Using transistors on the rows and columns doesn't seem to affect my display brightness at all either in the 8x8 or the 5x20 version..... and IMO, scanning columns is better especially if using rows as the data latch..... to each his own.....


B. Morse

 

Is 4mA average current really enough to light an LED? I suppose it is, if they're a high efficiency type. But all those resistors...

There is a serial power driver, the A6279, made by Allegro that has the same logic as a 74595 would have if it had 16 outputs, but it has constant-current drivers with up to 90mA available. You only need 1 resistor per 16 LEDs.

http://www.allegromicro.com/en/Products/Part_Numbers/6278/6278.pdf

Digi-Key carries the A6279 at $2.05 each. Apparently it's available as a DIP but they only stock it in surface mount.

 

Interesting. Ill have to look into the A6279. That would make things pretty easy.

I am also in the begining phase of building 2 LED matrix signs.

I am considering using SPI for display communications from a power meter to the signage.

Updating the display message is maybe the easiest part of the project.

 

If you are doing a multiple matrix display, why not use an IC designed to do just that?? Check out Maxim's line of LED display drivers, there are some that are SPI controlled and they have other versions with different interfaces ....

MAX6952 Specs....
High-Speed 26MHz with SPI-/QSPI-™/
MICROWIRE™-Compatible Serial Interface
2.7V to 5.5V Operation
Drives Four Monocolor or Two Bicolor Cathode-
Row 5 . 7 Matrix Displays
Built-In ASCII 104-Character Font
24 User-Definable Characters Available
Automatic Blinking Control for Each Segment
36uA Low-Power Shutdown (Data Retained)
16-Step Digital Brightness Control
Display Blanked on Power-Up
Slew-Rate-Limited Segment Drivers for Lower EMI
36-Pin SSOP and 40-Pin DIP Packages

I used the 6952 Version for 4 5x7 LED matrixes....

B. Morse

 

Is 4mA average current really enough to light an LED? I suppose it is, if they're a high efficiency type. But all those resistors...

There is a serial power driver, the A6279, made by Allegro that has the same logic as a 74595 would have if it had 16 outputs, but it has constant-current drivers with up to 90mA available. You only need 1 resistor per 16 LEDs.

http://www.allegromicro.com/en/Products/Part_Numbers/6278/6278.pdf

Digi-Key carries the A6279 at $2.05 each. Apparently it's available as a DIP but they only stock it in surface mount.

The A6279 and a myriad of other serial-to-parallel constant-current sinking drivers are excellent alternatives.

The 74HC595 in the examples I posted were specifically designed for 'economy' hobby displays. And yes, 4-ma provides a lot of brightness but using sinking drivers instead to get 10 to 20 ma "average" current would let you see the displays much better in a brightly lighted room or in sunlight.

Regards, Mike

 

Using transistors on the rows and columns doesn't seem to affect my display brightness at all either in the 8x8 or the 5x20 version..... and IMO, scanning columns is better especially if using rows as the data latch..... to each his own.....


B. Morse

Hi Brent (is it?),

You're right. Lots of ways to do it and it's absolutely amazing how much light you can get out of an LED with the most miniscule amounts of "average" current.

I apologize if you feel I was criticizing your design. Your design is great. I was just hoping to provide an alternative design philosophy that doesn't require more I/O pins to expand and doesn't increase the display duty cycle and reduce brightness as you expand it. For example, my MacMux™ method uses a small powered buss for a modular and expandable matrix. The first module could contain the mcu, row driver transistors, and a column driver IC. Additional modules use the same modular printed circuit board but only require a column driver IC. All this on a 10 pin bus. The MacMux method also features complete fade-to-black PWM brightness control.

The second graphic below depicts modular 2"x2" PCBs for some Fairchild 2.2" by 2.2" monocolor displays (the displays plug into the two horizontal sockets).

Cheerful regards, Mike

 

Does your MacMux max out at 8 for the addressable areas?

There are many good ideas around.

My first design was an 8x8 matrix and I use software and straight pin to pin, no muxing.

That was years ago, and for the new project, a modular design makes sence. A little more research into what to do is required.

Both of you have great ideas. I do suppose it is to each is own.

A decision will have to be made.. And for now, knowing both work, the deciding factor will be part count/cost.

 

Part count/cost--and construction method too. That Allegro part I suggested looks great and doesn't cost much, but if it's only available in surface mount and you want a circuit you can build on perf board, then you might decide you have to do it another way. It's getting way too common that DIP packages are hard to find.

Then again, it's possible to get cheap PC boards made in China and shipped direct. So life's not all bad.

 

I am comfortable with SMD. Everything will be on homemade PCB.