Quote Originally Posted by Dave View Post
mekohler, I don't think your design using an 74LS174 will be able to drive the current you are anticipating using. Even with only 20 Ma. per led string in parallel, thats 40Ma. An normal 74LS series part can at best case source 5Ma. You are going to need a high power sourcing device such as a PNP transistor to source the required 40+ Ma. I beleive after your design has been prototyped you will find that if you are multiplexing the displays at any kind of rate you will lose almost all of the intensity. Therefore you will have to increase the current to the display system. Most wallboard displays you see in stores for advertising use anywhere from 100 to 500 Ma. as the display is multiplexed and switching at high speeds. I suggest looking at Allegro semiconductor as they have a variety of sourcing drivers available...

Dave Purola,
N8NTA
Actually, it is labeled incorrectly. My book says to use 74S138:

"In this application I used a 74S138 decoder to provide the column negative power supply select. The selection of the S technology decoder was selected over the more common LS technology to sink the most current (LED's fully lit) as possible"

Does that make any difference? The low level output current for the 74s138 is 20mA...still, according to you that is way too low. The ULN2803A is the transistor array for the "rows", which http://www.electronelec.co.uk/uln2803a.htm says outputs current to 500mA

Should I just place a second ULN2803A after the decoder?