These instructions will teach how to make a cheap door alarm using basic circuitry and easily accessible materials.

The materials needed are:

-   A piece of cardboard, about 6'' x 6''
-   9 V Battery
-   Battery snap connector
-   DC Mini Buzzer (Ideally a 9V but if not available resistors can be used to adjust)
-   Resistors, size depending on the Buzzer purchased (Use next step to calculate)
-   Notecard
-   1 ft of ribbon or string
-   Clothespin
-   Large rubber band or hair tie
-   2-4 push pins
-   Tape
-   Scissor
Hole Puncher

Optional Materials:

-   Pliers
-   Alligator clips

Step 1: Preliminary Calculations

Ideally, you will have a 9 volt buzzer, but if thats not available you will need to calculate how much, if any, resistance is necessary. You will be using a 9 volt battery but if your buzzer uses less voltage than that, you could potentially burn it out.

The formula for voltage is V = IR with V being the voltage in volts, I being current in amps and R being resistance in ohms.

V = 9 volts
I = Can be found on the package as shown in the image

V = I * R   -->   R = V / I

EX.
45 mA = 0.045 A so:
R = 9 V / .045 A = 200 ohms

Resistor values can be rounded if exact value is not convenient (if you need 180 ohm, you can still use a 200 ohm resistor).

Step 2: Separator

Cut out a piece of the note card that is about 1'' x 1''. Punch a hole close to one edge and tie one end of the string to it. Tie the rubber band to the other end.

This piece will be referred to as the separator. You won't need it until later in the project but it's easier to have it ready before you need it.

Step 3: Battery

Connect the battery to the battery clip.

Tape the battery onto the cardboard piece. It doesn’t really matter where everything is placed as long as they are connected in the right order, but if you want to keep it more organized, tape it inside any quadrant (as shown by the dotted square) of the cardboard with the wires facing inward.

Step 4: Buzzer

Tape the buzzer in the quadrant caddy-cornered to the battery with the wires again facing inward.

Step 5: Connecting the Resistor

Attach one side of the resistor, it doesn’t matter which, to the red wire coming from the battery and the other side of the resistor to the red wire coming from the buzzer.

The wires only need to be touching, but to prevent them from accidentally disconnecting you can go ahead and twist the two wire ends together. You can use your hands or if your having trouble pliers or alligator clips can be used as well.

Step 6: Mounting the Resistor

Tape the resistor to the cardboard in any of the remaining quadrants. Use two pieces of tape and fully secure the connecting ends of the resistor.

Step 7: Secure the Clothespin

Securely tape the clothespin in the last quadrant facing away from the buzzer.

Step 8: Attaching the Wires

Tape the black wire from the battery to the bottom pad of the clothespin. Make sure the tape is not covering the exposed metal end of the wire. Then tape the black wire from the buzzer to the top of the clothespin, again with the metal end exposed.

Step 9: Completing the Circuit

To complete the circuit, wrap aluminum foil around the top and bottom of the clothespin covering the buzzer wire. When the clothespin is shut, you should be able to hear the buzzing. Stick the separator into the pin to stop the buzzing. Your are now ready to test your door alarm.

Step 10: Testing

To test your completed alarm use the push pins to pin the cardboard to the wall next to a door. Then place the rubber band around the door nob. Now simply open the door.

If your door alarm does not work remove the separator and:

-   Check the connections to the resistor
-   Check that the foil is pressed tightly enough against the two wire ends

is a battery snap connector a 9V battery connector right?<br>and<br>did u use a resistor??
<p>Yes, 9 volt batteries use a snap connector to clip onto the end of the battery, not a normal battery box like AA and AAA batteries use. I think he used a resistor too, please see my last reply in this post.</p>
Hey my dear author,<br>What's the role of resistor in your instructable ?
<p>I am not sure if you will be interested in my reply, since it was 1 year ago that you posted that, but I will try my best to explain. He used a resistor because he had a 9volt battery, but a buzzer with a lower voltage (probably 6volts) So basically, you would be in danger of frazzling a 6 volt buzzer if you used power that was too much for it to cope with. The resistor makes sure that the buzzer only recieves the ammount of electricity that it needs, no more. I hope that helps you understand the role of the resistor in this instructable.</p>
Nevermind. Got it.
<p>The pushpin wall mount is pretty clever. Have you read "Little Brother"?</p>