Resistor Substitution Box

When you are designing analog circuits, you may find that you are always running to the parts bin for random resistors. It gets tiresome. And when you are biasing transistors, you would rather just wire it up and adjust for quiescent current than do all math first. It gets the job done faster. I found myself at this point alot lately. Solving equations for resistor values, then finding preferred values. Then picking out resistors from their bins and putting them into the breadboard. Take them out, replace with nearest value plus or minus. It was slowing me down, and a real inspiration killer. So I decided I should build a resistor substitution box.


Micro Professor Clone - Intro


About a year ago amongst the many puzzles and board games at a local thrift store I found an original Micro Professor MPF-1B for a measly $10. I vaguely remembered them from years and years ago, but I didn't recall what exactly it was. Finding a Z80 CPU on board, a hex keypad, six 7-segment LED displays and a button overlay that had writing on it vaguely reminiscent of BASIC, I was sure that this was a computer (in the original sense). I immediately bought it, fearing someone else might know what goldmine I had in my hands, and try to take it from me. Once I got it home and fired it up I instantly realized what potential this computer had. I had been looking to get into some assembly programming, and this was perfect for the job.
After running some simple programs on it (hand assembled Z80) and getting acquainted with the hardware I figured why not just make my own version of the MPF that is smaller, battery powered, portable and uses only SMD parts and can run the original monitor/BASIC program. What a hell of an idea.