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I have used a bunch of diodes to do this in the past, assuming you have a spare gang on the switch all you do is have segments (via a resistor each) for each required segment for each digit. Mine was for a Suzuki motorcycle gear indicator which was a 6 way single pole switch, six speed box with neutral a separate lamp in the speedo. ICs, you want a decimal to BCD encoder 74147 and then a bcd to 7 segment decoder 7447. Diodes would be more fun as you can decide on the numerals look and it would work on any available voltage.