Functional Description

The Pololu QTR-1A reflectance sensor carries a single infrared LED and phototransistor pair. 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 20-25 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.

Because of its small size, multiple units can easily be arranged to fit various applications such as line sensing and proximity/edge detection.
Specifications

* Dimensions: 7.62mm x 12.7mm x 2.54mm (without header pins installed)
* Operating voltage: 5.0 V
* Supply current: 25 mA
* Output format: analog voltage
* Output voltage range: 0 to supplied voltage
* Optimal sensing distance: 3 mm
* Maximum recommended sensing distance: 6 mm
* Weight without header pins: 0.23 g

Interfacing with the QTR-1A Output

There are several ways you can interface with the QTR-1A output:

* Use a microcontroller€™s analog-to-digital converter (ADC) to measure the voltage.
* Use a comparator with an adjustable threshold to convert the analog voltage into a digital (i.e. black/white) signal that can be read by the digital I/O line of a microcontroller.
* Connect the output directly to the digital I/O line of a microcontroller and rely upon its internal comparator.

This last method will work if you are able to get high reflectance from your white surface as depicted in image 2 above, but will probably fail if you have a lower-reflectance signal profile like the one in image 3 above.

Included Components

This module has a single mounting hole intended for a #2 screw (not included); if this mounting hole is not needed, this portion of the PCB can be ground off to make the unit even smaller. The reflectance sensor ships with a 3-pin 0.1" header strip, which can be soldered in the conventional perpendicular orientation or parallel to the PCB, as shown in image 4 above. You can also solder wires, such as ribbon cable, directly to the pads for the smallest installation.