I've been trying to figure this out for the past week, so I figured it was time to seek out some help.
I'm trying to use a max7219 & arduino to drive a large 2.3" 7 segment LED. I'm getting the correct digits displayed, but they are very, very dim, probably 1/10th the brightness the display should be. The important specs for the red 7 segment LED are: forward current per segment: 30ma (within the 40ma maximum of the 7219) VF typical 6.8V VF maximum 8V The 7219 can only directly drive displays that have a maximum 5.5V / 40ma per segment, but Maxim does describe a number of methods to use the 7219 to drive higher voltage displays. This is the info they supply: http://www.maxim-ic.com/appnotes.cfm/an_pk/1196 I've wired the 7 segment to the 7219 according to figure #3: I'm using a 2N2222 as the Q3 transistor and a 60v p-channel mosfet as Q2. The driver current is 7.5V.
Anyone have any idea why the LEDs are nowhere near full brightness? I know the problem isn't with the 7 segment, because I wired it directly to the power supply to see what the brightness would be like (REALLY BRIGHT). I'm also certain that there is no trouble with the arduino code or in the order the 7219 pins are connected to the 7 segment, as I tested all of that with a lower current 7 segment that I ran off the arduino's power.
any help would be appreciated.