I wanted to retask some of the dollar (ten for $10) digital voltmeters I got off Ebay. Ignoring the decimal place, these voltmeters can make general purpose 3-digit LED displays, using just ONE I/O pin. I did some curve-fitting and came up with a linear conversion to make a variable into its corresponding voltage. That is, value 1=0.01V, 37=0.37V, 127=1.27V, etc. It's surprisingly accurate over the whole range, 0-2.55V.