I am working on a project including Piezo Vibration Sensors (https://www.sparkfun.com/products/9196). To connect them to my Arduino, I used the same circuit as in the KnockSensor tutorial (http://www.arduino.cc/en/Tutorial/KnockSensor), except I used 100K resistors instead of 1M.
Now, for my project it is important that I get very accurate values from the sensors. This seemed to be working fine - I got 0's when I did nothing, and values up to 800-1023 with vibrations. However, as soon as I plugged in the power chord of my Notebook, I did not get 0's any more, but a lot of values between 0 and 10. Touching the arduino at any metal part (the USB plug, for example), would result in values between 0 and 8, which still is not satisfying. However, as soon as I plugged in a projector (via HDMI), the values would be at 0 again (when doing nothing, of course).
The project will eventually run on a PC, not on a Notebook, so I thought I would try and test on my PC. Now everything seemed to be working fine - most of the time. At times, the random values from 0-10 return, then suddenly stop, without me doing anything, and then continue after a short time. I noticed that when I turn on the lights or a speaker in the same room, for example, I always get some higher values (~25), but this happens just one time when I plug in, so this is not a problem.
Now, I thought it would certainly be a problem with the USB power supply being noisy, so I bought a Low-Noise Power Supply http://www.aqvox.de/usb-power_en.html from a music store (these are usually used for USB Mics and other audio stuff), thinking this would certainly solve my problem - which it did not. It didn't change anything.
The really weird thing is that it happens only at times, sometimes it would run hours without problems, then, I get random values again and don't know why. I tried plugging everything out except for the PC, turning every light off, without success.
Could this be my PC's problem? Maybe simply bad quality motherboard? Or the power adapter? If so, why does the USB Low-Noise Power Supply not help? I don't really want to use resistors with less Ohm, as it works usually works perfectly fine, until the problem returns.
Thank you a lot in advance! I will keep you up to date.