EC probe causes AC interference with pH probe, should i put the EC on a MOSFET ?

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1

Hi,

i've got an Uno running temp, ec and ph probes but the ec probe interferes with the ph values (due to AC?).

So i need to turn off the ec probe while the ph probe is reading, is it best to do this with a MOSFET, i can't disable power to individual analogue inputs can i ?

Thanks.

2

We won't be able to help much without a schematic circuit diagram and your code. WIthout that it's speculative guesswork. Please post a diagram and code. There's heaps of people here that would love to help.

3

Please provide a link to the EC probe. How much current does it use? What voltage?

Does the pH probe interfere with EC values?

4

Thanks both, i'll do a block diagram this weekend.

If i have 2 glasses of water and measure pH and EC separately there's no interference, but when they are both in the same glass the pH rises, i've read this is due to the AC field created by the EC probe.

This is the EC probe : http://www.cqrobot.wiki/index.php/TDS_Meter_Sensor

This is the pH probe : Liquid PH0-14 Value Detect Sensor Module + PH Electrode Probe BNC for Arduino US | eBay

I did read by wvmarle here i think it was that even a grounded screening plane would still cause interference so separate power supplies were mentioned., but i don't see how that would work if it is the AC field causing the interference.

5

The specs for the EC probe show an Input Voltage of 3.3 to 5.5 volts, and a "Working Current" of 3 to 6 mA. I don't know what "Working Current" means, but if the probe actually uses 3 to 6 mA then I would be tempted (after verifying the current with my own meter) to connect the Vcc wire of the sensor to an output pin of the Uno (ensuring that the pin mode is set to OUTPUT).

I would initially set the pin to LOW after setting the pin mode to OUTPUT.

To take a reading, I would set the pin HIGH, possibly delay, take the reading, and then set the pin LOW. Another delay may be needed here after setting the pin LOW.

The reading uses a channel of the ADC so multiple readings may be needed due to the channel changes.

6

Thanks vaj, i'm still learning, are you suggesting a sort of chopped-DC approach ? I'm relying on the module so i'm now thinking maybe industrial units have shielding of some sort.

7

No, I was NOT suggesting a chopped approach. I suppose what I was saying is that the FETs in the Uno's microcontroller may be all that you need. When you need an EC measurement, the EC probe gets turned on, you make the measurement, and then the EC probe gets turned off. That way, the EC probe is off when you measure pH or other things.