Hi all and best wishes for Merry Christmas,
I'm being puzzled by a strange issue and need your valuable advise.
I'm trying to drive 8 x Common Anode 2.3'' tall 7-segment displays. These require 8Vf and 20mA.
MAX7219 datasheet has ways that his can be done with FETs and transistors. But searhing, I run into this post from a guy wanting to do just the same.
So this is what I built:
The top left IC is of-course a MAX7219 driven by an Arduino which is not shown here.
I'm using the LedControl library for this.
I had this tested on breadboard, using only 2 out of the 8 displays (as DIG-0 and DIG-1) and it was working fine (so I thought, but I should have tested more digits to discover there is a problem)
So went on and created a PCB out of it. Now that I have connected 4 of the 8 displays I'm having a problem which I will try to explain with example as I can not come up with another way to describe it.
So lets say I only have DIG-0 enabled and showing number 1 ( lc.setDigit(0,0,(byte)1,false); )
Then DIG-0 will work normally and show 1 and all other digital are of, as expected.
if I do the same for DIG-1 the same thing happens. The number appears on DIG-1 and all other digits are OFF.
When I try to display something on DIG-2 however, the same thing will ALSO be displayed on DIG-0. So if DIG-2 where to display number 3 the number three will also be displayed on DIG-0 as well.
The same holds true for all other Digits. Whatever displays on all digits (except for DIG-1) it will also be displayed on DIG-0.
In the above picture the DIG-0 should have been displaying number 1. But instead it displays additional segments from DIG-2 and DIG-3 (but not DIG-1).
I was thinking of a short-circuit, but wouldn't this mean that when I display something on DIG-0 the same thing would be displayed on all other (except DIG-1) digits? Moreover, if there was a short-circuit, since all digits (except DIG-1) are interfering with DIG-0 wouldn't they interfere among themselves? This is not the case as all other digits display their data only to themselves and to DIG-0.
Sorry, I can not explain it any better than this, I hope you understood me...
I would really like your advice as to what should I look in order to troubleshoot this...
Thank you for taking the time to read this.
PS. Just realized that the diagram shows a bit small. If you go the the image address it will show larger.