This popular sensor made by Sharp produces an analog output that varies from 2.5V at 20cm to 0.4V at 150cm.
The formula to translate SensorValue into Distance (the formula is only valid for a SensorValue between 80 to 490) is:
This sensor connects to:
The sensor plugs in the 1101 - IR Distance Adapter using the 30cm interface cable that comes with the 1101.