Are there any tested/documented benefits in using BOTH SN74AHCT125N and a Sacrificial LED ?
Or are they simply interchangeable?
Main differences/use cases?
Already did a lot of testing here and seems like the SN74AHCT125N covers pretty much all use cases but would like to establish my “standard” for future use.
Thank you
A SN74AHCT125 is guaranteed by the manufacturer to meet TTL voltage and timing standards as documented in their spec. sheets. They are ubiquitous in the electronics industry and used in a vast array of products from computers to TV’s to life support equipment and more.
A WS281x series LED is reasonably well documented by it’s manufacturer and used in many lighting products. Performance of individual parts will vary as will adherence to “standard” specs by various manufacturers. Personally I would trust the 74AHCT specs over the WS281x any day of the week just by the sheer volume of testing of the 74 series devices in a huge array of applications (relative to the WS281x).
The real world has shown many cases where the sacrificial LED works to solve problems. But if you’re designing a product to go into production, use devices with guaranteed specifications (74AHCT) to compensate for situations where you can’t necessarily trust the specs of other devices (WS281x).
I’ll take the 74AHCT over the sacrificial LED any day of the week.
As I said in an earlier thread if you need distance, substitute the MAX485 and get level shift and distance in one device with proper design specifications.
Did you witness any case where the Sacrificial pixel would work while the SN74AHCT125 wouldn’t?
Since I already have a number of SN74AHCT125 handy, just wondering if there is any reason to keep a “sacrificial pixel” in my wled toolbox.
Ps: do you have a buy link to the MAX485? To be sure I refer to the correct component.
Thanks.
If you can find a scenario where the sacrificial LED works over the 74AHCT, I submit you have a more serious case where you don’t really understand the problem you’re solving (not trying to be condescending here).
I’ve described the sacrificial LED as the “poor man’s level shifter”. You use the technique when you can’t get parts or can’t fit another part in the space or are just too lazy to do it properly.
Perrsonally, I almost never use level shifters at all. I put the MCU close to the strip or use RS485.
Does this look right?
I really dont mind 4 wires over 3 wires if it adds longer run reliability + level shifting in a single package.
BTW, i guess this would also work for any I2C devices ( like the BME280 for instance ) which have a very limited tolerance to wire lenght?
Got a bunch of these MAX485 boards on order at the moment.
Have to see how well they work in real life, but there’s quite the number of similar Amazon variants at reasonable prices. The actual IC part# is MAX485.
It might work for I2C in some cases but that’s a different kettle of fish.
The I2C bus can be very bidirectional in nature which the RS485 devices don’t always handle.
The WS281x protocol id defined as one-way, you transmit to the 1st LED, it receives and that’s it.
Extending the I2C bus is really a different design problem with different solutions.
No offense at all, that was exactly my perception. At a few cents a piece for the SN74AHCT125, not sure how there can be a “poor man’s solution” over it… 
Ok thanks, I’ll test it with limited expectations
I agree with everything divsys said. One scenario where a sacrificial pixels would probably be preferable is to boost signal where there was a significant gap between runs.
As for keeping sacrificial pixels in your toolbox, I seem to end up with one or two more every time I cut a strip to size.
Mmmm. That would imply the WS2811 pixel does a better job at boosting signal than the SN74AHCT125N. doesn’t seem to be aligned with Divsys statement above. Is this verified or an assumption?
Thx
I’ll jump in and ask:
Define “better”?
I can just about guarantee that a sacrificial pixel is easier to wire inline somewhere near your strip.
But I’d also bet, that people try it and if it works they think “OK problem solved, on to the next”.
Likely it works but I’d argue it’s not the solution to the underlying problem.
But hey - it’s fast, cheap, simple, and sometimes works - so that makes it “Better” right?
If you really want to know, you’ll have to try both options with varying lengths of Output wire and see the results.
Agreed, that’s why I was wondering if anyone had actually tried and compared results in a real life environment with other parameters unchanged.
Will post back if I eventually try / get results worth posting.
The problem is that you don’t have easy access to the 3v3 reference voltage required by the level shifter in a trench 20m down your driveway (and from your controller).
I was originally referring to mcu – first led
but yes it does make sense between runs as the level shifter is not really a viable/logical option. Good point.
But in this particular case I guess we’re talking about adding a short run (read: single pixel, 12v or 5v - same as rest of strip) in the middle of a longer hop (vs the so called “sacrificial pixel” 5V with diode, close to the mcu).
Can the MAX485 be used as a level shifter also, or are you just using it for serial communication?
It works well as a level shifter since the specs for a Hi input on a transmitter are down around 2V, well within the output range of the 3.3V devices even when it runs on 5V. You get full swing outputs on the receiver with lots of drive capability.
The net effect is you can put the transmitter right beside the MCU and the receiver right beside the LED’s.
No data line drops and you often can “steal” 5V power for the RS485 pair from either the LED’s, the MCU or both.