pH and conductivity probes

Other Hardware Science and Measurement
1

Hello,
Im doing a project with my nephew for his school, the project will be an automatic hydroponic system using peristaltic pumps (as dosers for nutrients and pH balancing) and a few sensors for monitoring.

I have a few questions regarding the probes that perhaps some more advance users or experienced person can clear out :slight_smile:

I know that those probes have a limited life time (usually 6 months to a year), and I wonder if I can perhaps do better (or atleast be on the longer side, rather than the 6 months replacement schedule...)

Right now the farm is powered by a 12V input (for the pumps) which is then dropt to 5V for the arduino and the sensors.

I have 2 approaches for powering the probes (pH and eC) which are prone to wear over time.

first one is wiring the probes directly to the 5V output of the converter, that means that they will be powered on 24/7 and constantly be monitoring the solution regardless if I need the readings or not,

the second option is to wire the probes V+ pins directly to the arduino (GPIO pins, not 5V!) , and every time I would want to take a reading I have to firstly toggle that pin to HIGH, wait for the output to stabilize, and only then take my measurement

now - I know that obviously the second option (atleast theoretically) should result in longer life time for the probes, but I really wonder - am I overthinking the wirings here? is the differences between the two negligible to the point where it doesnt make much difference ?

thank you :slight_smile:

2

Hi, @iSoda

Using an I/O port to power is not a good idea, depending on the model controller you should not be drawing more than 20mA

Using the outport to turn on the 5V supply to the sensor is a better idea.
If all your sensors are 5V and your Arduino is 5V logic, then you could use a P-CH MOSFET to turn the +5V supply on to the sensor.
The MOSFET must be a logic level device.

Tom.. :smiley: :+1: :coffee: :australia: :santa:

3

Tom, thank you for the quick and thorough reply :slight_smile:

given that im using 5V (for both sensors and arduino) , do you think a NPN transistor be valid replacement for the mosfet?

happy holidays :slight_smile:

4

Hi,

Using an NPN would mean you would be gnd switching rather than + supply side, this is not recommended due to currents that can flow from the still connected positive of the sensor to ground via the sensors output connection.
Also when turned ON you will have a volt drop across emitter/collector that will affect the output readings of your sensors.

Using a P-CH MOSFET will give you a lower volt drop source/drain.

Tom.. :smiley: :+1: :coffee: :australia: :santa:

5

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