Application Notes

Table Of Contents

QTR-1RC reflectance sensor schematic diagram.

1. QTR-xRC Sensor Output (Intended for Digital I/Os)

Each QTR-1RC [http://www.pololu.com/catalog/product/959] and

QTR-8RC [http://www.pololu.com/catalog/product/961] reflectance sensor

phototransistor output is tied to a capacitor discharge circuit as shown

on the right, which allows a digital I/O line on a microcontroller to take

an analog reflectance reading by measuring the discharge time of the

capacitor. When you have a microcontroller’s digital I/O connected to a

sensor output, the typical sequence for reading that sensor is:

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

2. Allow at least 10 us for the 10 nF capacitor to charge

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

4. Measure the time for the capacitor to discharge by waiting for the

I/O line to go low

The following three oscilloscope screen captures below demonstrate the result of this procedure. The sensor was

positioned 1/8" above a whiteboard-like surface with a 3/4" thick piece of black electrical tape on it. The first

reading was taken over the white portion of the surface, the second reading was taken at the edge of the tape, and the

third was taken while fully over the black tape. The yellow oscilloscope channel is the sensor output and the blue

oscilloscope channel is the output of a mega168 AVR microcontroller representing its interpretation of the sensor

output. A 5 V blue signal indicates that the AVR is measuring the sensor output as “high”; a 0 V blue signal

indicates that the AVR is measuring the sensor output as “low”. In an actual application, the important value is the

width of the positive blue pulse. As you can see from the screen captures, the shorter the pulse, the more reflective

the surface. Medium-width pulses occur from moderately reflective surfaces, or as you transition from a white

surface to a black surface (or vice versa).

QTR-1RC output (yellow) when 1/8" above

a white surface and microcontroller timing

of that output (blue).

QTR-1RC output (yellow) when 1/8"

above a white/black interface and mcu

timing of that output (blue).

QTR-1RC output (yellow) when 1/8"

above a black line and microcontroller

timing of that output (blue).

Please note that these data are affected by the specifics of the test. Reflectances depend on the surfaces, and each

microcontroller will have its own trip-low threshold. In our specific example, you can see that at a height of 1/8"

above our surface, white results in a high pulse width of 120 us and black results in a high pulse width of 2.6 ms. As

the pulse width varies between 120 us and 2.6 ms, you can tell that you are approaching or leaving the line.

The screen captures below show the results of the same test conducted at a sensor height of 3/8".

1. QTR-xRC Sensor Output (Intended for Digital I/Os) Page 2 of 5