Bag Alarm

Introduction: Bag Alarm

Today we will be showing you how to make your very own bag alarm! This alarm system can be integrated into purses, backpacks, suitcases, pretty much any kind of bag. It can then be used to protect your personal belongings from theft, misplacement, and even nosey siblings. This system will use photoresistors to tell when the bag is open then send power to a small speaker to deter the theif, and alert you and anyone around you that your bag is being opened. This will come in serious handy when you need to leave your bag around some untrustworthy classmates and or co-workers. By following the next simple steps you can secure your favorite personal belongings for a lifetime!

Step 1: Materials

1. Parrallax Breadboard
2. Wires of varying lengths
3. 1 220 ohm resistor
4. 20 gauge wires
5. 1 photo resistor
6. 7-14 volt speaker
7. 1 TIP120 transistor
8. Parrallax breadboard enclosure (optional)
9. 1 103 capacitor
10. Basic Stamp Editor v2.5.3.
11. Switch

Step 2: Step One

1. Begin by creating a basic photo-resistor by using a: 220 ohm resistor, 103 capacitor, photo-resistor, and 1 20 gauge wire
Install the parts as depicted in the picture above.

Step 3: Step Two

2. Now you will need to test the amount of light inside the bag you plan to use, (in a completely sealed bag it should be around 4000 to 10000). Use the code provided to test the light constant.

' {$STAMP BS2}
' {$PBASIC 2.5}

timeleft VAR Word
timeright VAR Word

RCTIME 5, 1, timeleft
RCTIME 3, 1, timeright
  DEBUG HOME, "timeleft =", DEC5 timeleft
  DEBUG CR, "timeright =", DEC5 timeright


Step 4: Step Three

3. Now install your speaker by running another wire from another PIN on the breadboard. You will use: 2 20 gauge wires, 1 TIP 120 transistor, and your 7-14 volt speaker.
Install as shown above.

Step 5: Step 4

4. Now you have to write your code in basic stamp to test your device and make sure its working. Use the code below and substitute the left constant variable for your own. ( in a brightly lit area, the speaker should come on after around 5 seconds).

' {$STAMP BS2}
' {$PBASIC 2.5}

timeleft VAR Word

leftconstant CON 4000

PAUSE 5000
  GOSUB read_p
  GOSUB alarm

    HIGH 5
    PAUSE 3
    RCTIME 5, 1, timeleft


  IF (timeleft < leftconstant) THEN
      FREQOUT 4, 2000, 4000

  PAUSE 20

Step 6: Step Six

6. You've now created your own bag alarm! You can use your enclosure if you wish. Just place your board inside, run the switch, your speaker, and photo-resistor to the outside of the enclosure like we did. ( we drilled holes for these).

barely fit in our enclosure

Step 7: Step 7

7. Done! Now you have a fully portable alarm system for your bag. Just place your valuables into a pocket along with your alarm. Now if anyone tries to open your bag without your consent, they will be greeted with a very unpleasant and aggravating sound.

- Joe, Cody, Jordan

DIY Audio

Participated in the
DIY Audio

Be the First to Share


    • For the Home Contest

      For the Home Contest
    • Big and Small Contest

      Big and Small Contest
    • Game Design: Student Design Challenge

      Game Design: Student Design Challenge


    Dashing Rainbow Dash

    Use a reed switch to activate and deactivate it. Also add a method of sensing vibration/movement so people can't just run with your stuff.


    Yes, this was an issue we ran into when we came up with this design. But there are a couple easy fixes for this! You could run the switch on a longer wire and make a hole somewhere on the outside of the bag so you can flip it on and off without opening or disturbing the device inside. Or you could make an electronic remote switch and use that if you wanted to make it extremely convenient.Thanks for the feedback!


    Reply 10 years ago on Introduction

    A less obvious way could be using a magnetic switch like are used on windows and a magnet to operate it. It would need to be the normally closed type switch and would open when the magnet was placed over it.


    cool, it would be really neat if you made it so that there was a special trick to opening the bag w/o an alarm, so you can access your own bag with no problems.