Is it magnetic field?

Hi all, as you see on video the values get change when i near my finger to the PH circuit. I don't touch anything.

Is it magnetic field? What do you suggest to fix it?

http://youtu.be/yyX6-uFW5tg

The input circuits are very high resistance - it takes very little current to make them change. The electric field of your body is enough to do it. If you want a stable reading, add a 100K resistor to +5 or Gnd to hold the level High or Low, and the PH circuit can overcome that little bit of resistance for the real input. (your video is blocked for me, network problem on my end).

beingobserver: Hi all, as you see on video the values get change when i near my finger to the PH circuit. I don't touch anything.

Is it magnetic field? What do you suggest to fix it?

No, its an electric field, specifically a varying one (you are oscillating at mains frequency in a typical indoor environment).

If an input is responding in this way you simply don't have a low enough source impedance, so the circuit is picking up any nearby field by capacitive coupling. The analog inputs should be driven with 10k or less impedance.

PH probes and it's associated input conditioning circuitry has an extremely high input impedance and can be subject to EMI type noise. Your body can couple 60 or 50Hz energy into such a circuit without even having to actual touch a terminal. It's not unlike in the old days touching a input terminal of a magnetic phono input channel on a high gain preamp and hearing the 60Hz hum in the speakers.

Lefty

Thank you so much for your fast replies. This problem made me really so tired. I will try to add a resistor tomorrow.

Another question: Do you think a pcb circuit can solve this problem?

beingobserver: Thank you so much for your fast replies. This problem made me really so tired. I will try to add a resistor tomorrow.

Another question: Do you think a pcb circuit can solve this problem?

Usually PH probes and their conditioning input circuit is sold as a package although replacement probes can be purchased. Working at such high impedance and noise sensitive measurement makes this a very challenging design and build project, even for people with experience in electronics. Building your own is probably not a good idea.

Ok. There will be nearly 7-8 sensors (PH, EC, ORP, DO, Temp, Humidity, Light etc) with the arduino + some wifi or gsm shields. And i will control some high voltage devices. In a nutshell this will be greenhouse automation device.

What if i work with an experienced engineer and make a "main board" for all the parts?

Do you think this is the right decision?

Shielding will help. I can’t see the video right now, either, so that’s about all I can say in addition to what everyone else has said.

Thank you for the answers.