Support SEN0161 ?!

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frenesik
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Joined: 21 Oct 2017, 11:00

Support SEN0161 ?!

#1 Post by frenesik » 21 Oct 2017, 11:02

Hello,

Are you planning to add the sen0161 (ph meter) module to the ESP easy project?

Sincerely, Yo.

papperone
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Joined: 04 Oct 2016, 23:16

Re: Support SEN0161 ?!

#2 Post by papperone » 21 Oct 2017, 12:29

I am not sure you need a specific plugin as far as I understood this sensor gives an analog output so you can use it with the Analog reading of ESP8266 or via a more accurate analog port expander (like ADS1115)...
My TINDIE Store where you can find all ESP8266 boards I manufacture --> https://www.tindie.com/stores/GiovanniCas/
My Wiki Project page with self-made PCB/devices --> https://www.letscontrolit.com/wiki/inde ... :Papperone

Shardan
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Re: Support SEN0161 ?!

#3 Post by Shardan » 21 Oct 2017, 13:17

Hello,

this pH module gives an analog value as output.

I'm not sure if that circuit may give negative voltage values as an output, the ESP can't work with that.
It has a voltage converter to -5V on board so this is possible.

You may check this using the pH calibration solutions ph4 and pH9, test the output of the module
with a multimeter. (NEVER test the probe directly! The input impedance of the multimeter is too low.)
If it gives positive values with ph4 and pH9 solution it can be used with the ESP's analog input.
Depending on output values it might need a voltage divider between module and ESP-Input.

So if the module gives positive voltages only -i assume it will - it can be used with the ESP without specialized plugin.
The analog input plugin and a formula will do.
If it gives negative values there is no simple way to use it with the ESP due to hardware limits.

I've checked the schematics found at http://image.dfrobot.com/image/data/SEN ... %20SCH.pdf

They definitely made some effort but i would not recommend using this for permanent metering in a fish tank or pool for example.
The CA3140 OpAmp has an input current of 2pA and an offset input current of 0,1 pA.
Nice values, but for a pH probe it is too much for permanent use and will shorten the probe's life.
For long probe life and permanent use it should have below 0,01pA.

A second point: The input amplifier is single circuited so it will take any hum or other electrical noise, for example produced by a filter pump.
To avoid this they put a 2,2nF capacitor on the input plug - definitely a no go. Every pH change will generate a loading current into the capacitor,
this shortens the probe's life.

So for short pH metering this circuit will work, the deviation by offset current is far below the given tolerance of 0,1pH.
Just disconnect the probe after metering.

For permanent metering it needs some more effort. Input should be an instrumentation amplifier that filters noise without capacitor,
An OpAmp with extremely low bias and offset currents shoud be used like the LMC604x-types.
Anyways this would make such a module very expensive.

Regards
Shardan
Regards
Shardan

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