Before explaining "how to make?"

I'll tell you answers for "Why to make?"

Has your child ever opened the front door and walked outside without your knowledge?
Do you know if someone trying to open your door from outside?

So its better to have a door alarm for safety purposes.

There is ON/OFF provision to avoid frustration

My Alarm is different from the cheep magnetic one.

I am latching the relay to sustain the buzzer till the switch is manually turned OFF.

So definitely you will be notified if some one tries to open the door that's why this one is Reliable.

I developed this circuit from patrick's (engineering shock) idea by implementing latching concepts which i learned from mechatronics. Check his trip wire circuit you will understand.

Step 1: Requirments

2N2222 - NPN Transistor (any npn will work fine) - 1
1k ohms resistor - 1
100 ohms resistor - 1
DPDT relay - 1 (if you follow circuit - A)
DPST realy - 2 (if you follow circuit - B)
LED diffused red - 1
12 v piezo buzzer
9 v battery with snap
duct tape *optional (step -4)
magnetic reed switch with small piece of magnet *optional (step -4)
Tools to work

If you got DPDT relay stick with CIRCUIT - A

If not... stick to CIRCUIT - B

Step 2: Use Circuit Wizard to Make Pcb (optional)

select --> PCB layout at the bottom of UI
then select --> Artwork
print the doc with a laser printer
then transfer toner to copper clad board
use ferric chloride to etch
make holes
solder

Search for a tutorial to make PCB

I've just used a permanent marker to draw the Artwork in copper clad board.

Don't forget to clean the clad board with IPA (isopropyl alcohol ) before starting.

Properly dispose of used ferric chloride solution.. sorry it will become cupric chloride (copper 2 chloride) after etching copper anyhow dispose of it.

Download the attachment and open with Circuit Wizard to simulate (circuit - B)