LED display

I have a homebrew bit of kit with a dodgey display, one digit (3rd)
has failed.... :-)

It is using a common anode 4 digit LED display. Rather than go buy a
new one (I know only about £2) I have some single 7 seg displays of
about same size and started to play about to do a swap and some
fiddling with the wiring. Then after a bit of playing I had an idea as
I also have some larger (4") ones that are nice and bright with 7.4V
and 40mA per segment I bought at Rishworth last year, one even has
RN1-4002LO8W on it so I found a data sheet!

The multiplexing etc is a bit of fun.

On the original common anode the digits are controlled with Digit 1
pin 12, Digit 2 pin 9, Digit 3 pin 8 and digit 4 pin 6.

a-g pins 11,7,4,2,1,10,5

The 7 seg are:

a-g pins 7,6,4,3,2, 9,10

I bridged the original wiring to the new needed.

12 goes to 1 1st digit
9 goes to 1 2nd digit
8 goes to 1 3rd digit
6 goes to 1 4th digit

bridge the a-g segment pins too 11 to 7 etc.

The original or the smaller uses about 2.2V forward and draw is about
25mA per segment.

If I wired in the smaller LED I do not need to do anything else, they
work. If I were to try the larger LED how would you safely raise the
voltage and current draw? I thought add in a simple NPN transistor as
if I were driving a relay and add a stable voltage of 7.5V (use LM317)
to the display pin?

BTW I will order 11 extra sets of components for the Cumbria DSP
boards from Ron I'll contact those who expressed an interest with
details tomorrow.

It is also valve day tomorrow so hope to find you on air.