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I see that there're vibration sensors and that piezo can detect vibrations. Which is better (more sensitive) for detecting knocks over a piece of wood? I'm looking for a small package, so I'm only looking at small piezo. I've tried tap-enabled accelerometers, but they don't seem to be very sensitive. Should piezo or vibration sensors have higher sensitivity than accelerometers?

Thanks

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Piezo is very good for detecting vibrations, bias it to a non-zero voltage would also make it suitable for vibration in both ways. You can check this post, where a few people helped me with a good circuit diagram.

Piezo into 0-5v ADC, with offset center voltage?

note : to protect the input of the opamps, use a 10k in series into the opamp input.

note2 : with a bouncy material (like wood and rubber), it would have a lot of a small vibrations after the initial knocking, you would need to work it in the code to isolated the 1st knock to avoid multi triggers in a very short times.

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With the right amplifier, piezos can be very sensitive. A piezo can be used as a contact mic, and it sounds like that's pretty much what you want. For good sensitivity, you may want to glue your piezo to the surface.

However, keep in mind that you need to protect the input of your preamp, because if a piezo receives a direct/large impact, it can generate a nasty voltage transient that will blow away sensitive op-amps and the like.

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  • \$\begingroup\$ For non-direct contact, is a small piezo like 1cm wide still more sensitive than a vibration sensor? For instance, if the device was soldered on a PCB which itself is inside a case. \$\endgroup\$ – Kar Jan 3 '16 at 6:55
  • \$\begingroup\$ What sort of vibration sensor are you referring to? And would the case be in direct contact with the piece of wood? \$\endgroup\$ – uint128_t Jan 3 '16 at 6:56
  • \$\begingroup\$ Like that on this which is probably same as this. Not sure if there're better ones though. The case will be in contact with a foam tape which is in contact with the piece of wood. I'd imagine that the foam tape will absorb a fair bit of vibrations though. \$\endgroup\$ – Kar Jan 3 '16 at 7:01
  • \$\begingroup\$ Those mechanical switches are going to require a fair bit of travel to activate. If your "knock" is a fairly large, slow impact, those would probably work. However, if your "knock" is a sharp hit, and the wood won't visibly move, I think piezos are your best bet. The foam is a bit of an issue though; the stiffer you can make the mechanical junction between the piezo and the wood, the better. \$\endgroup\$ – uint128_t Jan 3 '16 at 7:14
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If you want to go digital - try ADXL312 3-axis accelerometer. Cheap one, 3 kHz. At ±1.5g range and it's 12 bits - it has to be really sensitive.

If you want to stay analog - you may give a try @ electrostatic microphone. There is knock sensor definition in the automotive industry (engines). It sounds like what you need. I believe, that it's piezo based however.

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  • \$\begingroup\$ Are accelerometers meant to be more sensitive than vibration sensors or piezo for knock detection though? Also, the 312 costs like $7 each on digikey. \$\endgroup\$ – Kar Jan 3 '16 at 8:19

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