Most of the damages made by the burglaries are due to the people that forget to lock their house. Even if the door was locked, damage of illegal invasion by picking is not few. In this study, we developed monitoring system of the lock-key using photo reflector. The system judges the locked state of doors in house by using a photo reflector. The user can confirm the Lock-Key state of doors by smartphone from anywhere. And the system has a device that trigger the alarm when user is going out

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... ut locking the door. This system sends the data of the Lock-Key states to the computer through wireless communication.