QTR-3A Reflectance Sensor Array is intended as a line sensor, but it can be used as a general-purpose proximity or reflectance sensor. The module is a convenient carrier for three IR emitter and receiver (phototransistor) pairs evenly spaced at intervals of 0.375" (9.525 mm). Each 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 resistors are set to deliver approximately 17 mA to the LEDs when VCC is 5 V, making the total board consumption just over 50 mA.
Pololu QTR Reflectance Sensor Application Note: https://www.pololu.com/docs/0J13
Pololu AVR C/C++ Library User’s Guide: https://www.pololu.com/docs/0J20
Arduino Library for the Pololu QTR Reflectance Sensors: https://www.pololu.com/docs/0J19
Pololu AVR Library Command Reference: https://www.pololu.com/docs/0J18
Pololu Web Site: https://www.pololu.com/product/2456