Do you have people constantly bugging you? If so try using this so you know when they're coming.
Step 1: Get Your Parts
1. Arduino uno
2. Some connecting wires
4. Piezo buzzer
6.code (for arduino, go to make some.com for code.)
Step 2: Build
Connect the wires like in the picture shown.
Step 3: Test
I hope it works for you. Good luck!