I'm kicking around the idea of creating a 64x64 led matrix using smaller 8x8 led matrices. My plan is to use shift registers and darlington arrays in order to control the matrix.
My design is such that I'll loop through the rows tying them to ground, and then I'll supply current to the correct columns in that row. This way at any given moment in time I'll have at most 64 leds lit (this would be when an entire row is lit)
Here's my first round of questions
1) I'm looking at using ULN2803A darlington arrays to act as the current sinks for each row. Every 16 leds in a row would be on its own darlington array (meaning I'd need 4 per row, 32 chips total). I decided on this configuration because each output of the array supports up to 500ma, assuming each led is around 25ma, 16 * 25 = 400ma. Is this a safe assumption / design? Is there any way I can lower this number?
2) I'm also looking at using the 74HC595 shift registers to control the rows / columns. The 74HC595 is an 8bit serial shift register, meaning I'd need 8 for the rows and 8 for the columns (16 total). For the rows, I'd be shifting just 1 bit at a time going from row 0 to row 63. For the columns I'd be clocking in the correct string of bits for the currently enabled row. Each 74HC595 that is tied to a row will be going through the darlington arrays, so the current output doesn't really matter. However, for the columns, since only one row will be tied to ground, only 1 led will be driven by each output of the shift registers. Does this mean I'll be safe with the 74HC595's output of ~25ma per output? or will I need darlington arrays for the columns as well?
Let me know if any of this is confusing, as I'd love to get started as quickly as possible. My ultimate plan is to get this using bluetooth as a data transfer medium, and hopefully get it displaying some simple images/video soon