I am using an QTR-MD-04A Reflectance Sensor Array at 3.3V to detect a line for a small line following robot. Unfortunately I am only getting a 2% variation (of the 0V - 3.3V range) in output value with line compared to without.
For these tests the sensor was 3-4mm above a piece of white printer paper with a black area created using a sharpie.
I have used both a microcontroller and multimeter to read the output, though on the microcontroller the noise outweighs the variation to a large degree.
The product is listed at operating between 2.9-5V, so I assume I have overlooked something…