QTI Line Sensor 


The Parallax QTI senor uses a QRD1114 infrared (IR) reflective sensor to determine
the reflectivity of the surface below it. When the QTI sensor is over a dark surface, the
reflectivity is very low; when the QTI is over a light surface, the reflectivity is very
high and will cause a different reading from the sensor.

 available from Parallax: $5.95


  • Phototransistor Output
  • No contact surface sensing
  • Unfocused for sensing diffused surfaces
  • Compact Package
  • Daylight filter on sensor


PCB Length: 1” Overall Length: 1 1⁄4” PCB Width: 3/8” Thickness: 5/16”

pic and schematic
QTI Line Lensor and schematic


The QTI sensor is activated by placing 5 V (Vdd) on the W pin. This will cause current to flow through the 470 ohm resistor to the LED side of the QRD1114. IR light reflecting of the surface below will cause a change in the ability for the current to flow through the phototransistor side of the QRD1114. The transistor, in effect, behaves like an IR controlled resistance.


A Closer Look at the QTI

The QTI module is designed for close proximity infrared (IR) detection. Take a look at the small square black box just above the QTI label. It’s nested below the capacitor and between the two resistors. That’s a QRD1114 reflective object sensor. There’s an infrared diode behind its clear window and an infrared transistor behind its black window. When the infrared emitted by the diode reflects off a surface and
returns to the black window, it strikes the infrared transistor’s base, causing it to conduct current. The more infrared incident on the
transistor’s base, the more current it conducts.

two modes:

When used as an analog sensor, the QTI can detect shades of gray on paper and distances over a short range if the light in the room
remains constant. With this circuit, you can read the values into an analog pin to measure how long it takes the capacitor to discharge
through the IR transistor. Since the IR transistor conducts more or less current depending on how much IR it receives, the analog measurement can give you an indication of distance or shade of gray.

If all you want to know is whether a line is black or white, the QTI can be converted to a
digital sensor by adding a 10 kΩ resistor across its W and R terminals. After doing so, the
QTI behaves similarly to the circuit on the right. When W is connected to Vdd and B is
connected to Vss, the R terminal’s voltage will drop below 1.4 V when the IR transistor sees
infrared reflected from the IR LED. When the IR LED’s signal is mostly absorbed by a black
surface, the voltage at R goes above 1.4 V. Since the micro controller interprets any voltage
above 1.4 V as 1 and any voltage below 1.4 V as 0, this circuit gives us a quick and easy
way to detect a black line on a white background.

analog vs digital circuit

Building the Sensing Circuits

If you apply 5 V to a QTI's W pin, its R pin will rise above 1.4 V if it detects a black surface, or fall below 1.4 V if it detects a white surface. In other words, the QTI sends a binary-1 if it does not see its IR reflection or a binary-0 if it does. Only one QTI should be turned on at any given time to make sure that one QTI doesn't see the reflection of another QTI's IR signal. Turn each QTI on one at a time, read the digital pin, and then turn that QTI off again. If the QTI that receives 5 V sees it reflection, it will drive the voltage at digital pin low; otherwise, it will be pulled high by the 10 kΩ resistor.


Processing gray scale reader + freq out screen shot
  • read distance, though not far, at most maybe .5"

Trouble Shooting Tips

Make sure that the QTI sensor is properly installed by matching up the pins. The “B” connects to Vss. The “R” connects to pin 9 of the BASIC Stamp and the “W” connects to pin 10 of the BASIC Stamp.

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