Pololu QTR-8RC - Manual (Page 8)

Note: Do not short the 3.3V BYPASS pins together while using a Vcc of 5 V. While this would make
the LEDs even brighter, it would exceed their current rating and put them at risk of burning out.
LEDON
This pin is connected to a MOSFET that delivers power to the IR LEDs, so its state determines the whether the LEDs
are on or off. When this pin is driven high or left disconnected (it is internally pulled high), the LEDs are all on; when
this pin is driven low, the LEDs are off. You can use this pin to save power by turning the LEDs off while you are not
taking a reading. Additionally, you can connect this pin to a high-frequency PWM to control effective LED brightness
and decrease power consumption.
QTR-8A Sensor Outputs
The QTR-8A reflectance sensor array has eight distinct sensor outputs, one from each LED/phototransistor pair.
These outputs are analog voltages ranging from 0 V to Vcc (which must be between 3.3 and 5 V). With a strong
reflectance, such as when the sensor is over a white surface, its output voltage will tend towards 0 V; with very weak
reflectance, such as when the sensor is over a black surface, its output voltage will tend towards Vcc. To get a good
range of readings between white and black surfaces, we recommend you mount your sensor no more than 0.25" away
from the surface. In our tests, the optimal distance between the sensor and the surface has been 0.125". Please see our
QTR Sample Output Data document [http://www.pololu.com/docs/0J13] for more information.
QTR-8RC Sensor Outputs
The QTR-8RC reflectance sensor array also has eight distinct sensor outputs, one from each LED/phototransistor pair.
In the 8RC sensor model, each phototransistor uses a capacitor discharge circuit that allows a digital I/O line on a
microcontroller to take an analog reflectance reading by timing how long it takes the output voltage to decay due to
the phototransistor. This format has several advantages over the 8A sensor model:
• 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
When you have a microcontroller digital I/O connected to a sensor output, the typical sequence for reading that 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, such as when a sensor is over a white surface, the decay time can be as low as several
dozen microseconds; with very weak reflectance, such as when the sensor is over a black surface, the decay time will
typically be a few milliseconds, but can be several dozen milliseconds in the worst case. The exact time of the decay
QTR-8A and QTR-8RC Reflectance Sensor Array User's Guide © 2001–2014 Pololu Corporation
5. Module Connections Page 8 of 12