Hi Al,
Those low cost panel meters are usually built around the ICL7106/07 (LCD/LED) chips and their derivatives. Check out their data sheets and application notes. These chips are rather fussy about their input's range relative to the power supply voltages, so separate power supplies is an easy solution.
Good luck, Peter
if you have any old 10-base-2 network cards (thin coax ethernet) in your junk box they have on them a wide-DIP shaped isolated powersupply
5v in 9v out, 10 seconds on the underside with a blowtorch and they just fall out.
Since this is apparently a one-off, observe that MPJA has 4-1/2 digits for $18.95
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