Tag Archives: Motion-Activated Alarm

A Pro Trinket-based movement alarm for your bag


A portable, battery-powered device that sounds an alarm when your bag is moved. 


Your pocketbook. Your backpack. Your gym bag. Each of which hold a number of expensive, personal belongings. And, as you know from walking around the mall, through campus, into the gym, or throughout an event like Maker Faire, lugging around a hefty bag can be a burden. Wouldn’t it be much easier to set it down, give your shoulders a rest and have a peace of mind that no one will take it? Well, a Maker by the name of “MakerSelf” has devised a solution that will allow us to do so: a motion sensing bag alarm.

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Based on a Pro Trinket (ATmega328), the Movement Alarm is a portable, battery-powered device that sounds an alarm when your bag is moved. Once armed, it can only be turned off by your secret code.

For those who may not know, the Pro Trinket is a sort of break out board for the fan-favorite ATmega328 MCU. A “big” sister to the original, uber-mini Trinket (ATtiny85), the board offers the familiarity of an Arduino Pro Mini with more pins and a USB thrown in the mix. With the Pro, Makers have the choice of either programming with the Arduino IDE, using AVRdude with the “-c usbtiny” programmer flag, or flashing the chip directly with an AVR programmer like the AVR Dragon.

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In this case, MakerSelf hooked the Trinket to a GY-521 accelerometer to enable the detection of movement. Therefore, when the Trinket senses that the device has been set into motion, such as picked up or removed from its resting place, it emits a high-pitched alarm from its built-in piezo speaker.

“It is 9V battery operated, but without an on/off switch, otherwise the potential thief could just hit the off button. As a result, I have an ‘arm’ button, and then you have 20 second to but the bag + device stationary,” MakerSelf adds.

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Once armed, if the bag/device is moved for more than 5 seconds and above a certain threshold level, it sounds an annoyingly loud alarm until the correct code is entered. The secret code uses a four-button interface, but the code itself can be any length and easily modified in the Trinket software. After the passcode is entered, the status LED will turn solid indicating to the user that the device must be placed stationary. The status LED will turn off after the designated time period, advising that the alarm is now armed and listening to the accelerometer.

If the bag is moved for more than five seconds at a time, the alarm will sound. Just in case that someone needs a piece of gum, phone or some other item from their bag, entering the secret code after or during the settling time, the status LED will blink once long, followed by three short, and then the device will turn itself off without turning the alarm on.

Intrigued? Head over to the alarm device’s official Hackaday.io project page here.