/diy/ - Do It Yourself

I made a color sensor from an Atmega328, a RGB Led and a photodiode. It's for sorting objects in a conveyor belt. While it works well enough to measure red, green and blue levels and from that obtain the hue of the object with a simple algorithm, it seems like the voltage difference between different light levels is quite small by using the photodiode like I am currently doing. What kind of opamp circuit can I use to turn the small difference (4.27 to 4.5v) into a bigger difference so that I can have a bigger resolution and less influence of noise? This is for sorting the objects by hue and I can't use a camera or a sensor like TCS3200 (I mean, I could, but the sensor chip alone costs more than the arduino I pulled the MCU from)