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).
Each phototransistor uses a capacitor discharge circuit that allows a digital
I/O line on a microcontroller to take an analog reading of reflected IR by
measuring the discharge time of the capacitor. Shorter capacitor discharge time
is an indication of greater reflection.
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.
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 in the images tab:
  |
| 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
Like the Parallax QTI, the QTR-8RC module has eight identical sensor outputs that require
a digital I/O line capable of first charging the output capacitor (by driving
the line high) and then measuring the time for the capacitor to discharge
through the phototransistor. 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 all eight sensors is possible
with most microcontrollers
- Parallel reading allows optimized use of LED power
enable option
The typical sequence for reading a sensor is:
- Turn on IR LEDs (optional)
- Set the I/O line to an output and drive it high
- Allow at least 10 us for the 10 nF
capacitor to charge
- Make the I/O line an input (high impedance)
- Measure the time for the capacitor to discharge by
waiting for the I/O line to go low
- Turn off IR LEDs (optional)
These steps can typically be executed in parallel on multiple I/O lines.
With a strong reflectance, the discharge time can be as low as several dozen
microseconds; with no reflectance, the discharge time can be up to a few
milliseconds. The exact time of the discharge 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 AVR library provides functions that make it easy to use
these sensors with our Orangutan robot controllers
and other AVR-based controller boards such as Arduinos.
Please see section 11 of
library command reference for more information.
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. |
