On Thu, Sep 11, 2014, at 08:56 AM, Ravi wrote:

> Metal polyester. At low frequency, the waveform is 50% duty cycle. At=20
> higher freq the duty cycle changes. This also could be one of the=20
> issues. I will investigate more on this.

Hi Ravi,

I've designed a lot of circuits that use electrolytic sensors. Your
situation seems to have a lot of parallels. A small amount of DC bias is
catastrophically bad. But also having a duty cycle that isn't 50% is
also bad, even with capacitive coupling. Although the capacitor removes
any DC bias, the chemistry of the sensors is not like a capacitor, and
the sensor is still ruined. It just takes longer.

You can make a 555 timer 50% duty (or very close) by using the output
pin instead of the discharge pin. It uses one less resistor.
Simply attach your capacitor from ground to pins 2 and 6. Attach your
sensor from pin 3(output) to pins 2 and 6.

The duty cycle will be very close to 50%. You can trim it with a high
value trimpot(100K to 1 megohm), wiper to pin 5 and ends to V+ and
ground.

This circuit isn't perfect but you already have the parts.

You can take the output from pin 3, or if you don't want to load it
anymore, use pin 7 with a pullup resistor.

Unfortunately one thing I didn't think about is if your circuit has a
true ground reference and if your water also has a ground path.

If It does it might be a good idea to make a better circuit to begin
with.

Cheerful regards,
Bob


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