The Item Detector detects items passing through it, and must be placed in-line between two pieces of tube.

It will not remove the colour tags of the items passing through it.

It has three different modes:

• Item Mode: Will emit a redstone pulse for every item, e.g. 64 pulses for a stack of dirt.
• Stack Mode: Will emit a single pulse for every stack of items passing through it, e.g. a single pulse for a stack of cobblestone.
• Stuffed Mode: Will emit a constant redstone signal whenever there is an item stuffed back into it, which happens if there are no available destinations left for the item.

Recipe

Sources

RedPower2 Wiki