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In the days where a single-chip computer costs less than a gallon of gasoline, being able to work with microcontrollers is invaluable. This project serves as my introduction into the 8-bit PIC MCU world, with a scrolling LED display.

I put in a large order from Digikey to secure some PIC16F677s, along with a bunch of other components (including more transistors, finally!) to make a 16x24 bicolor LED display.

The display will be composed of 6 8x8 bicolor LED modules. They are 8x8 arrays of red-green LEDs. These display modules are dirt cheap! I am hoping that I can mix the red and green to obtain yellow light, but might need a diffuser.

The columns will be driven by 6 74595 serial to parallel shift registers with latches. They will be cascaded in order to hold one bit per color per column (24x2=48 bits, 48/8=6 chips).

The rows will be driven by a 4 to 16 converter. The display will scan rows, so only one row is lit at any given time.

The microcontroller will take input from various sources (digital for a few buttons, analog for analog things) and display information on the display.

While waiting for my 8x8 module to arrive, I put together a test circuit with 8 7-segment display modules. They are driven in nearly the same way I intend to drive the display: The 7 segments are serially brought into the shift register, and then latched to a display. The display is chosen by sinking its commmon cathode via a 4 to 16 decoder. Therefore, only 1 display module is lit at any given time, but switching between them fast enough makes it appear they are all on at once.

The drawback of this design is that each display is slightly dim. It is still readable, but probably not in bright light. The setup requires 9 pins and can drive 16 by any number of LEDs (by cascading shift registers.) The displays are also darkened slightly by the sink current capacity of the decoder: displays with more segments lit appear dimmer than displays with fewer segments lit. This gives me a good idea about how my final display will behave.

This differs from my final display in a few ways: Instead of one, there will be 6 cascaded shift register will sink, not source, the 48 (24x2 color) columns. The rows will be sourced by a PNP transistor on the decoder, instead of the decoder itself. I still need to get these transistors, and 25 more resistors (oops, I only got 25 the first time).

1. Get Parts
2. Prototype on breadboard
3. Tranfer to perf board
4. Final casing
5. Decide what to do with it!