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I have the ground for an SD card module, LED, and piezo sensor all plugged into the same ground rail. When doing this I get a low "hum" in the piezo readings where instead of reading ground voltage (0) when there's no vibration, it cycles between 4 and 6.

However, when I switch the piezo's ground onto ground rail connected to different ground pin on my Arduino, the hum goes away. If I keep the piezo on original ground and move the SD card module's ground, the hum comes back.

I'm very new to electronics, so I'm wonder what's going on?

EDIT: Here's a diagram of the circuit.

enter image description here

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    You have a ground loop. Without seeing the actual physical layout it is impossible to determine the cause. This is a common problem in audio circuits. Fixing this is as much an art as a science. The usual solutions involve a single earthing point, earthing at one end only or in extreme cases common mode rejection techniques. – Milliways Mar 6 '16 at 4:36
  • I have added a diagram of the circuit. How could this result in a ground loop? – user1569339 Mar 6 '16 at 6:03
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    This is an idealised diagram. These kind of problems are often related to the physical layout e.g. parallel wire runs. I agree that there appears to be nothing amiss. As I stated this is a black art. I would suggest separating the low level analog circuit from the rest of the wiring - i.e. DO NOT share any earth cabling with any other circuit. This appears to be what you have done when it worked. You should also minimise any other source of interference by using twisted wires to the sensor. – Milliways Mar 6 '16 at 6:23
  • I copy-paste @Milliways comment as an answer. Please, accept it to close your question. – user31481 Oct 28 '17 at 21:50
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@Milliways remarked:

You have a ground loop. Without seeing the actual physical layout it is impossible to determine the cause. This is a common problem in audio circuits. Fixing this is as much an art as a science. The usual solutions involve a single earthing point, earthing at one end only or in extreme cases common mode rejection techniques.

Yours is an idealised diagram. These kind of problems are often related to the physical layout e.g. parallel wire runs. I agree that there appears to be nothing amiss. As I stated this is a black art. I would suggest separating the low level analog circuit from the rest of the wiring - i.e. DO NOT share any earth cabling with any other circuit. This appears to be what you have done when it worked. You should also minimise any other source of interference by using twisted wires to the sensor.

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    I suggest that you make this a Community Wiki answer. – sa_leinad Oct 29 '17 at 1:54
  • It's not my answer. I'm not qualify in this subject. – user31481 Oct 29 '17 at 7:48
  • I refer to this post by Nick Gammon. – sa_leinad Oct 30 '17 at 0:51

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