595 power question

I have 9 of these http://cgi.ebay.com/10pcs-8x8-Dot-Matrix-3mm-dia-Bicolor-LED-Display-/250669533751?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item3a5d118a37#ht_2730wt_983

I am going to matrix then and use 9 595 shift registers to control them. I'm played around with 2 595s and understand how they work. I have a power question.

I assume that the vcc pin on the 595 is the power is sends to the output pins. Now, how does the power/resistor combo work if I want to light them all at once and then turn them all off but 1.

If I use a resistor at the end of each row and rate it for 1 led being on, won't they all be dim when I turn them all on? And if I rate it for all of them at once, won't 1 led blow up when I only turn 1 on?

one thing your forgetting is the 595 is a logic chip, and not a driver, you can only pass (on average) about 70ma through the thing before it starts to die

back to the question if you use a resistor to limit the entire row yes they will be dim, worse is that they will be uneven (ie row 1 only has the even led's lit up, row 2 has all of them, 1 row will be half as bright as the other)

otherwise if you go too low then yes you may burn up your led, but they have a peek current input (vs constant) so if your scanning though them fast they can take some hard hits and still be ok

so there is some info to chew on, meanwhile there are about a billion different ways to handle this and I am sure everyone will share their favorite

I'm now discovering the MAX chip, thanks

Whoa, MAx chip + ledcontrol library = win

Also whoa they are $10, even at digikey

yea I actually have 2 of those chips, and they rock, or so I think cause I tried sniffing the free sample fumes and they sent the wrong chips

the ones I have (and I dont know the package name) have the lead's under them and the whole chip is about 5mm square :o

if you still want to use the 595's your going to have to put up with a uneven display (example = http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1267250236)

which is more obvious in real life


figure out a buffer so the 595's are switching logic levels, this can be done with transistors, which eats up a ton o space, but they also make multiple transistors in a dip chip which may fill the requirements

otherwise you will need to look into driver type chips so that one side sinks current the other sources it, using the 595's to deal with the logic

its sorta like what you were looking at in the max chips, but they are all in one + a lot more

I dont know them off hand as I am not trying to be a led matrix guru, though I have a 1024*1 bit (ghetto)matrix hehe

I got my MAX7219 from here, they are probably on Ebay.com as well, I bought several things from here and have had no problems:-


$8 for 5!