Add to Wish List

QTR-8RC Reflectance Sensor Array

SKU: POLOLU-961 Brand: Pololu
This sensor module has 8 IR LED/phototransistor pairs mounted on a 0.375" pitch, making it a great detector for a line-following robot. Pairs of LEDs are arranged in series to halve current consumption, and a MOSFET allows the LEDs to be turned off for additional sensing or power-savings options. Each sensor provides a separate digital I/O-measurable output.
$16.80 AUD, inc GST
$15.27 AUD, exc GST

In stock, ships same business day if ordered before 2PM
Delivered by Fri, 12th of Aug

Quantity Discounts:

  • 10-25 $14.66 (exc GST)
  • 25+ $14.20 (exc GST)
- +

2 from local stock, 1 supplier stock; your order will dispatch between Dec 1 to Dec 10. And yes, stock levels and lead times are accurate!

Favourite product

Shipping:

  • $6+ Standard (5+ days*, tracked)
  • $10+ Express (2+ days*, tracked)
  • FREE Pickup (Newcastle only)

Shipping costs may increase for heavy products or large orders.

Exact shipping can be calculated on the view cart page.

*Conditions apply, see shipping tab below.

Note: The QTR-8RC reflectance sensor array requires digital I/O lines to take readings. The similar QTR-8A reflectance sensor array is available with analog outputs, and the reflectance sensor is available individually as a QTR-1RC reflectance sensor or QTR-1A reflectance sensor.

Functional Description

The QTR-8RC 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 eight IR emitter and receiver (phototransistor) pairs evenly spaced at intervals of 0.375" (9.525 mm). To use a sensor, you must first charge the output node by applying a voltage to its OUT pin. You can then read the reflectance by withdrawing that externally applied voltage on the OUT pin 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 coupled with the ability of the QTR-8RC module to turn off LED power:

  • 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
  • Parallel reading allows optimized use of LED power enable option

The outputs are all independent, but the LEDs are arranged in pairs to halve current consumption. The LEDs are controlled by a MOSFET with a gate normally pulled high, allowing the LEDs to be turned off by setting the MOSFET gate to a low voltage. Turning the LEDs off might be advantageous for limiting power consumption when the sensors are not in use or for varying the effective brightness of the LEDs through PWM control.

This sensor was designed to be used with the board parallel to the surface being sensed.

The LED current-limiting resistors for 5 V operation are arranged in two stages; this allows a simple bypass of one stage to enable operation at 3.3 V. The LED current is approximately 20–25 mA, making the total board consumption just under 100 mA. The schematic diagram of the module is shown below:

For a similar array with three sensors, consider Pololu's QTR-3RC reflectance sensor array. The sensors on the QTR-8RC are also available individually as the QTR-1RC reflectance sensor, and the QTR-L-1RC is an alternative designed to be used with the board perpendicular to the surface.

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: 2.95" x 0.5" x 0.125" (without header pins installed)
  • Operating voltage: 3.3-5.0 V
  • Supply current: 100 mA
  • Output format: 8 digital I/O-compatible signals that can be read as a timed high pulse
  • Optimal sensing distance: 0.125" (3 mm)
  • Maximum recommended sensing distance: 0.375" (9.5 mm)
  • Weight without header pins: 0.11 oz (3.09 g)

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

Interfacing the QTR-8RC Outputs to Digital I/O Lines

The QTR-8RC module has eight identical sensor outputs that, like the Parallax QTI, 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. Turn on IR LEDs (optional).
  2. Set the I/O line to an output and drive it high.
  3. Allow at least 10 µs for the sensor output to rise.
  4. Make the I/O line an input (high impedance).
  5. Measure the time for the voltage to decay by waiting for the I/O line to go low.
  6. Turn off IR LEDs (optional).

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 of all 8 sensors. If lower-frequency sampling is sufficient, substantial power savings can be realized by turning off the LEDs. For example, if a 100 Hz sampling rate is acceptable, the LEDs can be off 90% of the time, lowering average current consumption from 100 mA to 10 mA.

Pololu's Pololu AVR library provides functions that make it easy to use these sensors with Pololu's Orangutan robot controllers; please see the QTR Reflectance Sensors section of Pololu's library command reference for more information. Pololu also have a Arduino library for these sensors.

