16 x 24 Dot matrix LED display

[Izzy]

Hello,

I wanted to use 6 of 8x8 dot matrix LEDs to form a 16x24 panel, so that I can display the accelometer reading of a car to the LEDs as a meter, as well as to display some other data. I think I should mention I am very new to dot matrix LEDs.

I have done some reading on dot matrix and it sounds quite interesting. However I was wondering if there is a single chip that can do 16x24 dot matrix or even 16x16. Or do I have to use multiple 8x8 chips? If you guys have any suggestion for chips please let me know.

Any information related with dot matrix LEDs will be very helpfull.
Thanks in advance.

[Izzy]

I went through few articles about dot matrix and I have seen people using different Chips but I am not sure which one would fit the best for mine (16x24 dot matrix).

Also, how many different ways are there to run dot matrix?Looks like the most popular one is scanning method. I have heard people doing multiplexing, is it same as scanning way?

One thing I am confused about is, do I only need Shift register/BCD chips? or do I need transistor array chips too? I dont think the shift registor/BCD chip can provide enough voltage /current for the whole row or column. I know there are some LED driver chips like

TPIC6B595
TLC5921
Are they same as having shift registor chip and transistor array in one package?

The chips that I have come across are:
Shift registor:
74HC595

Transistor array:
ULN2803

I am pretty sure lots of people here have worked with Dot matrix. I would appreciate if you guys can share some information.

Thanks

[Izzy]

Anyone?

[Izzy]

Hello once again,

Alright, I think I have done enough reading and I am planning to source the LEDs with the 74HC595, and sink the current though the ULN2803 transistor array which will scan the rows.

My question is should I be sourcing the leds with the transistor array or with the shift registor?I can rotate each of those 8x8 block and make the transistor array source the leds. But I dont think it really matters, does it? Because the shift registor can only sink or source 25-35mA.

Also I have been reading that the driving chip should be able to source around 100mA per channel in order for LEDs to golw bright, because of the duty cycle. The transistor array can sink or source 500mA but the shift registors have only about 25-35mA, which is what each LED will get. So I am lil bit confused here.

I would appreciate if someone can shed some light here.

Also, can anyone verify my small drawing, if it makes sense.

[FvM]

I think, your basically on the right track, but didn't conclude your calculations, which is obviously necessary for a consistent design.

I understand, that you are intending row-multiplexing (or scanning, which is just the same). You considered a LED current of 100 mA. This means, that each LED has a brightness equivalent to 100/14 = 7.1 mA DC, or somewhat lower, cause efficiency usually drops with higher pulse currents. To my opinion, this value is at the lower end of useful display currents, but let's simply base the calculations on this value.

100 mA LED current means, that the row driver must sink 24*100 mA = 2.4 A. Obviously an ULN 2803 can never handle this current. A solution would be e.g. to use ULN2803 as column driver (driving one LED at a time) and individual PMOS transistors as high current row drivers, with reversed LED polarity. The 500 mA, you mentioned with ULN2803, are not valid for simultanous load of all outputs, by the way. Read the datasheet thoroughly.

You also need a means of current limiting, e. g. series resistors at the column drivers, or constant current drivers.

Best regards and Merry Christmas,
Frank

[Izzy]

Thanks Frank for replying.

The 100mA that I had mentioned was what I read somewhere in an article. The article said, due to multiplexing the the equivalent current will be low, so in order to compensate for that we need to have high current. (By the way, each block was supposed to have 8x8 matrix. My bad)


What I could do is, on the column side I could connect 3 x ULN2803 after the shift registors which will drive theose ULN2803. And on the row side I could use only 2 ULN2803 for multiplexing.

So like you said if my Shift registors could drive 100mA then the equivalent current will be 100/16 = 6.25mA, which is definately low.
But I have seen people driving 16x16 array or even more with ULN2803 and 74HC595, without using extra transistors.

But the question, if I only use shift registors on the column it can only provide upto 35mA. With multiplexing that 35mA will be equivalent to 35mA/16 = 2.xxmA. Thats too low, how did other people do that?

Obviously I cannot provide 2.4A because I will be running this off from the Car's power socket(used for cell phone charging and etc). I have the power cable for my GPS which regulates the 12V from car to 5V. So I am not sure if that can provide 2.4A. Even if it can I want to limit the total current consumption within 500mA - 1A max.

Your though on this would be very helpful!

Thanks

[FvM]

The required curent depends on matrix size and LED efficiency, too. When calculating the supply current, the percentage of LED in on state matters. For a number display, it may be about 30 %. So, with 100 mA LED peak current, the average supply current would be below 1 A.

HC595 has also an absolute maximum VCC/GND current rating of 70 mA, so you shouldn't have higher output currents than 8 mA with simultanous load. They can be used without external driver at low LED current and low multiplex ratio only.