Do i need to use an op-amp with this sensor to sense the path?
The idea is that you use whatever the 1->0 threshold is for your input pins, and let the capacitor discharge, and measure discharge TIME rather than VOLTAGE to see how illuminated the sensed position is.
Typical digital inputs are high impedance (> 100 kOhm) and thus aren’t affecting the RC discharge enough to matter. And, even if they were, that’s OK, as long as all of them are affected roughly the same, as you’ll simply calibrate the time it takes for the sensor to read “0” from turning off the charge. Just make sure your pull-ups aren’t on
This is a lot like old-school IBM PC joysticks, btw. Which were great for the time, but became a PITA with faster CPUs, because waiting SEVERAL MILLISECONDS just to read two digital values sucked down an inordinate amount of CPU cycles.
As jwatte describes, the QTR-8RC outputs are read by timing how long it takes for them to go low after being driven high. A basic description of how to use them is on the product page under “Interfacing the QTR-8RC Outputs to Digital I/O Lines” and in section 5 of the user’s guide under “QTR-8RC Sensor Outputs”, and some more details are in this app note.