Sharp IR Sensor - GP2Y0A21YK0F is a distance measuring unit, can be used for obstacle detection for robots. It has measuring range of 10 to 80 cm. The sensor gives output voltage proportional to the distance measured. 
The sensor adopts triangulation method hence cannot be easily influenced by variety of reflectivity of object or ambient temperature and provides reliable readings. The module can be used in obstacle detection for robots, sensor for energy saving, amusement equipment (like arcade game machine) and many such applications.
Its high obstacle detection range makes this sensor an excellent alternative to sonar in some applications. 
Features : Composed of an integrated combination of PSD (position sensitive detector), IR LED (Infrared light emitting diode) and signal processing circuitSmall compact size and easy to use.
 
How it works: Infrared is sent out by the sensor which bounces off objects. The analog voltage that is returned determines how close the nearest object is. The closer it is, the higher voltage is returned.
 
Specifications
General Specifications
Current Consumption
30mA
Interface
Analog (Voltage)
IR Range
10 to 80 cm
Update Rate
38 ± 10 ms
Power Supply
Phase Voltage
5V