Hello All,

First of all, this is a hobby project for me, and I'm not an electronic engineer. I'm building a device that displays ground speed, engine speed, and a few other things. It needs to be used in direct sunlight so I decided to use a combination of Super-red 16-segment and 7-segment LED displays from Kingbright. I am using MAX6954's to run the 16 segment displays and they work great.

The problem lies with driving the 7-segs. I chose a 2.3" super-red model (I am trying to drive 3 digits) but each segment actually has 4 LEDs in series. By my calculations and subsequent testing each segment likes to have 7.4 volts (1.85v/LED) and ~30ma. That was bright enough to light up my spare bedroom so it should work great in direct sunlight.

I know I can't use the MAX6954 chips because they will not drive over 5 volts (I'm using 3.3 for the supply on them for the small displays to keep the dissipation down on the ICs). One of these chips does have additional capacity for 4 more 7-seg displays.

I got a few Toshiba TB62709s. These looked good because it will run with a 17v supply line and use 5v for logic but it barely illuminates the displays (have to turn off the lights in the house to even see them). I have tried between 7 and 12 volts on the supply line and vaired the Rext (sets the current) from zero to 1k, no change. I'm beginning to think that maybe I didn't read the datasheet carefully enough!

I am trying to keep pincount and wirecount down and keep tasks off the uP (at89c51ed2, btw) as much as possible. Does anyone have any advice? I would love to use the spare MAX6954 capacity but I don't know what extenal circuitry that requires and the Maxim folks didn't want to help me out with that. I have both common anode and common cathode displays to experiment with.

Some datasheets to peruse:
-2.3" displays:
http://www.us.kingbright.com/product.a...C23-12SRWA

-MAX6954:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3410

-TB62709:
http://www.marktechopto.com/PDFs/Tosh...ACF42F.pdf

Thanks in advance,
Andy