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I am hacking together a prototype wrist band, with BLE as the main component.

I'd like the device to detect a bump or a wave. What would be the most energy efficient way to do so? I'd like the charge on the device to last at least 6 months.

Any ideas would be appreciated.

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Is it a specific type of motion or any type of motion? for a specific type of motion (gestures, wave etc), an accelerometer and/or a gyro (There are devices which contain both) is the most suitable. For any type of motion, a rolling ball switch or vibration detector has lower power requirement and lower complexity.

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  • \$\begingroup\$ +1 on this. Just run the MCU in sleep mode until the motion sensing IC sends an interrupt, when that happens just be all, "HEY THERE'S MOTION DUDE!" \$\endgroup\$
    – Funkyguy
    Commented Jul 30, 2014 at 5:11
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Depending on how gentle a motion you need to detect many cheap toys use a small light spring that is positioned vertically on a PCB with a pin running up its center. Any knock from the side causes the spring to bend and touch the spring.

This is a very cheap option with effectively zero active power consumption, it is commonly found in flashing super balls.

EDIT: Missed the part about this being a wrist band, might not be suitable

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