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 17-Segment Alphanumeric LED Displays Issues
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By: tripod (offline) on Wednesday, February 10 2010 @ 02:59 PM PST (Read 5402 times)  
tripod

I recently bought some 2.3" 17-Segment Alphanumeric LED Displays from the EMS shop - but I'm having troubles to get them working. From the specs of the BL-S230B-12UE-11, it should operate on 2.1-2.5 V on 25mA. Unfortunately the diagram is not very clear about which pin is #1, and the case has no numbers on it neither. but i reckon that the upper left or upper right are either 1 or 11, so i tried setting one of the corner pins to +2.2V and any of the middle pins to GND. but no luck.

anyone some advice?
regards, toby


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By: Windell (offline) on Wednesday, February 10 2010 @ 06:20 PM PST  
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The forward voltage (and current) specified on the datasheet are *per LED chip,* and the diagram in the datasheet shows how these LEDs elements are arranged within the digit.

The decimal point contains a single LED element. Some of the segments have two LEDs in series, and some have two parallel sets of two LEDs in series. With 2.2 V, you should be able to light up the decimal point, but that's it. The other segments want roughly twice that.

The pin numbering is indicated on the data sheet: If you orient it such that the decimal point is on the lower right, then pin 1 is on the lower left. And it is also the same as for regular integrated circuit packages: If you hold the display such that you can read the label, then pin 1 is on the left hand side of the row closer to you.


(Also: We have a driver circuit specifically for these underway, but it's high in surface-mount parts. Could become an advanced kit.)


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By: tripod (offline) on Thursday, February 11 2010 @ 12:51 AM PST  
tripod

Thanks for this in detail explanation. Now that you mention it, I can see the "Pin 1" indicator on the diagram, too :-) Don't know why I missed that before.

Apart from your yet-to-come kit, what driver IC would you recommend ?


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By: Windell (offline) on Thursday, February 11 2010 @ 02:14 AM PST  
Windell

The best way is to use two separate constant-current serial drivers, one of which gives 20 mA per segment and the other of which gives 40 mA per segment.

You can also *build* your own drivers from scratch, using a few 74HC595's for the serial drivers, each of which drives a 2N3904 to switch the current, with a different load resistor depending on the segment/DP configuration. Smile


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By: tripod (offline) on Sunday, February 28 2010 @ 12:34 PM PST  
tripod

do you think it's possible to use 1 constant current driver and just add a 210ohm resitor in parallel to the 20mA segments?


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By: Windell (offline) on Sunday, February 28 2010 @ 02:06 PM PST  
Windell

I haven't worked with any single constant current drivers that have more than 16 channels, but I suppose that they are out there.

One of the 20 mA segments-- the decimal point -- is different from the others; I assume that you can handle that case separately.

For the remaining 20 mA segments, yes something like that could work. You may want to try this, with an ammeter in series with the LED portion, and trim the resistance to find the actually correct resistance for your particular LEDs.


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By: tripod (offline) on Sunday, February 28 2010 @ 02:11 PM PST  
tripod

for my application i don't need the DP. I got this driver:
http://www.allegromicro.com/en/Products/Part_Numbers/6276/

i'll give it a try. thanks for your quick answer.


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