Êlectronics: Mechanical rotation detection alarm

Design the electronics for low voltage mechanical rotation detection device that triggers an alarm. Provide an electrical diagram and instructions how to build a prototype using easily accessible components (eg from eBay). Electronics should be as simple as possible, and the build instructions must be understood by an amateur with practical skills but little electronics knowledge.

This is a low budget, non-commercial project and does not require a high level of presentation (sketches are acceptable).

Inputs
2 mechanical rotation detection switches.
Alarm triggered when one switch is turning but the other is stationary.
No alarm when both inputs are rotating
No alarm when both inputs are stationary