QTR-L-1RC Reflectance Sensor (2-Pack) Note: The QTR-L-1RC reflectance sensor requires a digital I/O line to take readings. The similar QTR-L-1A reflectance sensor is available with an analog output. Functional Description The Pololu QTR-L-1RC reflectance sensor carries a right-angle infrared LED and a right-angle phototransistor, both pointing toward the front edge of the board. To use the sensor, you must first charge the output node by applying a voltage to the OUT pin. You can then read the reflectance by withdrawing the externally supplied voltage and timing how long it takes the output voltage to decay due to the integrated phototransistor. Shorter decay time is an indication of greater reflection. This measurement approach has several advantages, especially when multiple units are used: No analog-to-digital converter (ADC) is required Improved sensitivity over voltage-divider analog output Parallel reading of multiple sensors is possible with most microcontrollers 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- 1RC is sold in packs of two units. For a similar sensor that can be used with the board parallel to the surface, but with shorter range, please see the QTR-1RC reflectance sensor. We also offer arrays of three and eight sensors: the QTR-3RC reflectance sensor array and the QTR-8RC reflectance sensor array.QTR sensor size comparison. Clockwise from top left: QTR-3RC, QTR-1RC, QTR-L-1RC, QTR-8RC. Specifications Dimensions: 0.3 0.35 0.12 (without optional header pins installed) Operating voltage: 5.0 V Supply current: 17 mA Output format: digital I/O-compatible signal that can be read as a timed high pulse Optimal sensing distance: 0.125 to 0.25 (3 mm to 6 mm) Maximum recommended sensing distance: 1 (25 mm) Weight without header pins: 0.006 oz (0.2 g) Interfacing the QTR-L-1RC Output to a Digital I/O Line Like the Parallax QTI, the QTR-L-1RC module has sensor outputs that require a digital I/O line capable of driving the output line high and then measuring the time for the output voltage to decay. The typical sequence for reading a sensor is: 1. Set the I/O line to an output and drive it high. 2. Allow at least 10 us for the sensor output to rise. 3. Make the I/O line an input (high impedance). 4. Measure the time for the voltage to decay by waiting for the I/O line to go low. These steps can typically be executed in parallel on multiple I/O lines. With a strong reflectance, the decay time can be as low as several dozen microseconds with no reflectance, the decay time can be up to a few milliseconds. The exact time of the decay depends on your microcontrollers I/O line characteristics. Meaningful results can be available within 1 ms in