1)Your original message is quite misleading and throws Michael and Eric into this debate :)

2)It sounds like you are working with some relay outputs from your sensor (perhaps a microwave sensor or PIR alike? ). Usually, there are three output terminals - Normally Open (NO), Normally Close (NC) and Common (C). It would be hard to image that a device gives you +5V for "normal", but "float" for "alarm".

3)What makes you think a transistor is "bulky"?