Hello,
I'm building a driver for multiplexed LEDs in a 555 LED cube. I am using 4 udn 2981 transistors to source the current for 25 anodes (LEDs in columns) and 1 Uln 2803 darlington array to sink the 5 outgoing cathodes (LEDs in layers). All driven by arduino mega. Could anyone please check my circuitry in the image provided in the bottom? I am mostly concerned with the orange connections. Also, each Arduino GND in the drawing represents one Arduino GND pin, so all the GND wires go separately. But how much current is really flowing thru these and can the pins handle it?
From what I can tell you are fine, looking at the datasheet at http://www.datasheetcatalog.org/datasheet/allegromicrosystems2981.pdf it shows that the outputs of the udn2981 can source 350 mA. You only need around 20mA or less for an LED. (E = IR) so (I = E/R) where E is voltage, I is current and R is resistance. Also the voltage drop is 2V for a green or red LED or 4V for blue. I would like to know which color LEDs you are using but I'm sure your fine no matter what.
But lets just start with one LED and say its a green one. If you had a series resistor and the LED connected to 12V(voltage shown in your diagram powering the chip) you would have around 10V dropping across the resistor(small voltage is also dropped by the transistors in chip but is only around .6V). If you were to use a 330 ohm resistor you can say that (I = 10/330) which is a tad over 30 mili-amps which fine for the chip but is to much for an LED. Wanted to make sure I went over that because a lot of people use that as a standard but sometimes forget if using over 5V you need to make sure you aren't going to burn up the LED.
You want to figure it up by plugging in a value for the current you want to supply the LED which is 20 mili-amps.
You would say (R = E/I) Where R is the resistance your trying to find, 10 is the 10V minus the 2V dropping across LED and I is .02 the current you want. I didn't figure in the small voltage drop for the outputs of the chip but either way it will be fine to come up with a value for a resistor.
R = 10/.02 <---------------- .02 is the same as 20 mili
R = 500 ohms
You need to make sure your series resistance is 500 ohms or better to be safe for the LEDs
I think I read it correct on the datasheet you may want to doublecheck that or may someone else could take a look at it the link is at the top.