Multiple moisture probes on one sensor.

I am planning on condensing the hardware in my irrigation system control box. Currently, I am using the LM393 moisture sensor breakout boards x8, so the wiring inside the box is a bit cramped, and since I have started making my own PCB's I am looking at making one for this project.

What I am wondering is if I can use one LM393 on multiple probes using FETs to control which probe get read and when.

Using something like


N$2 = Sensor GND
N$4 = Sensor V+

The wires connecting the probes to the control box vary in length of 1-10m, will this affect the readings using the one sensor??

Many Thanks
Iain

You're are looking for an analog multiplexer/demultiplexer (p.e. 4051).

http://playground.arduino.cc/Learning/4051

pylon:
You're are looking for an analog multiplexer/demultiplexer (p.e. 4051).

http://playground.arduino.cc/Learning/4051

Thanks pylon.
Yeah I was looking at multiplexing and I am currently designing my circuit using it, but I was just wondering if the solution above was viable to cut down on a number of sensors and if not why??

Many thanks
Iain

Looks viable enough to me. You just have to dedicate a digital output per sensor, hope you have enough pins for all that!

wvmarle:
Looks viable enough to me. You just have to dedicate a digital output per sensor, hope you have enough pins for all that!

Cheers wvmarle, I am only going to be using one sensor in the setup, just multiple electrode probes in different places, using the the one sensor to read each probe individually at seperate times.

OK, one digital pin per "probe" then, plus whatever you need to read the "sensor".

Just looked up more details on that sensor - it's a soil moisture sensor - to prevent corrosion make sure you switch them off when not reading data. It's probably best to disconnect both wires, so two MOSFETS or transistors per probe. This prevents current leakage to the ground (the literal ground in this case). There exist very convenient SMD chips that contain both an N-channel and P-channel MOSFET.

wvmarle:
There exist very convenient SMD chips that contain both an N-channel and P-channel MOSFET.

Oh god no, lol, I haven't even thought of venturing into the SMD world yet....... Only just got a decent magnifying headset, let me see if I can pluck up the courage.......

Thanks for the advice bud.
Iain

You said "making my own PCB" so I automatically assumed SMD.

The smallest stuff for me so far has been SSOP8 packages (1.27 mm pitch - the back of hte breakout boards are for the VSSOP packages, that must be hard to do, maybe try to use a skillet for that) and 0603-size resistors. I'm really thinking I should be getting a proper soldering iron - still using a cheap 40W unregulated one for all this.

It's a bit daunting at first but not that hard, really. You do need fine pointed tweezers and a magnifying glass at the very least.