I am connecting a baby-sense sensing board (which I am quite sure is piezo-electric sensor) to an Arduino Nano, using the arduino to analyze the signal (i.e instead of the baby-sense "brain" which I do not have). The circuit is quite simple - I have a resistor in parallel with the sensor, and 2 diodes that prevent the input voltage to the Arduino's analog input from going below 0V or above 5V.
It works ok, but when the arduino is powered using an external power supply (I am using the 9V/230mAmp transformer that I got with the baby-sense), I get a low-frequency noise at about 2.5Hz, which interferes with the readings.
I can solve it by defining a threshold level, since the noise level is about 6-8 (units of the Arduino's 10bit analog readings) and when my child sleeps with the sensing board under his mattress the signal level is about 15-20 --- but I would like to know the source of the noise and maybe eliminate it.
I attach 2 plots, one of the time domain noise, and one the frequency domain data in which the 2.5Hz noise is clearly evident.
Have you considered adding a high pass filter (digital or analog)?
If you go for an analog filter, you may need to make it active depending on the impedance you have.
You do need to know what your sampling frequency is, ... And what are the frequencies you are interested in. Also you should see where this it is coming from. Such low frequencies are normally a pain.
This could well be 60 Hz line noise (or 50 Hz in some parts of the world) that looks like a much lower frequency due to a phenomenon called "aliasing". It depends very much on how fast you are sampling your signal. If you sample faster than 140 samples per second (theoretically 120 sps) you should see the 60 Hz noise. If you sample at lower than 120 sps you will see aliasing (due to Nyquist's theorem). If you sample at 70 sps, for example, the 60 Hz line noise will look like a 10 Hz oscillation.
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons
