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By: Anonymous: dave () on Wednesday, July 01 2009 @ 04:32 AM PDT (Read 27747 times)  
Anonymous: dave

Hi,

I am planning on mounting the LEDs away from the PCB, by taking a line out from each of the rows and columns to recreate the matrix remotely. Can you see any issues with with doing this? I will most likely be using two lengths of 25core cable around 10m long to accomplish this.

Secondly I am using LED's that are 3.4V @ 30 mA, do I just need to alter the value of the resistors?

thanks.





       
   
By: karlgg (offline) on Wednesday, July 01 2009 @ 08:48 AM PDT  
karlgg

Ten meters sounds like an awfully long run for such a small circuit. You can't mount the board closer to the display?


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By: Windell (offline) on Wednesday, July 01 2009 @ 09:41 AM PDT  
Windell

You did not say, but I'm presuming that this is a Peggy 2 based project.


>I will most likely be using two lengths of 25core cable around 10m long to accomplish this.

So you're just bringing out wires from the rows and columns and hand-wiring the full matrix on the far side?

10 m *is* a long way. If the resistance of each wire pair can possibly be an issue-- be sure to measure it, and make sure that it's less than 10 ohms for the round trip.

There will be increased capacitance from the extra wire. This may limit the speed at which you can operate reliably; you may need to lower the refresh rate in the firmware.

>Secondly I am using LED's that are 3.4V @ 30 mA, do I just need to alter the value of the resistors?

You do not need to alter anything to use those LEDs-- they will work well without any mods at all.
You *can* alter the resistor value to draw slightly more current if you want to, however.


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By: Anonymous: dave () on Thursday, July 02 2009 @ 04:25 AM PDT  
Anonymous: dave

Yeah sorry this is a Peggy 2 based project.

Its a bit mad but I am hand wiring the full matrix.... I was planning on keeping all the business end in one place with all the other bits and pieces, but I guess the Peggy PCB could be remote as well - do you know if there would be any issues with running a 10m TWI connection to the Peggy? (im using an arduino as a serial/twi converter, and the video hack)

thanks.





       
   
By: Windell (offline) on Thursday, July 02 2009 @ 10:02 AM PDT  
Windell

>do you know if there would be any issues with running a 10m TWI connection to the Peggy?

Yes. That almost certainly won't work. TWI is only good for very short connections. The "smart" way to do this would be to put the Arduino board right next to the Peggy and run a longer serial cable. BUT, even that is tricky at 10 m if you are using USB-- the maximum length of a USB cable is only 5 m. This really would be a good job for a serial (RS-232) based Arduino-- that could handle it.


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By: Anonymous: dave () on Thursday, July 02 2009 @ 11:35 AM PDT  
Anonymous: dave

> the maximum length of a USB cable is only 5 m

I have successfully used a usb wireless g adapter on a 15m cable before, just to check I dug out two 5m extensions and my mouse receiver is working fine on a 10m.

Oh well looks like I have to give up and build a waterproof enclosure for all the gear and give up any ideas of keeping things separate!

thanks for the advice.





       
   
By: Windell (offline) on Thursday, July 02 2009 @ 11:38 AM PDT  
Windell

>I have successfully used a usb wireless g adapter on a 15m cable before

If you think that a USB extension can do this, that would be more likely to work than TWI over that distance.


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By: MonsieurBon (offline) on Thursday, July 02 2009 @ 02:55 PM PDT  
MonsieurBon

Quote by: dave

Hi,

I am planning on mounting the LEDs away from the PCB, by taking a line out from each of the rows and columns to recreate the matrix remotely. Can you see any issues with with doing this? I will most likely be using two lengths of 25core cable around 10m long to accomplish this.



Have you considered putting 2x 25+ pin connectors on the Peggy from the rows and columns? It could be nice to not have the cables hardwired onto the board and the LEDs. I was thinking of doing something similar and figured I'd use scavenged IDE cables and headers, which you could probably do as well with the cable you're using.


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By: Windell (offline) on Thursday, July 02 2009 @ 07:00 PM PDT  
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>Have you considered putting 2x 25+ pin connectors on the Peggy from the rows and columns?

No-- this is definitely the exception not the rule; I'm not aware of anyone else doing it. As we say, Peggy is meant to be hacked-- so you're totally OK with us doing it, but that doesn't mean that we need to build it in. Wink

Meggy Jr RGB has a similar architecture, although it's set up to do 32 rows by 8 columns. The middle part-- to drive the 24x8 matrix LED matrix *is* fully socketed. So, if you were doing a smaller array, that might be more suitable. (Meggy Jr also has a serial port that's available while running.)


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By: MonsieurBon (offline) on Thursday, July 02 2009 @ 09:14 PM PDT  
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Quote by: Windell

>Have you considered putting 2x 25+ pin connectors on the Peggy from the rows and columns?

No-- this is definitely the exception not the rule; I'm not aware of anyone else doing it. As we say, Peggy is meant to be hacked-- so you're totally OK with us doing it, but that doesn't mean that we need to build it in. Wink



Sorry Windell, I was suggesting that to Dave, not to you.

But do you think you could add a function to the Peggy that makes free ponies for everyone? Smile


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By: karlgg (offline) on Friday, July 03 2009 @ 01:19 PM PDT  
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Free ponies! Yes!


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By: Windell (offline) on Friday, July 03 2009 @ 01:27 PM PDT  
Windell

>Free ponies! Yes!

Err... do you have a digi-key part number for those?


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By: karlgg (offline) on Monday, July 06 2009 @ 07:24 AM PDT  
karlgg

Apparently DigiKey refuses to carry livestock, no matter how important you say they are for your "projects". Maybe better luck with Mouser? Big Grin


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By: Anonymous: dave () on Thursday, July 16 2009 @ 03:57 AM PDT  
Anonymous: dave

ended up running the 50 wires... all working nicely although its far to dim at the moment. The round trip is 10-11 ohms, taking this into account what would you recommend changing the resistor values to to get the max possible brightness? the trim pots do not seem to do anything either.

LED specs:
Forward Voltage (V): 3.0-3.4
Forward Current (mA): 20-30

thanks.





       
   
By: Windell (offline) on Thursday, July 16 2009 @ 04:40 AM PDT  
Windell

10 ohms at 20 mA takes up a maximum of 0.2 V-- this is probably OK, assuming that your power supply is not running low on batteries (or otherwise low voltage). Sounds like you've got a multimeter, so you can check your power supply voltage.

You should see some effect from the trimpots. To check, adjust one all the way up and the other all the way down-- there should be a clear difference. If not, something else may be going on.

>Forward Current (mA): 20-30

You *do not* want to exceed the actual maximum at any point. The default config is set up to be safe for 20 mA rated devices; if you're not sure what the actual rating is, you probably do not want to mess with that parameter.

Columns 0-15 are controlled by LED driver chip U4. The control resistance for U4 is the sum of the variable resistor (trimpot) VR1-- which can be as low as zero-- and resistor RA1.

Columns 16-24 are controlled by LED driver chip U5. The control resistance for U5 is the sum of the variable resistor (trimpot) VR2-- which can be as low as zero-- and resistor RA2.

The datasheet describes the relationship between control resistance and programmed current. For 30 mA, the correct control resistance *appears* to be approximately 625 ohms. HOWEVER, if you want to mess with this and not destroy your LEDs, it's a good idea to measure the actual currents involved, and use that as your true guide.


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