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Topic: Impedance matching 4ft of 20awg wire (Read 4031 times) previous topic - next topic

Boardburner2

#15
Apr 16, 2016, 06:37 pm Last Edit: Apr 16, 2016, 06:45 pm by Boardburner2
I would still like to try it. Someone said it worked well for them, but there wasn't enough detail. 1k across data and clock is what the said, but is that across one another or to ground?

Probably data to ground and clock to ground.

I did this with a neopixel strip which has only 1 signal though.

Values , hard to say its suck it and see with an oscilloscope really, tweak until ringing is reduced.

But if you are able to control your strip OK its not necessary.
Once the first LED gets a good signal the rest of the strip regenerates the signal anyway.
Normally only a problem with long data feed wires.

I haven't tested this. But if I set up the LEDs to only get written once in setup, will that achieve the same thing?


The reason i suggested it is once they are on , physically disconnect the signal wires.

That may eliminate any possible glitches from the arduino end being the culprit.

Qdeathstar

#16
Apr 16, 2016, 07:11 pm Last Edit: Apr 16, 2016, 07:13 pm by Qdeathstar
Well, I will try that but I've noticed if you touch data and clock lines you can get led strips to light up at random, I I think disconnecting the data and clock lines will introduce noise that will change things from the get-go.

I also have breaks in the line about 1/2m in length every 10m or so... Maybe I should put a resistor there (though it would be difficult)
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Boardburner2

#17
Apr 16, 2016, 07:22 pm Last Edit: Apr 16, 2016, 07:25 pm by Boardburner2
Well, I will try that but I've noticed if you touch data and clock lines you can get led strips to light up at random,


You act as an aerial ant put out random noise.
That noise will be faithfully amplified all the way up the chain until a pixel interprets it as data.

1K to ground on both signals should fix that, preverably right next to the strip input.

Putting resistors anywhere past the first pixel may be a bad idea. possibly damage the amp output of the previous pixel that way.

Qdeathstar

A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Qdeathstar

You act as an aerial ant put out random noise.
That noise will be faithfully amplified all the way up the chain until a pixel interprets it as data.

1K to ground on both signals should fix that, preverably right next to the strip input.

Putting resistors anywhere past the first pixel may be a bad idea. possibly damage the amp output of the previous pixel that way.
How do the terminating resistors work then?
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Boardburner2

#20
Apr 16, 2016, 08:11 pm Last Edit: Apr 16, 2016, 08:13 pm by Boardburner2
They are there to match the characteristic impedance of the cable to the load.

Without them reflections occur.
This distorts the signal.

With a square wave that appears as ringing which can cause clocking errors.



Eg a video cable with an impedence of 50 ohms should be terminated with 50 ohms.

Likewise an audio cable is normally terminated with 600 ohms.

Pixel to pixel distance is so short to make them unnecessary between pixels.

Boardburner2

#21
Apr 16, 2016, 08:16 pm Last Edit: Apr 16, 2016, 09:10 pm by Boardburner2
I should add that it is essential to avoid ground loops for this to work properly.

I think disconnecting the data and clock lines will introduce noise that will change things from the get-go.

I also have breaks in the line about 1/2m in length every 10m or so... Maybe I should put a resistor there (though it would be difficult)
Unplugging the arduino psu would be just as good i think to avoid the soldering iron.

Your earlier offer of a video may be helpful, if only to see the physical layout.
At 50M length a circuit diagram does not always tell the whole story.

Qdeathstar

Ok, I will take a video tonight and YouTube it
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Qdeathstar

As promised, video. This is a ten second clip of one of the blinks. I recorded video for about 2.5hrs, and got three blinks, but this is one. (they all look roughly the same). I have it play twice, once it real time, and one slowed down 20x. You can see the blink "start" at the end of the strip, and work its way back toward the middle.

https://www.youtube.com/watch?v=Ad3DHnSiE1k

I also got a rough schematic. The red arrows represent data and clock flow, i've labeled the MCU's location, and the blue dots represent power injection points. I have those protected with ptc fuses and filter noise out with caps.

http://s22.postimg.org/7av1gye4x/Image2.png

hopefully a video is worth infinate words.
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Wawa

#24
Apr 17, 2016, 07:34 am Last Edit: Apr 17, 2016, 07:34 am by Wawa
You can see the blink "start" at the end of the strip, and work its way back toward the middle.
I think that means that you can rule out data and clock line problems.
Data/clock signals can only move forwards.

I would think it's a power problem. Spikes from long supply lines. Too fast for normal electrolytic caps.
I would start with a 5volt TVS diode across the supply at the very end of the strip.
And if problems persist, some more at various other easily accessible points on the strip.
Leo..

Qdeathstar

#25
Apr 17, 2016, 07:49 am Last Edit: Apr 17, 2016, 08:00 am by Qdeathstar
thanks for your response, the supply voltage is 12v, so should I get a 12v one?

I also see they can be bidirectional or unidirectional, does it matter which I get?
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Wawa

Yes. 12volt TVS for a 12volt strip.
Uni-directional. Cathode (ring) to +12volt.

e.g this one.
http://nz.rs-online.com/web/p/tvs-diodes/6295254/
Leo..




Boardburner2

#27
Apr 17, 2016, 01:46 pm Last Edit: Apr 17, 2016, 02:42 pm by Boardburner2
When you said blink i assumed you meant off.

Were the leds at full brightness when that happened ?

The fact that all the leds kept the same colour makes me suspect that was caused by a sudden burst of power.

EDIT

I suspect the percieved traveling of the lights is an artefact of the video recording.

Qdeathstar

the leds were not at full brightness. they were at 10 percent.  I think they probably get to full brightness during the flash. what do you mean by "artifact". what do you think it actually looks like? even at full speed you can kinda see the last five or so pixes go wonkers, followed my more than half...

thanks

A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Boardburner2

#29
Apr 17, 2016, 03:34 pm Last Edit: Apr 17, 2016, 03:39 pm by Boardburner2
Video is scanned from the top down so if at the time of the flash the scan is half way down that frame does not 'see, it, the next frame picks it up however.

This gives the perceived effect of movement where there is none.

A similar faster effect occurs in the horizontal scan direction.

Stop motion, (frame by frame) is needed to be sure, that event is just too fast.


Does your power supply allow the output to be floating, that is not connected or referenced to mains earth.

Lab supplies normally do this but i am not sure about server PSU's

EDIT

Trying to video with the camera on its side may give more insight.

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