I designed a new power stage and a new display stage. I only simulated the circuits since I am away from my laboratory, but they are very simple and there should be no problems. The powew stage is similar as the previous but it has only one transistor.
Also the display stage is more simple with no zener diode, it is modular and it takes only a transistor for each LED. If you want to use only one led then this stage is not necessary, because the LED is already present in the DC amp stage and this turns on when the voltage at the output of IC1A is about 1V. But to have more complete information you need at least 3 LEDs total, I suggest the following combinations:

LED1 = 1V    LED1 = 1V       LED1 = 1V
LED2 = 4V    LED2 = 3V       LED2 = 2V
LED3 = 8V    LED3 = 5V       LED3 = 4V
             LED4 = 8V       LED4 = 6V
                             LED5 = 8V
Components:

R1 - 10K                V    2    3    4    5    6    7    8
R2 - 1K                 R4   33K  56K  39K  33K  33K  33K  33K              
R3 - 22K                R5   33K  27K  10K  5.6K 4.7K 3.9K 3.3K
R6 - 1K


C1 - 100 nF               
C2 - 100 microFarad
C3 - 100 nF
C4 - 22 microFarad

TR1 - PNP - BC307B or similar
TR2 - NPN - BC547B or similar
IC1 - SN78L12 or SN7812
DL1-DL2 = Led diode