Stop Bike Thieves Dead in Their Tracks! Make a Magnetically Controlled Bike Alarm

Bikes are a great form of transportation. They use human energy more efficiently than any other machine. You can keep it in your closet or hallway. You can even take it on the train in a pinch. However, this portability is also the bike's biggest draw back. If you own a bike in the city, chances are it will be stolen. Locks barely deter thieves armed with bolt cutters and crowbars. Throw the bike thieves for a loop and make a tilt-sensitive alarm. It will hopefully startle your bike's assailant enough to make them think twice about stealing your precious!

Materials

• Cardboard
• Hot glue
• Tape
• Aluminum foil
• 9 volt battery
• Piezo buzzer
• Enclosure or box
• Hard drive magnet

Make a Tilt Sensor

Tilt sensors are electronic components that sense when an item shifts from one angle to another. Usually, a rolling steel sphere will complete an electrical connection at either end of a tube. When turned one way, the sphere connects the left side. When turned the opposite way, the sphere rolls down the tube and connects the right side.

While tilt sensors are cheap enough to buy, I decided to make one from cardboard. My tilt sensor is roughly "L" shaped.

Below you can see the parts lain out on the floor. The steel washer will rest at the top of the cardboard enclosure. When tilted, the washer will roll down to the lower section and be stuck there.

Hot glue the side pieces making sure to keep the inner edges clear of glue so that the washer can roll smoothly.

Tape a stripped wire to the back side of a piece of aluminum foil. Glue the foil to the bottom section of each wall.

Make sure the two aluminum sheets do not make contact while you glue the sensor closed.

Circuit

Our circuit has only three components. The positive lead of the battery connects to the tilt sensor. From there, the other tilt sensor lead connects to the positive lead of the piezo buzzer. The negative lead of the buzzer connects to the battery ground, completing the circuit.

Install the Alarm

Though this alarm is installed in a box, you could just as easily conceal the whole circuit under your bike seat. I placed my alarm in the front bike basket. Set up your bike so that it is leaning the correct way, plug in your alarm, and try to ride away!

The alarm should start producing a constant high pitched tone. The great part about this alarm is that you can disarm it by turning it upside down or by using a magnet to hold the washer in place inside of the sensor.

Below, a hard drive magnet keeps the washer away from the foil contacts at the bottom of the tilt switch.

Thoughts

There are many ways to make tilt sensors and bike alarms. This is merely the simplest. Adding a microcontroller could give you a lot more options for buttons, noises, lights, etc. If you really want to startle your bicycle thief, I suggest rigging up some of these car horns.

Tilt sensors are great for triggering pranks, too! Show us what you do with your alarms by posting on the Mad Science corkboard. If you are stuck on this project, don't hesitate to message me directly or ask in the forum.