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Hi. I'm not quite sure what you mean. I'm guessing you are trying to cascade 8 MAX7219, each with a 8x8 LED matrix? By "segment" you mean one MAX?
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One thing comes to mind.... make sure you connected those 100nF decouple capacitors as close as possible to the MAX ICs and...
Thanks!
They usually say size doesn't matter. But it did matter a lot for this project.. ;-)
4 metres you say? The room where my display will end up isn't even 4 metres in any direction... ;-)
Mine will have 8 numerical digits and a 8x64 LED matrix for text (another 8 MAX7219) ;-))
OK, I'm happy to report that this works. :lol:
I only had to change the limiting resistors for the LEDs. I hadn't taken into account the voltage drop through the 2803s and 2981. So these are now 22R and 560R for the decimal point. But you would want to recalculate these for your displays...
Ah, yes, of course! Thanks for the correction.
I ordered a few commoned resistor networks with different values and will try this out as soon as everything has arrived. The pin-out of the 2803s and 2981s makes it real easy to add these resistors on a breadboard.
I'll keep this thread updated...
Thanks for that. I'm using DesignSpark for my schematics now. It's free and a lot more fun than, say, MS Paint. :D
I thought I had calculated the 4k7 pull-ups for the 2nd 2803 from the datasheet. Vin 3.85V / Iin 1mA = 3.85kOhm. But I have probably not fully understood that datasheet for the...
OK, I changed the schematic to include the ULN2803. Actually 2 of them. Since the ULN2803 is inverting I had to invert the inputs again. I prefer to use another ULN2803, mostly because I prefer its pin-out to, say, a couple of 7404s.
Do you think this will work this way?
I ordered a few...
yes, they are 3 inch displays. 6 LEDs in series for each segment (2 for the decimal point).
So I would add a ULN2803 into the digit lines just like the UDN2981 but connect pin 9 to ground and pin 10 to the same 12V supply as the UDN2981?
The 12V voltage should have dropped to zero at this point...
I made a schematic adding the UDN2981 to the segment drivers. Could you please have a look at the attached schematic and see if I understood this correctly?
Is it OK to sink the LED current through the digit drivers of the MAX or would it be absolutely necessary to add another external driver...
Thanks for that.
Since I'm within 40mA I assume the sink side (digit drivers) can be used directly, is that correct?
So I only need to put one UDN2981 into the segment lines and put 9V and ground onto the UDN2981?
Hi.
I'm looking for a solution to almost exactly the same problem. I need to drive common cathode 3 inch 7 segment displays with a MAX7219. Each segment requires 9V 20mA. So, as opposed to the original poster, I'm still well within the max 40mA per segment that the MAX supports. I only need...
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