Application Notes

Table Of Contents

2. QTR-xA Sensor Output (Analog Voltages)

Each QTR-1A [http://www.pololu.com/catalog/product/958] and QTR-8A [http://www.pololu.com/catalog/product/960]

reflectance sensor phototransistor output is connected to a pull-up resistor as shown below to form a voltage divider

that produces an analog voltage output that ranges between 0 V and the supplied voltage (which is typically 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 the

supplied voltage.

QTR-1A reflectance sensor schematic diagram.

To demonstrate what the QTR-xA sensor output looks like as it passes from a reflective surface to a non-reflective

surface and back again, we set up a motor to spin a white paper circle with a piece of black electrical tape on it as

shown below.

Experimental setup for QTR-1A and QTR-1RC oscilloscope outputs.

The following three oscilloscope screen captures show the output of the QTR-xA reflectance sensor during the

period where the black line passes by the sensor. The oscilloscope is set to 500 mV per division. The only difference

2. QTR-xA Sensor Output (Analog Voltages) Page 4 of 5