Hallo Ehigie,

assume you want to display 10 characters, each consisting of 5X7 matrix. Between every two characters there should be a little space. So, you would need a 6X7 matrix per character and dispaly matrix consisting of 7 rows and 60 columns. You could use the following scheme, but keep in mind that only a small fraction of all needed components is shown, of course:




With lower HC574 you can decode 8 different HC238, resulting in 64 different columns. With upper HC574 you define which LEDs per column are turned-on.

Displaying text works as following: Write LED code of first character and first column of 5X7 matrix into upper HC574, while not having any HC238 enabled. Then select first column of display by writing according address into lower HC574. Enable HC138 (and by this the according HC238) for about 100µsec.
Then, write LED code of first character and second column of 5X7 matrix into upper HC574, while not having any HC238 enabled. Then select second column of display by writing according address into lower HC574. Enable HC138 (and by this the according HC238) for about 100µsec.
And so on.

100µsec duration for each column of display gives 60 x 100µsec = 6msec for the whole display, gives an updating frequency of about 170Hz. So, according to Erik, no flickering should occur. There's even enough time to manipulate text, in order to achieve some certain 'text running' over display. The best moment for this is, when all columns just have been written. Then, eyes do not see any interruption. But look, that display updating frequency does not fall below about 100Hz.

Using standard LEDs, at least about 50mA per LED should flow. But for a bright display a much higher current should flow. This current must be delivered by each PNP transistor. Choose a base current of about 5mA, then upper HC574 is able to drive all PNP transistors.
NPN transistors driven by HC238 must be able to sink at least 7 x 50mA = 350mA. According base current for securely driving transistor is much too big for HC238. So, you should use here some Darlington transistor. There are nice arrays arround (e.g. ULN2003), doing the job quite well. But a solution using discretes only is also possible.

Have a look at datasheet of LED and find out what current they can tolerate for short intervals. High efficiency LED would be highly recommended. But, of course, price can be much higher.

Let's demonstrate the advantage of high efficiency LEDs:

Assume you use standard diffuse 5mm LEDs, like 590-446 (red) and 590-452 (green) from RS components. These are cheap standard LEDs, giving a brightness of 5mcd at 10mA.
If you, on the other hand, choose high efficiency diffuse 5mm LEDs, like 585-573 (red) and 585-589 (green), which are only a bit more expensive, situation looks totally different. These LEDs give you a brightness of 30mcd (red) and 25mcd (green) at same current, namely 10mA.

The advantage is now, that these high efficiency LEDs only need 1/6 (red) and 1/5 (green) of the current of the above standard LEDs to produce the SAME brightness!! This helps you in dramatically decreasing total current consumption, without suffering from lower brightness.

Last hint:
Do only use LEDs with diffuse characteristic, because only these have a wide view angle and can be seen from nearly every direction.


Good luck,
Kai