Shake Flashlight

I'm Khaliil and i'm engineering student. I'm doing this project called
Shake flashlight which is using Faraday's Law. I know the basic idea and
how it works but i want to add some modification to slow down the drain
of the capacitor to allow the LED to light more time…I have an idea but
i don’t know to apply it, so can you help me with this please.

My question is:

How can I add a voltage sensor which can measure the voltage of the
capacitor and than resets the resistor automatically which is connected
2V LED?

Example

I want to keep the LED 2Volts when the capacitor is fully (I’m using
super-capacitor 16v) charged and I’m using resistor to protect the LED.
when the capacitor reaches 5V i want the resistor to be 4-3ohm's and
maintain the LED 2V(fully bright) but the capacitor reaches 3V, i want
the LED to be like 1.6V(start getting dim and dimmer till its full OFF)

i can show you the circuit diagram if you want it…

I would real appreciate your help.

With Regards,

Khaliil