Ground Interference On EMF Detector

Hi Members,

I am new to both Arduino and this Forum so please take it easy on me ;) I'm using Arduino Uno to learn electronics (i'm a computer programmer by trade) and my first project was to follow the below links to create an EMF detector: http://www.instructables.com/id/Arduino-EMF-Detector/ http://www.aaronalai.com/emf-detector

This works perfectly when powered from the Laptop over USB or from a 9v battery. But when I plug it into a USB charger or more importantly into my Raspberry PI then I see the EMF reading jump all over the place and give consistently high readings.

I need to connect to the PI over USB so I can use the USB Serial to send EMF data to the PI. I tried even using the battery at the same time as the USB connection, knowing that the Arduino would use the Battery first. However this still gave the interference.

What I have also found is that touching the USB ground shield drops the interference significantly and therefore I presume this interference is coming from the ground source.

Does anyone have any ideas on how to either stop the ground interference or shield the EMF sensor from it?

Any help is greatly appreciated.

Gary

Bump.

I've since found that using an Apple iPhone USB charger minimises the noise. It's still there as I see the EMF fluctuating slightly with no contact from an EMF source, but it is much better than with other USB chargers. The noise is eliminated completely if I use battery power.

Anyone have any idea on how to minimize or eliminate noise travelling from the AC USB inlet up to the Raspberry Pi then through the USB cable to the Arduino Uno?

Thanks, Gary

Found the solution. So thought I would reply back so that the solution is out there.

Sorry about the lack of electrical knowledge. Please feel free to correct me ;)

Basically the interference was coming from the induced current from the transformer travelling up the shield on the USB cable. On the PI and Arduino the shield is connected to the same common ground. Therefore the induced current was able to travel up all the way to the Arduino.

As I posted I found I could dramatically reduce the interference by using an Apple charger. This is because the Apple charger, while being shielded for induction from the transformer also has an earth connection where many stray currents can travel (all my other USB chargers did not have an earth - UK pins). However there was still a slight interference coming form the Apple charger and this was because the USB Shield was not grounded back to earth so some of the stray currents could still travel.

Therefore the following solved this: 1) Use the Apple Official USB Charger Minimises the induced currents 2) Hack the Apple USB Charger to connect the ground shield of the USB to the Earth/Ground connection on the plug. All Stray currents now travel back to ground. 3) Use a USB to micro USB cable (connections to the PI power) which only transports power and does not transport the shield (i.e. is unshielded). All stray currents now travel back to ground through the hacked charger and do not travel up to the PI and then the Arduino.

Hope this helps someone with the same issues.

Cheers, Gary