Breaking the Module in Two

If you don’t need or cannot fit all eight sensors, you can break off two sensors and still use all 8 sensors as two separate modules, as shown below. The PCB can be scored from both sides along the perforation and then bent until it snaps apart. Each of the two resulting pieces will function as an independent line sensor.

Included Components

This module ships with a 25-pin 0.1" header strip and a 100 Ohm through-hole resistor as shown below.

You can break the header strip into smaller pieces and solder them onto your reflectance sensor array as desired, or you can solder wires directly to the unit or use a right-angle header strip for a more compact installation. The pins on the module are arranged so that they can all be accessed using either an 11×1 strip or an 8×2 strip.

The resistor is required to make the two-sensor array functional after the original eight-sensor array is broken into two pieces. This resistor is only needed once the board has been broken.

Solder the included resistor to the 2-sensor array piece as shown to make the separated piece functional.

How it works in detail

For more information about how this sensor works, see the “How it works in detail” section of the QTR-1RC product page.

People often buy this product together with:

TB6612FNG Dual Motor Driver Carrier TB6612FNG Dual Motor Driver Carrier
Pololu Ball Caster with 3/8 Pololu Ball Caster with 3/8" Metal Ball
QTR-8A Reflectance Sensor Array QTR-8A Reflectance Sensor Array

Identifying markings

PCB dev codes: irs02a1

Notes:

1
Note: this does not differentiate between the A and RC output types.

Documentation and other information

File downloads

Recommended links

This product is listed in:

Sensors>Reflectance

Product Comments

Exact shipping can be calculated on the view cart page (no login required).

Products that weigh more than 0.5 KG may cost more than what's shown (for example, test equipment, machines, >500mL liquids, etc).

We deliver Australia-wide with these options (depends on the final destination - you can get a quote on the view cart page):

  • $3+ for Stamped Mail (typically 10+ business days, not tracked, only available on selected small items)
  • $6+ for Standard Post (typically 6+ business days, tracked)
  • $10+ for Express Post (typically 2+ business days, tracked)
  • Pickup - Free! Only available to customers who live in the Newcastle region (only after we email you to notify your order is ready)

Non-metro addresses in WA, NT, SA & TAS can take 2+ days in addition to the above information.

Some batteries (such as LiPo) can't be shipped by Air. During checkout, Express Post and International Methods will not be an option if you have that type of battery in your shopping cart.

International Orders - the following rates are for New Zealand and will vary for other countries:

  • $11+ for Pack and Track (3+ days, tracked)
  • $16+ for Express International (2-5 days, tracked)

If you order lots of gear, the postage amount will increase based on the weight of your order.

Our physical address (here's a PDF which includes other key business details):

Unit 18, 132 Garden Grove Parade
Adamstown
NSW, 2289
Australia

Take a look at our customer service page if you have other questions such as "do we do purchase orders" (yes!) or "are prices GST inclusive" (yes they are!). We're here to help - get in touch with us to talk shop.

Have a product question? We're here to help!

Write Your Own Review

Videos

View All

Guides

The Maker Revolution

The Maker Revolution celebrates the creation of new devices and the modification of existing ones - ...
The Maker Revolution celebrates the creation of new devices and the modification of existing ones - ...

Getting Hands-on with Sensors

What good is a robot if it can’t interact with the environment around it? Sensors are the back...
What good is a robot if it can’t interact with the environment around it? Sensors are the back...

Projects

Backyard Birdcam

IntroductionI had been thinking about how best to capture clear close-up photographs of some of the...
IntroductionI had been thinking about how best to capture clear close-up photographs of some of the...

Smart Watering System

Introduction This began as an idea after building a very basic single-channel unit. Commercial sma...
Introduction This began as an idea after building a very basic single-channel unit. Commercial sma...

Autozem

Introduction So now everything is available in this day and age, and all those childhood dreams ca...
Introduction So now everything is available in this day and age, and all those childhood dreams ca...
Feedback

Please continue if you would like to leave feedback for any of these topics:

  • Website features/issues
  • Content errors/improvements
  • Missing products/categories
  • Product assignments to categories
  • Search results relevance

For all other inquiries (orders status, stock levels, etc), please contact our support team for quick assistance.

Note: click continue and a draft email will be opened to edit. If you don't have an email client on your device, then send a message via the chat icon on the bottom left of our website.