One of the biggest headaches with building a robot with a large number of sensors is integrating all of the sensors despite the various voltages everything runs at. The motors generally run at battery voltage while the sensors end up usually being both 3.3V and 5V.
Sparkfun has a tutorial about interfacing sensors.
EDN also has a nice article about the problem.
Personally I like to use the TI "Dual Bi-Directional I2C-Bus and SMBus Voltage Level-Translator" Part# PCA9306, also known as Digikey Part# 296-17988-1-ND, for all my bi-directional level shifting.
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