posted by simon
Ok so here is a bit of info on the salinity circuit as used in the Salinity Sampler Sequencer.
It function is pretty simple. It measures the saltiness of water by putting a signal through it. The more salty the water, the stronger the signal.
Its simple but the complicated bit is doing this in a way that does not destroy your probes, and in a way that gives you a nice smooth voltage output that can be put into a microcontroller.
The starting point for this circuit was the following article. There used to be a discussion thread here too but it seems to have been spammed to death.
This is a fantastic start, but unfortunately we could not get the whole thing to work. So we ended up changing the second part of the circuit to have a more standard AC / DC converter. And for our purposes it seems to work. You may want to play with it a bit if you are using it to keep fish alive, but here is how it works.
The circuit is based around a Quad OpAmp TL074 IC, This has 4 opamps in it.
The first stage of the OpAmp forms a Wien bridge oscillator to produces a sine wave that should looks something like this.
The second the second OpAmp forms part of gain loop. This part of the circuit is described well in the link above.
The probe makes up one leg of the voltage divider across the op-amps's gain loop. So long as there is no conductance between the probes, the op-amp's output will equal whatever is on the + input pin 5. As conductance increases the amount of negative feedback (which is what determines op-amp gain) decreases because some of the signal is being sinked to ground via the probe. Less feedback means more gain and more amplification.
So if you put the wires in and out of a salty solution you should see the wave form go up and down in amplitude.
OpAmps 3 and 4 were used to rectify the AC signal into a smooth DC signal that can be plugged straight into a microcontroller such as an Arduino.
So out of OpAmp 4 you should have a nice flat line that goes up and down like this depening on the saltiness of the water. You then need to play about with the gain loop resistor value and probes to make something that is callibrated to your needs but it seems to work pretty well. Again this is described well in the link above.
There is also another op amp in place to invert the signal but I never used it.
Have fun with it. Here are the the eagle files and some photos of what you should see on an osciloscope.