I made a project based on the above idea and I thougt I would share it here. Also I am using a 9V battery to power this and the batery runs out quite fast, because of the IR transmitter I suppose. Is there any way I can make the battery last longer.

http://www.ingenstech.com/projects/letter-box-monitoring-alerting-system/

 

Is there any way I can make the battery last longer.

Have the micro sleep most of the time and only wake up occasionally to power-up the IR Tx for a very brief period. OR...... discard everything except the battery and buzzer and use a micro-switch to activate the buzzer .

 

Is there any way I can make the battery last longer.

Eliminate the linear regulators.
Run the entire circuit on the same Vdd.
Reduce the number of LEDs.
Use an ultra low-power uC.

 

Alec has the correct solution. Turn on the transmitter only when a mechanical switch changes state. I would use a mercury switch taped to the inside of the mail box door.

 

Have the micro sleep most of the time and only wake up occasionally to power-up the IR Tx for a very brief period. OR...... discard everything except the battery and buzzer and use a micro-switch to activate the buzzer .

that is a good idea, will do this