[Staff note: The original poster asked the Maker Forums community for advice on a mechanism to count kicks of a hacky sack. He has now requested that all his content be deleted from Maker Forums.]
I supposed that is probably still too large? It’s the smallest integrated package I know of with rechargeable battery and accelerometer in one package. It has bluetooth and wifi and could send message to your phone.
The MPU I used in Time Turner: Time Tracking Tool in a dodecahedron - #4 by mcdanlj they haven’t released information on how to use the interrupt function, which would be exactly what you would want in this application.
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Now I have to get one to play with! I know Don needs one too!
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I DO NOT NEED THESE!, I DO NOT NEED THESE!,I DO NOT NEED THESE!,I DO NOT NEED THESE!..
https://docs.m5stack.com/#/
https://docs.m5stack.com/#/en/uiflow/uiflow_home_page
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There are motion-sensing switches that are basically a weighted wire spring inside a loop of wire that is the other contact that are designed to connect when moved. I don’t know what they are called to link to a part, though. It would be possible to use such a sensor instead of an MPU but it could miss hits that are aligned with the axis of the sensor wire.
I think most accelerometers have “fall detection” built in.
Yeah, I just discovered sadly that the one I used recently, the MPU-6050 that I thought was open-source-friendly, lets you use half the features (basically all the ULP features) if you get their proprietary API and tools. That really messed up my project.
If I were trying to design one to do energy harvesting I’d more likely use 433Mhz than audio. Using the battery seems easier to me. I’d expect ultrasonic signal to be muffled by beads in a sack. YMMV.