive 40 pixels. I have the ATtiny85s that I am going to use to program a couple of modes into. I have a small prototype already running and I already have the 12v strips. I do understand that there may be some noise and I have seen a few solutions to counter that. Since I am only doing 40 pixels and the animation is not high speed, I figured that using the WS2811 would work and be cheap. I am doing this for my church's lighted backdrop. Right now it is using 5 100w halogen spot lights with a color gel over them. They get really hot and the gels are slowly burning and fading from the heat. So hear I am designing a new led color backdrop. This project is going to expand as I go. I was inspired by several smaller addressable light display projects and I figured there is always a way to scale up. I already have my power supplies, wire, chips and LEDs. I have a plan and now I need to get a solution working. My plan is not carved in stone but I am funding this project on my own and so far the parts I have bought were inexpensive through eBay. I hope that helps paints a better picture of what I am trying to accomplish. Thanks and I appreciate the help and expertise. Don

Robert Peter Oakes · Answer

you need to be careful about current for a starter, each LED at full brightness is 60mA (3 LEDs per pixel at 20mA each), running from a 5V supply 1000 LEDs = 1000x0.06 = 60 Amps using 12V and 3 LEDs in series will reduce it by 1/3 so you're still looking at about 20Amps. you would have to feed power to the strips every so often so the current draw does not diminish the voltage too much along the strip The timing requirements I think may also limit you to 1024 devices in series, technically this may mean 3000 LED's if there the 12V strips with 3 in series per chip i would suggest you simply use a separate data line for each 500 or so LEDs to keep the single strip current down and the refresh rate up

theriddler · Answer

You can use a ws2811 with either a fet or Darlington ic. You need to replace the led with pull up resistors ,10k on a 5 volt setup will work run the output from the rgb of the ws2811 into a 74ls04 hex inverter .. you can then drive a fet ( make sure you have a 1k resistor on the output of the hex inverter to limit current ) or drive a Darlington IC .. no current limiting resistor required for this.. the trick to get the output of the ws2811 to swing logic high and low when it uses current limiting to drive the LED ( in this case about 18.5 mA) is to use pull up resistors on the rgb output . Otherwise the output of the 2811 floats I have assembled a 11 channel board to drive standard RGB strips ws2811. —-> 74ls04 —> uln2003 its is limited to 500mA per colour which was s fine for what my use was for ( about 1 m of rgb led strip ) https://ibb.co/tYFWf4y https://ibb.co/G9gYxyx I did design and build the board without any prototyping when I decided to read the ws2811 data sheet to find that the output was current limiting .. the design does work without the pull up resistors , however if you want to keep the input a clean square wave for the hex inverter I will be fitting them into the led footprints instead of the LED .. installing the LEDs on the control board does stuff things up as I over looked the vcc to the led needs to have the current draw limited with a 100ohm resistor .. either way I dont need the led onboard as a reference but instead putting 10k resistors in place

dmilton2004 · Answer

I am going to use RGB LEDs. I already have 30 LEDs per meter 12v strips (analog type). The strip has 10 sections per meter and each section has 3 5050 SMD RGB LEDs with a chip resistor per color (150ohm for Green and Blue, and 330ohm for Red). This diagram is from here . After seeing this, I figured that I can do the same thing from the PWM outputs from the WS2811. And since the MOSFETs are allowing the current to come from the 12v supply, I could use a 10 sections from a strip (30 LEDs) which will draw 1800mA at full brightness. What I am not sure of is that the MOSFETs in this diagram appear to be depletion type so they are acting in "normally closed" mode. The blog is not open for questions so I cannot ask if that was an error in the diagram. The spec sheet for the part labeled does say that it is an "enhanced" type rated at 100v but only 750mA so it is acting in "normally open" mode (all from this wonderful site that has a great tutorial on how MOSFETs work). So based upon this, it seems very plausible that this should work. And I have seen these 24v large color pixels which uses WS2811 to drive 12 5050 SMD RGB LEDs.

ipdotsetaf · Answer

hey there i know this question was on 2014 but i'll answer any way the thing is that i had the same idea and wanted to do some thing like this cuple of weeks ago BUT i actually knew that it is imposible to connect the ws2811 ic directly to mosfets and thats because the pwm input signals of the mosfet (0-5v usually) are different from the output pwm signal of a ws2811 to fix that you can basically use a rgb amplifier witch are preaty cheap OR you can make your own rgb amplifier i found a picture taken from one of these amplifiers and its basicaly a inverting op-amp (LM324 witch has 4 op-amps inside) and three n-channel mosfets (in this case A2sHB) the op amp translates the pwm signal and the mosfets amplify each r, g and b channels link to the rgb ampifier page :: https://perso.aquilenet.fr/~sven337/english/2015/01/19/Tearing-down-a-RGB-LED-strip-amplifier.html i'am also planning to create something similar so i'd be happy to know that if you could completed the project ? if so, how was the results.

Help Using WS2811 to drive 12v RGB LED strip

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