Pololu Reflectance Sensor (2-Pack)
These things are pretty awesome! They're tiny and effective. The only bad thing about them is that they are so small there just isn't enough room for a screw hole. So you have to tape them, wire them or otherwise make some sort of adapter to hold them in place. I've designed a fast printing little piece that works pretty well. You can download and print one from Thingiverse.....or I'll do it for ya' for a buck. Heck, I'll even solder on the pins and toss in a wire for a little extra change..
The QTR-L-1A reflectance sensor incorporates a right-angle infrared LED and a right-angle phototransistor in an inexpensive, tiny 0.35″ × 0.3″ module that can be mounted almost anywhere and is great for edge detection and line following. The reflectance measurement is output as an analog voltage. This sensor is sold in packs of two units.
Visit Video 1 on YouTube directly.
The Pololu QTR-L-1A reflectance sensor carries a right-angle infrared LED and a right-angle phototransistor, both pointing toward the front edge of the board. The phototransistor is connected to a pull-up resistor to form a voltage divider that produces an analog voltage output between 0 V and VIN (which is typically 5 V) as a function of the reflected IR. Lower output voltage is an indication of greater reflection.
The LED current-limiting resistor is set to deliver approximately 17 mA to the LED when VIN is 5 V. The current requirement can be met by some microcontroller I/O lines, allowing the sensor to be powered up and down through an I/O line to conserve power.
This sensor was designed to be used with the board perpendicular to the surface being sensed, and narrow-angle lenses built into the infrared LED and phototransistor packages allow it to be effective to a range of about 1″ (25 mm). Because of its small size, multiple units can easily be arranged to fit various applications such as line sensing and proximity/edge detection. The QTR-L-1A is sold in packs of two units.