Here's a sensor array for line following robots. Has 8 IR LED/photodetector sensors (QRE1113).
Driven by twin I2C I/O expanders (PCA9554A) so only needs two Prop pins to work.
I picked 42 Ohm resistors to limit LED current to ~25mA when powered by 3.3 V.
It will also work as is at 5V, but you should increase the resistance of these resistors to maybe 100 Ohms or so.
(Otherwise the LED current is 54 mA, which exceeds the 50 mA absolute maximum. Still works though!)