QTR-L-1RC Reflectance Sensor (2-Pack)

Description

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.

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. Top row: QTRX-HD-07, QTR-HD-07; middle row: QTR-3, QTR-1, QTR-L-1; bottom row: QTR-8.

Specifications

• Dimensions: 0.3" x 0.35" x 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:

• Set the I/O line to an output and drive it high.

• Allow at least 10 us for the sensor output to rise.

• Make the I/O line an input (high impedance).

• 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 microcontroller’s I/O line characteristics. Meaningful results can be available within 1 ms in typical cases (i.e. when not trying to measure subtle differences in low-reflectance scenarios), allowing up to 1 kHz sampling. The following table shows some typical decay times (from 5 V down to a 2 V threshold) of the sensor over different surfaces and at different distances:

Ambient light, especially sunlight, can affect the sensor readings significantly. If the decay time of the QTR-L-1RC is consistently low, you might need to add shielding around the sensor or mount it in a different location to reduce interference from outside light sources.

Our Pololu AVR library provides functions that make it easy to use these sensors with our Orangutan robot controllers. We also have a Arduino library for these sensors.

How it works in detail

With only five components, the operation of this sensor is relatively basic. The emitter side is just an IR LED with an appropriate current-limiting resistor. The light from the emitter leaves the sensor, reflects off a nearby surface, and returns to the detector.

The detector side is a resistor-capacitor (RC) circuit, where the resistance comes from the phototransistor and is a measure of the incident infrared light, and the decay time is proportional to the resistance. The first step of the sensor-reading process-driving the sensor output high-discharges the integrated 10 nF capacitor and puts both sides at the same voltage (VIN). Alternatively, you can think of this as "charging the output node", and it is functionally equivalent to charging a capacitor with one side connected to ground. Once you are no longer supplying an external voltage to the output pin, the capacitor can slowly charge through the phototransistor, with the rate of charging being a function of the phototransistor’s resistance (which is in turn a function of the incident IR). As the capacitor charges, the voltage on the output side drops, eventually reaching zero when the capacitor is fully charged. Alternatively, you can think of this as "discharging the output node", and it is functionally equivalent to discharging a capacitor with one side connected to ground.

The 220 Ω resistor on the OUT line serves to limit the current flow, making it possible for a microcontroller output to safely charge the output node prior to each reading. It has very little effect on the sensor output.

QTR-1RC and QTR-L-1RC reflectance sensor schematic diagram.

QTR-1RC output (yellow) when 1/8" above a white/black interface and microcontroller timing of that output (blue).

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