Custom built 6 digit 7 segment display and power

[Bodman]

Hi all,
I have been experimenting with a MAX7219CNG and 7 Segment displays and had some success. I am now scaling it up and made a custom 6 Digit 7 Seg prototype using vero board (strip board) - see attachement and 5mm LED's (2 in Parallel per seg). I worked out that the LED Segments of a standard 7 Segment Display are around 1.8V - 30mA(Measured at 20mA). The 5mm White LED's are in parallel ( 3.2V 20mA) Using the attached circuit diagram I have set Rset at 27K (I tried higher and lower but it didnt seem to change the current, or brightness much), although i know the rule of thumb seems to be around 10K.

So My Problem, if i run and power this from my Pi Zero i can display nearly all numbers apart from 0, 1 or 7, however if I plug a zeroseg in i can display those numbers no problem..

Is this a power issue, or something to do with the fact I am only using 6 digits instead of 8



Cheers
Bod

[jojopi]

So My Problem, if i run and power this from my Pi Zero i can display nearly all numbers apart from 0, 1 or 7,

What happens when you try! Does it make a difference how many digits you display, and in which slots?

Those glyphs are not the most power hungry; 1 is actually the least. Those glyphs are the only ones that do not use segment g.

[LTolledo]

dont get power from RPiZ,
split the input power
one for the RPiZ
and one for the MAX7219

for your reference:

RPi1A Max7219 x 6 Matrix display.jpg (252.26 KiB) Viewed 290 times

[Bodman]

So My Problem, if i run and power this from my Pi Zero i can display nearly all numbers apart from 0, 1 or 7,

What happens when you try! Does it make a difference how many digits you display, and in which slots?

Those glyphs are not the most power hungry; 1 is actually the least. Those glyphs are the only ones that do not use segment g.

If I send 88888888. I get

IMG_5557a.jpg (99.45 KiB) Viewed 271 times

If I send 87654321 i get

IMG_5558A.jpg (135 KiB) Viewed 271 times

and if i send 88780812 i get

IMG_5556A.jpg (149.17 KiB) Viewed 271 times

The LED's are all the same brightness, its just the camera angle

[Bodman]

dont get power from RPiZ,
split the input power
one for the RPiZ
and one for the MAX7219

for your reference:
RPi1A Max7219 x 6 Matrix display.jpg

Thanks, I just tried that and unfortunately i get the same results, im wondering do i need to change something when only using 6 digits instead of 8

[jojopi]

It does look like, when you try to display a glyph with no central segment, that entire digit does not illuminate. I cannot think of an obvious wiring mistake that would give that result, but what happens if you unhook the segment g wire from the display?

Are you using exactly the same code with the ZeroSeg, so your display is also common-cathode, and you are driving it in digits mode, rather than as a matrix?

im wondering do i need to change something when only using 6 digits instead of 8

No. Obviously you do not connect the digit 6 and digit 7 wires. Obviously you will not see those digits. Optionally you can tell the chip to only scan the first six digits, which will slightly increase brightness and may affect the way the display appears to flicker on camera. There is nothing that will make the output wrong.

[Bodman]

It does look like, when you try to display a glyph with no central segment, that entire digit does not illuminate. I cannot think of an obvious wiring mistake that would give that result, but what happens if you unhook the segment g wire from the display?

Are you using exactly the same code with the ZeroSeg, so your display is also common-cathode, and you are driving it in digits mode, rather than as a matrix?

im wondering do i need to change something when only using 6 digits instead of 8

No. Obviously you do not connect the digit 6 and digit 7 wires. Obviously you will not see those digits. Optionally you can tell the chip to only scan the first six digits, which will slightly increase brightness and may affect the way the display appears to flicker on camera. There is nothing that will make the output wrong.

Thank you, Yes same code as Zeroseg and same library. I disconnected seg G but same result. im convincing myself now there is a fault on the board ive made, ill have a good inspect of that

[Bodman]

Out of curiosity how do i tell the chip to only scan the first six digits,

thanks
Bod

[DougieLawson]

Post the code you're using to drive the MAX7219 and a schematic of how your LEDs are wired to the output pins on the MAX7219.

The MAX7219 has two modes. One to drive 8x8 matrices, one to drive common cathode seven segments.

[Bodman]

yay, i eventually found a short on the bottom of my Max7219 chip...that only took hours to find