Thanks PaulRB for help and bear me.
PaulRB:
I did not say you have to light 4 LEDs at once.
Could you example me 1/4 duty cycle code. I think you are talking about led buffer?
PaulRB:
If you choose to light only one, you can do that. But if you truly only want only one led to appear to be lit at once, you do not need to multiplex
no it will be lit multiple leds. as i said i will provide row and col to lit an led. when i lit next led then previous led will not be off.
PaulRB:
120R is probably ok, but do not go any lower than that. The current into your leds is probably around 20mA, which is the maximum for most LEDs. In saying that, I am assuming the ULN chip will drop around 0.7V and the led's forward voltage is around 1.8V, so the current is ( 5 - 0.7 - 1.8 ) / 120.
I'm using 2.1v, 25mA LEDs, also i removed uln now i'm driving leds direct from shift register but multiplexing exists.