MAX7219 and higher voltage LED displays - how?

[khsoft]

Ok, thank you. I'm impressed with your project a lot! I wonder if there is the a way to get rid of one chip. Is there a sink driver without inversion? Will using common anode LEDs allow to use fewer chips?

 

[yesyes]

Thanks ;-)
The MAX7219 can only directly drive common cathode displays. Though it should be possible with similar electronics to drive common anode displays. But then you would have the same problem with the inversion, just on the segment drivers instead of the digit drivers.
You may be able to find a non-inverting (or rather double inverting) sink driver. I'm not aware of one but I haven't really looked around as I already had the 2803s "in stock". ;-)
You could also use 8 transistors to invert the signal, but that would probably take up even more space on the PCB.

 

[khsoft]

I tried the same scheme with common anode 7-segment display (MAX7221 + UDN2981 as digit driver + 2*ULN2803A as segment driver) - without success so far... The display defiitely reacts, but the segments do not work properly - For some reason, I cannot control individual segments in the way I need (there is a mess - all segments on all digits are either on or off).

I tried to pull up/down segment/digit outputs of MAX7221 (MAX7221 outputs, in contrast to MAX7219 outputs, are not pulled up/down internally), but without success. I can controll 8x8 dotmatrix display with MAX7221 (without UDN2981 and ULN2803A), so I am pretty sure that my software part works.

Did you leave pin10 of ULN2803 in your scheme unconnected?

 

[yesyes]

yes, it works for me with exactly the same schematic as I posted in post #18 (page 2) in this thread. Pin 10 of the ULN2803 is left unconnected.
I don't think it would work with common anode displays. Do you have a schematic of how you are trying to wire it?

 

[pwcross]

Chris, You've saved me the rest of my hair and hours of wasted time... I had the exact same LED issue and here's the answer - I just need to make it work now!

I'm an Arduino beginner and have definitely bitten off more than I can chew so thanks for documenting this so well. One question, I looked at your picassa images for the project which are excellent, but can you tell me what make of wire - pcb connectors you are using, they look so much better than the flimsy ones I have.

 

[yesyes]

Hi, glad I could save some of your hair. I wish I could say the same about mine.. ;-))

The connectors are called JST-XH and I bought them off ebay. They do require a crimp tool though. I did my first few with a very small pair of pliers but if you do more than just a few pins the crimp tool is very much recommended...

 

[AnonymousI]

How do I calculate resistors foe segments?

First off, i would like to thank you for providing such invaluable information on this type of project. This has really helped mine and is very understandable to those beginning electronics, like me.

Unfortunately, i made the same mistake as you in regards to the current limiting resistors. I have a similar setup as yours but i am driving only 3 digits. I am using 150 ohms for the segs and 560 ohms for the dp. I got 150 from 15 - (9 + 2 + 1)/.020 A (the 2 and 1 volts are for the udn/uln and resistors as you said in other forums. With this setup though, i only have the dp lighting. Using my voltmeter, i found that i am only getting less than 5 volts to the segments! I need to have at least 9-11 volts to power the segs.

Can you explain how to calculate the resistors for the segments and dp in this setup? A formula would be ideal. I'm planning to use 22 ohms like in your setup, but i want to make sure i dont burn out my digits but still supply 20 mA. (These took 3 weeks to arrive :O)
These are the digits i have: https://www.futurlec.com/LED/7SR50011AS.shtml

Thanks. I appreciate your help.

 

[khsoft]

yes, it works for me with exactly the same schematic as I posted in post #18 (page 2) in this thread. Pin 10 of the ULN2803 is left unconnected.
I don't think it would work with common anode displays. Do you have a schematic of how you are trying to wire it?

After two months I'm back to this project.

The only difference in my basic schematics (I alos tried different additional pull up resistors) from yours is that I have two ULN2803 (with pull up resistors) in place of UDN2981, and vice versa, and put common anode displays instead of common cathode. Why you think it would not work? Will MAX7221 drive my ULN's and UDN correctly after I have exchanged them? Now I have MAX7219 too, and will try with them, but it would be great to know if there are additional difficulties with driving common cathode displays that I do not realise. Very appreciate your help.

 

[mirceacra]

Also workink ok for me !! Thank you !!
I will post images later