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 0V and VIN (which is typically 5V) 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 5V. 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 parallel to the surface being sensed. 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: 0.3″ x 0.5″ x 0.1″ (without optional header pins installed)
• Operating voltage: 5.0 V
• Supply current: 17 mA
• Output format: analog voltage
• Output voltage range: 0  to supplied voltage
• Optimal sensing distance: 0.125″ (3 mm)
• Maximum recommended sensing distance: 0.25″ (6 mm)
• Weight without header pins: 0.008 oz (0.2 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 the top image, but will probably fail if you have a lower-reflectance signal profile like the one on the bottom.

 
QTR-1A output 1/8″ away from a spinning white disk with a black line on it.

QTR-1A output 3/8″ away from a spinning white disk with a black line on it.

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.  Each pack of two reflectance sensors includes sets of straight male header strips and right-angle male header strips, which allow you to mount them in the orientation of your choice (note: the header pins might ship as 1×6 strips that you can break into two 1×3 pieces). You can also solder wires, such as ribbon cable, directly to the pads for the most compact installation.