I doubt this is a long time solution. I assume there is no material that can widthstand DC currents for a long time.The real solution is to use probes made from a more chemically resistant material, for example stainless steel, gold or even carbon.
Hi,
* 12V is not dangerous, as long it is not inside your body.
* Nobody said you should use 12V
* you just need to avoid DC.
* you could generate any AC by toggeling a microcontroller pin...then feed it through a series connected C to avoid DC.
...or any other method to generate DC free AC.
Klaus
This is electrolytic corrosion. Basically, the water is decomposing one electrode and moving material to the other. With DC, the process is continuous but when the polarity is continuously reversed the action works for half the time in each direction and to some degree reverses itself.
The real solution is to use probes made from a more chemically resistant material, for example stainless steel, gold or even carbon.
The cheap fix is to generate the AC from the MCU but it requires some circuit changes. Instead of using one power feed and one input for each water level, you connect the power feed to one of the MCU pins and drive it with a square wave. Then remove all the transistors and connect the MCU pins directly (or through a ~10K resistor) to the sensor probes. What you do is drive the feed probe high and read the inputs (as at the moment) then drive the feed probe low and reverse the sensor pin directions so they are outputs then drive them high. As you alternate this, the probes have AC across them differentially so the corrosion should stop. Protect the MCU inputs by wiring Schottky diodes (BAT85 etc) from each input to +5V and GND. If you want to add extra protection add a resistor of say 100K from each input to ground as well.
Brian.
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