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Topic: Need help multiplexing high voltage 7 segments (Read 393 times) previous topic - next topic


May 29, 2013, 09:33 am Last Edit: May 29, 2013, 09:35 am by Kev_O Reason: 1
I am having trouble trying to multiplex 4 inch digits from Jameco. I am using 2 MAX7221 each controlling 3 single digits (a 4 inch and a 1 inch version). The big version requires a separate driver to power the 8.5 - 14 volt display @ ~20mA. So, I adapted my schematic from one of the users on this forum (yesyes). However, I made a few changes:
1. Replaced the UDN2981 with a TD62783 - Helps reduce/eliminate ghosting issues
2. Changed the 2.2 kohm pullup resistors on the ULN2803 to 10kohms - further reduces ghosting, i guess?
3. I am powering the big display via the VIN pin, supplying the arduino with a 15V regulated wall wart

So far, I am glad to see that of this setup works, sort of. I have my code setup to show the same information as in the small display. The only problem I have now is trying to display 3 different numbers on the big display. For example, if I send a 2 to digit 1 on both ICs, the small display lights the correct digit. However, the big display outputs 2 on all digits. The linked video shows the problem better. Essentially, the big display should mimic the small display.

I've tried swapping the max with a new one, changing scan limit, and connecting DIN, CLK, and LOAD directly to arduino but to no avail. Any ideas? Thanks for your help.

Video: https://docs.google.com/file/d/0B1V3EHXGfPYcNjkzQjdtaTFxNDQ/edit?usp=sharing
Schematic: https://docs.google.com/file/d/0B1V3EHXGfPYcVk9vR1U0VXBFSnM/edit?usp=sharing
Code (sorry its so long): https://docs.google.com/file/d/0B1V3EHXGfPYcdDRRQk5tYVlpWms/edit?usp=sharing

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