In SX Microcontrollers, SX/B Compiler and SX-Key Tool, tdg8934 wrote:

Can you post the schematic? I would be interested in seeing if there is a better way. Remember the 74HC138 will have an individual low output (1 of 8). You can use many different methods but seeing the end idea would really help. 

I have found that on the LED cathodes one can use any of the following (depending on how the hardware and software is dealt with): (highs in, lows out for the cathodes) UDN2803, A6821 (serial version of UDN2803 - much like a high powered 74HC595 with inverted outputs), etc.

If you use UDN2803's then you will need to drive them with something like 74HC595s, 4017 & 4081 combos, or direct with lots of I/O like a Parallax SX-48 microcontroller. If you use UDN2803's you will ultimately have lots of chips because you still have to reduce the I/O to something that the microcontroller has. However if you are only talking about 1-3 5x7 matrixs then you can use the I/O from an SX directly through the UDN2803's. 

My preference is to use the serial version being the Allegro Microsystems A6821 which only needs 3 inputs can provide the same 500 mA per output that the UDN2803. Less chips to deal with and they cascade easily. You can use SHIFTOUT statements for this type of SPI design.

Now for the LED matrix Anodes, you can use ULN2881/2/3 (that's right ULN not UDN) they give the parrallel outputs just like the UDN2803's but they take lows in to give highs out to the anodes. Just the opposite of the UDN2803s. You can also use Pn200's on the Anodes. However, you will soon find out that your LEDs just wont be all that bright UNLESS you allow more current to flow. This is when P channel FETS come into play. Use them on the Anodes. 

You will also need a higher current power supply 5vdc. If you use a higher voltage  and higher current (3-5 amps) like 12 vdc , then you will also need to add N channel FETS to the P channel FETS - gets a bit complicated doing it this way. I try to stay with 5vdc for everything. 

Another approach entirely is to use the popular MAX7219 which used SPI (SHIFTOUT command) for lighting up all 64 LEDs per led matrix. You can cascade them using a Basic Stamp 2 if you like. 

You may also want to look into a complete solution by Sparkfun.com. I had a link to them previously. They sell bi colored LED matrixs AND controllers as well as RGB LED matrixs and controllers. They are a bit pricey but all the hardware design is done for you and it is somewhat simple to write the microcontroller software (SX chip or Basic Stamp 2). I can help with that. 

Many other possibilities exist but lets see your schematic design and criteria for using this or can anything be changed on it.

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