switch off. The required time period can be selected by changing the values of the capacitor Cx. The max selected time can be any from 5 minutes to 15 minutes or more. 

A normally open reed switch is used as a sensor that can be placed with the circuit at the boarder of the door. A reed switch also requires a permanent magnet that should be placed on the door in front of reed switch. The operating voltage of the circuit is 4.5V DC. For operating with 3V replace the 555 IC to its lower voltage version 7555. The circuit will not use any power in consequent mode due to which the batteries will last for several months to years.

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This simple project can be used to make a door security system. It will alert if the door is left open and it is not closed after a set time period in the circuit. The circuit will indicate by activating an alarm connected at the output of the 555 timer IC, and when the door is closed it will automatically
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Door Left Open Indicator Alarm Circuit Diagram