"Normally" you would put the led in the collector of Q10.
In the above Ic = Vdrive / Remitter = Vdrive / 10 in this case.
So if you want max LED current to be Ix then
Remitter = Vdrivemax/ Ix (in amps). So for Imax = 50 mA
then Remitter = 5 / .05 = 100 ohms.
Then for shutoff use a small signal part to ground input of NI
OpAmp to power off. Use a R between Vdac and NINV input,
and use switch at junc of NINV input and R for disable operation.
What is supply you are using for LM324 ?
An LM324 has limited CM range input so you must observe that.
Or use a RRIO OpAmp.
Note many modern LEDs can glow at uA levels of current, and
LM324 when output driven low still at a few mV off ground. Just
be aware of that.
Here is a sim, you can see that the input CM range was violated of the LM324,
so power supply V would have to be raised from 5V, or use a RRIO part.
Note I used 3 diodes to represent RED LED as I had none in my simulator. To
get an ~ Vforward of a RED LED.
Herre I raised supply to 10V for the LM324, works fine.
Regards, Dana.