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I had a sensor who needs Ac in order to work.
What must I do in order to use AC with the arduino

thanks for your help
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You would need to provide a link to the sensor's datasheet or users manual for anyone to figure it out for you.

Lefty

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it is a home made sensor to calculate the onductivity of water
when you have a disolution of water and for example salt (NaCl), and you transmit to it DC, the water suffer a process called desionization, because of that i must use AC in order that water do not deionizat and with AC water can transmit electricity, that is what i need to make the sensor works.
I think  this is the best explanation i can tell you about this part of my proyect

thank you for your help
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Well then possibly a schematic drawing of the whole thing would help. Otherwise you aren't giving us much to work with.

Lefty
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I don't know what kind of sensor you have, but couldn't you do this more easily with a hall based current sensor?  If you hook a known voltage to one end of a hose full of the brine, ground to the other, you could run a current sensor into an analog pin of the Arduino.  With voltage and current know, of course, you could figure the conductance.

Might be an easier way.
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Simple answer: You need a full-wave rectifier, some capacitors, and a voltage regulator.

The arduino can only run on DC power.

If the sensor outputs an AC signal, I have no idea, sorry.
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Sounds like you're trying to do the same thing I just did, to check for the presence of water at a pair of electrodes. I used a 74HC14 Hex Schmidt trigger inverter to create a 1MHz AC signal that was on the output pin in the water. To receive that signal, I created something called an "Envelope Detector", which was simply a 1N4148 diode, a small cap with a 10mega ohm pulldown resistor to ground on the output of that diode (receive pin goes to one end of the diode, output is the striped end). Ran the output to an 2n2222 npn transistor, which when triggered by the "envelope detector" will pull the CPU pin low.
I followed these instructions for the Schmidt trigger oscillator: http://www.fairchildsemi.com/an/AN/AN-118.pdf
and these instructions for the Envelope detector: http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part9/page2.html

I used a pair of 2.2k resistors and a .001uf ceramic cap for the oscillator circuit, and a 1n4148, a .1uf ceramic cap and 10mega ohm resistor for the envelope detector circuit. It creates 1MHz AC..

credit: david in #SeattleRobotics.. smiley
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Brian from Tacoma, WA
Arduino evangelist - since Dec, 2010.

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