I'm not quite sure if I completely understand what you are trying to do, but I did something pretty similar actually once before. I was controlling a pump's speed based on the output of a 0-10VDC transducer. Based on the output, the pump speed was determined by an AD654 chip, which is a Voltage-to-Frequency converter. Try taking a look at that chip. Hope this helps.
I was looking for something similar to this circuit, but my idea is to implement a circuit that increases the brightness of a led by increasing the voltage softly until reach the total bightness and then for a specific time of timer to decrease the brightness and stays ON only at 25% of brightness.
I try to incorporate a 555 for the timer and at the output a capacitor/resistor delay for ON time that increases slowly at 12v (which is the arrangement of leds) this output is going to the input of a PWM to increase the brightness with 95% duty cycle (which is the total brightness of led in my case) and then after timer goes off then the circuit delay softly decreases the voltage thus the PWM will decrease the brightness of led and keeping it at 25%.
Sounds a little bit complicated and I think the last part that keeps the PWM at 25% duty cyle is my challenge.
Does anyone know how to implement this part? or have some path where to illuminate myself?