K8035 : changing 7 segment display for bigger ones

Kits Timers and Clocks
1

Hi everyone,

I’m using a 8035 kit and it works very well. I now would like to replace the standard 7 segment displays by bigger ones.
I have some common anode 2,3 inch (57mm) digit height (LTS-23805HRB) but of course it does not work that easily.

The first thing I tried is to lower R1 to R8 from 150 to 47 Ohm in order to increase the current in each segment. It barely reaches 4,4 mA… The highest current I can have is 24 mA by simply removing those resistors ! But it does not help lightening the segments.

My problem (or at least one of them !) is that the 7 segment displays are powered with 5V. It’s enough for the standard ones but each segment of a LTS-23805HRB contains 4 LEDs, wihch means that the threshold voltage is higher than the input voltage…
On top of that, there is a BC557 transistor in front of each 7 segment display (T1 to T4) and that greatly lowers the voltage arriving to each display. Before the transistor I measure indeed 5V but after the BC557 there is only 1,8V left to power each 7 segment display.
I have tried to change the resistors R9 to R12 from 10k to 2,2k but the voltage that arrives to the display only changes from 1,8V to 2,1V…

I have no idea on how to increase the voltage that powers the four 7 segment displays.

If anyone can help, I would be very grateful !

2

I forgot to add the schematic diagram that shows where the 5V input comes from (UA7805 linear regulator) !

So here is the complete schematic diagram :

3

I tried something !

I have replaced only the last two standard digit (DY3 and DY4) by my big LTS-23805HRB digits and to power them properly, I have connected the collector pin of T3 and T4 (the last two BC557 PNP transistors) directly to the main 9V power supply which powers the entire kit.
This way, I have the two first standard (small) digits powered with 5V from the UA7805 linear regulator, and the last two (big) digits powered with 9V from the main power supply.

The whole thing lights up correctly, the 4 digits display “0” and I have around 12mA going through the two big 7 segment display which is great.
BUT… Now when I start counting, the last two big digits display the same information ! And on top of that, all the segments stay on but not on the same brightness level…

for example, when I start counting one, the 4 digits should show “0001” but it shows “0011” and the segments B and C are brighter than the other, but the other segments are also on !

Any idea on what I do wrong…?