opAmp switching?

Hi Guys, forgive me if this is a stupid question but circuit design has never been my strong suite... I am wanting to get a nice setup on a mega to monitor my hydroponics setup, including pH, TDS, temp and maybe DO sensors. That's all well and good, but I need four or more zones, each zone with its own complete set of sensors. So all that said, I would love to get away without having to build 8 opAmp circuits to read the sensors.

My question is this, would it be feasible to front end the amplifier circuits with a set of transistors that I can trigger from digital i/o pins to basically switch the input to the amplifier to whatever probe I want to take a reading on? Am I just dreaming, or could that actually work?

Any advice or comments would be greatly appreciated!

I suppose it comes down to having sensors that don't need complicated driver circuits. If you do need to use a driver circuit, they're unlikely to be difficult of expensive to manufacture, so once you have got one working you should find it easy enough to produce another three copies.

Temperature sensors, for example, should be easy to interface directly to the Arduino. I'm using DS18B20 digital thermometers and they just connect straight up to a one-wire interface so you can piggy back as many of them as you like just using one pin on the Arduino. I have no idea what TDS and DO are, but if you can measure them with a sensor that has an analogue or digital output they should connect easily enough. So I think there's a good chance you won't actually need to use op amps or any other fancy driver circuits.

What you will need drivers for, is another that you want to have controlled by the Arduino. So if you want it to turn pumps or lights on and off you'll need to get a driver shield, or make up your own driver circuit.

Hi Peter, first off the TDS and DO are total dissolved solids (measures the nutrient mix strength) and dissolved oxygen respectively. Unfortunately for the TDS, PH and DO sensors, the current that needs to be measured is WAY to small for the arduino to pickup by it self. the driver circuits are there to "up-convert" it to something usable by the controller.

Here you can see the circuit for the TDS sensor http://blea.ch/wiki/index.php/PPM_Meter And here for the PH sensor http://blea.ch/wiki/index.php/PH_Meter

and yes, I could just build 4 sets of the amps, but at least the TDS is not exactly small... and 8 external amps just puts a bad taste in my mouth...

I've only looked into this very briefly for humidity monitoring (not hydroponics) and I saw a couple of sensors that were fairly cheap. One worked by measuring conductivity, the other worked by some sort of ion sensing I think. Both sensors had little driver circuits built into the module and presented the signal as an analoge voltage - absolute doddle to read with an Arduino. I suggest you have a look around for TDS and DO sensors that do this too. I would think that anyone using these type of sensors would want to read them digitally so this is probably a very common requirement and I'd be surprised if it hadn't already been addressed.

HAHA, well the funny part is that the opamp circuits on those links are the solution to get it running on a microcontroller...

Depending on how long the cable runs are, you may be better off amplifing/digitizing the output for communications over longer runs. The reason is because the signals are so weak and at >10Mohm even the slightest stray current could drive the reading off by a significant amount. (assuming a probe of 100M? (*1nA)=.1v or almost 2 ph units) Also if you choose a decent amp they can be pretty cheap, sure a cd4066 is dirt cheap, but so is an stTL072. You have a few options for amps and configurations, from the design you listed to even just a single amp in unity mode, or a multi stage-multi amp design with a high quality gain stage followed by a buffer, then an offset, then buffer.

pH interface tutorial http://www.sparkyswidgets.com/Projects/pHInterface.aspx We built upon those tutorials(you posted and a few others) a bit, adding a bit of background and a few designs. It is still a work in progress but hey its another source of information after all hehe. Hoping to finish some videos we have in the pipeline shortly for this and a few other widgets(tds, co2, temp and humidty, etc..) We also have a nice little pH unit that has an atmega328 onboard, you can use the design and modify it to your needs. We also carry a few built and bare in stock if anyone is interested in that avenue.