how can i simulate intensity loop control for led in simulink

 i am having 4 LEDs driving with a voltage Vdrive(taken as 5V).it is detected by 4 phototransistors and producing 4 different voltages(Vpt1...Vpt4).for different values of vdrive ,we got different LED current.plotted the graph and find an eqn ie Vdrive=(scaling factor*Iled)+bias and (Ipt/Iled)=constant and Ipt=Vpt/R.4 phototransistor o/p voltages are added in the summer circuit and the o/p of this is given as i/p to the PI controller.o/p of it will drive the led(vary vdrive to change intensity of LED)