JLB

Audioguru, Thanks again. If you mixed a level teaspoon of salt ( about 5 grams) in 19 oz ( 454 grams ) of water you had some 11,000 ppm solution. Your probe is simmilar to the one I use, so your info is very interesting. I will see if I can overcome the alinearity .......... or live with it. Best regards, JLB.

Sorry, I meant to type CD74HC132. By the way, do you think that wit the suggested modifications the circuit will show a linear response respect to the media conductivity? I plan to measure the conductivity of salts dissolved in water from 0 parts per million to 3000 parts per million. This is equivalent ( roughly) to a conductivity range of 0 mSiemens/cm to 4 mSiemens/cm. Will your changes provide a linear response to increased conductivity? ( Twice salt concentration = twice conductivity = twice output from the circuit ??? ) I plan to rectify the output signal to the LED.

Hi there Audioguru: I was a little bit shocked by the level of controversy that my simple question started. As a matter of fact I started to look into other circuits as this one ( as it was) had some additional problems like the 10% mark to space ratio that polarized the probe, and the fact that I need a linear output as the media conductivity changes. But with the changes you suggest, including the use of the CD74BC132, I will give it a second try. Your help has been great !!!! Don't be surprised if I request your help once again in order to finish my project ( sorry ! ) Best regards, JLB.

I have built the Plant Watering Watcher circuit presented under Projects, "science related" circuits and works to a certain extent. Problem is that I find that the LED has only two states ( ON or OFF ), it does not dims gradually as the description states. As a matter of fact I think that the circuit, as it is, will only put the LED ON or OFF, as mine does. I am mistaken ? It really should dim gradually ? If so, I will really appreciate if you can explain why. I plan to use it as a conductivity tester for salts disolved in water , in order to measure conductivity in micro Siemens, but not just High or Low. but continuously. Helpwill be appreciated. JLB.