You would want to build a circuit based around a PIC microcontroller. I was looking at doing one, but just lack the time right now. Also, by the time I bought the parts, designed a circuit, bought a programmer to program the chip, read up on programming....to me it made more sense to buy one from Fettronics or Shackman. I am confident that I could do it myself with time, but since I am not looking to mass produce and sell them, it just doesn't seem worth the time just to build one for myself.
