Improved Multipexed LED drive.

In a previous blog I tested a TTL multiplexed LED driver, however it is not optimal, as there is always a potential across the LED's even when they are off. While LED's are diodes they are much less tolerant of reverse, also since they are being driven from the 74LS139 their brightness won't be that good.

So then I added some PNP transistors to switch the cathode, this will allow each segment to get the full drive current.

I used some 910Ω to limit the base current. The picture doesn't show it but the display is noticeably brighter.

This is the method I will use on the 8279.