The QTI sensor by Parallax is a proven little module that lets you use either an analog or digital I/O line detect a light from dark surface. Each module uses a QRD1114 IR reflection module that charges up & discharge a tiny capacitor. By either monitoring the reflection level directly (analog), or the time it takes to change logic state (digital), you can detect a line on a surface. Here’s Parallax’s text:

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. The QTI Sensors were originally meant to sense the outer rim of a SumoBot® ring; however these little line sensors can be very handy for other applications, including line following and maze navigation.

Features:

• Capable of analog or digital outputs
• No contact surface sensing
• Unfocused for sensing diffused surfaces
• Daylight filter on sensor
• Easy to interface with any Parallax microcontroller

Specifications:

• Power requirements: 5 VDC
• Communication: Analog output of capacitor decay time depending on level of infrared reflectivity
• Dimensions: 1.25 x 0.37 in (31.7 x 9.4 mm)
• Operating temp range: -40 to +185 °F (-40 to +85 °C)