Strange shadow in LED display

[rascupanamuha]

Thank you !
You and Mike on this forum really helped me alot !

Now i am planing to do 128x8 display, for my neighbor. similar to this one but with ULN2803, 8 decoders, and master 3 to 8 decoder. I think it would be awesome. Parts are cca110$, and my work is also 110$ for that. Is that fair enough?
In my country on black market there are similar led panels for 330$, so i think it is ok

 

[dougy83]

Sure, no worries.

Regarding your 128x8 display, you may want to consider a different method of driving the rows/columns. With your 5x16 display each column is driven for 1/16 of the time, and each LED is driven using 16x its average current. If you increase the display to 8x128 and retain the method of driving a single column at a time, then each LED will be driven using 128x its average current. This can destroy LEDs. There's also the case of increased flicker; for 5x16 the column is updated every 16 'col update cycles', whereas 8x128 will be updated every 128 'col update cycles' and therefore have a frequency of around 8x lower.

Perhaps consider driving a full row at a time; use serial-in-parallel-out shift registers (e.g. 74HC595) connected to the SPI port to drive the columns instead of the decoders. This will use fewer pins also.

 

[rascupanamuha]

Sure, no worries.

Regarding your 128x8 display, you may want to consider a different method of driving the rows/columns. With your 5x16 display each column is driven for 1/16 of the time, and each LED is driven using 16x its average current. If you increase the display to 8x128 and retain the method of driving a single column at a time, then each LED will be driven using 128x its average current. This can destroy LEDs. There's also the case of increased flicker; for 5x16 the column is updated every 16 'col update cycles', whereas 8x128 will be updated every 128 'col update cycles' and therefore have a frequency of around 8x lower.

Perhaps consider driving a full row at a time; use serial-in-parallel-out shift registers (e.g. 74HC595) connected to the SPI port to drive the columns instead of the decoders. This will use fewer pins also.

Jep, that make sanse. But i was afraid of that because i am not as good in programming yet. You have seen how easy letters can be moved in my 8x12, just from 11 to 0. I thought to do the same because then it would be also that easy to move, 127 to 0..
I would not do that. How about programming these 74HC595? I would need to use shift bit, or what?

 

[dougy83]

Using the '595 is easy. Just connect it to the SPI port (the DataIn, Clock and Latch to the MOSI, SCK and SS pins), and send the data to it like you would for the other serial port. That means you just write a byte to a register and the SPI module will shift the byte out.

You will have to transpose the column and row data, which should be simple.

 

[rascupanamuha]

Ok, i think i understand. Now i have in mind something else. As i should implement uC in that new project, and i want something in-expensive (i dont need 4 ports, only 2), and i am not sure at89c51ed2 is good enough for that. It costs 20$ only for uP. So i am thinking of buying some other micro processor, and i dont know which one. Is arduino much better option? Or something else?
Also, i dont like serial connector that i have on current uC, i would want to make something with USB..
So can you please suggest me what to buy? (i would better like to build anything myself than just buy finished uC)

 

[dougy83]

Well, based on your requirements of: fewer ports and cheaper, yes the Arduino uC (which is an Atmel ATMEGA328) is suitable. The uC costs a few dollars and can be purchased with a bootloader (which means you don't need a dedicated programmer); though you could also use a cheap programmer if you find it easier. The ATMEGA can be programmed with the free GNU C/C++ compiler (for windows this is called WinAVR).

As far as USB connection goes, you can use a USB-serial converter and connect to the standard uC serial port.

Some examples of prices/products:
ATMEGA328 with preinstalled bootloader: **broken link removed**
USB - Serial converter: https://www.ebay.com.au/itm/USB-To-...e-Converter-Adapter-For-arduino-/180953299346
Optional programmer: **broken link removed**

 

[rascupanamuha]

Thank you very much ! I think i will buy first 2 of your 3 links + arduino uno for programming, and for learning how to do all this things. Then i could just put atmega 328 uC chip to one of my projects and it will work how did i programmed it
I have always wanted that