Once I figured out the inputs this instrument cluster needs in order to light up control lights and move needles, I started working on the interface board between the ATMega board and the cluster itself.
I need this interface board for the following purposes.
1. Converting the 5V TTL signals coming from the microcontroller, to 12V as expected by the instrument cluster.
2. Because some of the lights come on when their corresponding pin is high (12V), while others when it's low (Ground), I had to provide "source" of current for the first ones, and "sinks" for the others. This meant using two different kinds of buffers (integrated circuits outputting 12V when 5V are being input): the ULN2803 and the UDN2981.
The board features 2 connectors to hook up the ATMega board, and 3 to connect to the dash. A power connector was also installed to bring 12V and 5V voltages from the ATX source.