This compact module packs three IR LED/phototransistor pairs onto a 1.25″ × 0.3″ board. The sensors are mounted on a 0.375″ pitch, making this array a great minimal sensing solution for a line-following robot. Each sensor provides a separate analog voltage output.
The 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. With sensors spaced at intervals of 0.375″ (9.525 mm) along of the board’s longer axis, this array works well as a minimal detector for line-following robots, as line-following courses are commonly made using 3/4″ (19 mm) black electrical tape. The middle sensor is slightly offset along the short axis of the board.
Each phototransistor is connected to a pull-up resistor to form a voltage divider that produces an analog voltage output between 0 V and VCC (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.
More technical information at Pololu.