In elektor magacine, some years ago they explain a circuit to translate from an standard 4 digit 7 segments thermometer to the binary vaue.

You can read in 4+7 lines the value of the digit and wich digit is active.
With a simple table yoy can get the BCD value.
You can remove the leds displays and use optocoplers to interface the 8051, at the and both are leds.