Question regarding buffer IC for simple OctoWS2811 project

ladansedesdamnes

Active member
Hello!

I'm looking to do some testing using the OctoWS2811 library to drive some WS2812b leds.
I know I need to use a buffer to bump the 3.3V signals to 5V to be able to drive the leds, but have a small question that I've been so far unable to find answered anywhere.

The recommended IC is the 74HCT245, I'm wondering if the 74HC245 would work as well?
From what I can find on the spec sheets, the difference lies in the supply voltage, which should be ok for the HC version, am I wrong?

I know there is an adapter shield, but it's really overkill for what I'm trying to do, I just need something simple and straightforward to experiment with.
 
The recommended IC is the 74HCT245, I'm wondering if the 74HC245 would work as well?

From this page:
The only difference between HCT and HC is the input voltage levels.
For HC, anything below 1.5V is a low signal, anything above 3.5V is
seen as high (assuming 5V supply). For HCT, these levels have been
changed to 0.8V and 2.0V so that they can be driven by TLL outputs,
meaning that wether a HCT can be replaced with a HC or not depends
on what it is driven by.

Since a HIGH output of Teensy is 3V3 max, an HC part will not suffice.

Paul
 
Ah, that's unfortunate, I could source the HC locally, but not the HCT.

Well, time to order from abroad and just wait :(

Many thanks for the clarification!
 
Another possibility is to use one or more 74AHCT1G08's (a single 2-input AND gate).
This part is used on PJRC's Prop Shield as a level-shifter.

Paul
 
Another possibility is to use one or more 74AHCT1G08's (a single 2-input AND gate).
This part is used on PJRC's Prop Shield as a level-shifter.

Paul

I actually found that my simple 8-led board can be controlled using the normal Teensy output, so that covers it for now.
Nonetheless, I'll be looking into that for future
 
I actually found that my simple 8-led board can be controlled using the normal Teensy output, so that covers it for now.
Nonetheless, I'll be looking into that for future

On paper the WS28 series LEDs need 0.8 * VCC on the input pin for it to count as a high. Which means if you supply them with 5V you should drive the input to 4V to be sure of correct operation. That doesn't mean it will never work with a lower input voltage, just that for certain parts and certain temperatures it may not work.

So while it's working for you right now keep in mind that if you get a different LED board it may not work.

Just about any logic gate in the HCT family can be made to work. If you don't mind the signal being inverted a single transistor and a resistor will do the job, with two of each you can get the signal back the same way around.

Or without the correct parts there are a couple of tricks you can use, these aren't recommended but are sometimes good enough:
The threshold is 0.8*Vcc which means if you drop the supply voltage to the LEDs (they will run down to about 3.5, they can go lower but then the blue LED gets dim) then the required input voltage for reliable operation also drops. If you can drop the supply voltage for the first LED in the chain to around 4.5V (by say putting a diode in series, don't try this with a resistor) then the input requirement drops to 3.6V, still not guaranteed with a 3.3v output but a lot more likely to work. Since the output to the next LED in the chain will be close to the LED supply voltage you only need to do this for the first LED, the rest can run off 5V.

Similarly if you have a long chain of LEDs then the voltage will drop along it. If you supply power from the opposite end the the one driving the data that drop along the chain may ensure the threshold for the first LED is low enough, just remember that the drop will in part depend on how many LEDs are on so this could potentially get stuck if you turn most of them off.
 
Many LEDs with different controller chips have sold as "WS2812B", since they first appeared on the market around 2011-2012. Most of the ones from 2014-2016 really did require nearly 4 volts to receive as logic high. Today nearly all of them will accept a 3.3V signal. What the actual specs really are, who know?!
 
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