Building a Conductivity Tester with LED Indicators for Chemistry Class

[nemrehs1]

Okay...in my engineering class we have to build an invention. My partner and I are building a conductivity tester. We are using three levels of LED indicators, to tell how conductive the liquid is. I don't want an LED to light up when it isn't supposed to, so I am trying to use an Op Amp voltage level detector, so the voltage has to reach a certain point for the LED to light up. But the problem is, the different levels of salinity are so small there is no visual difference in the LED. I need to redesign a circuit that indicates the small differences in the levels of salinity. These circuits are being built for a chemistry class, so the circuit has to be easy to handle. Any ideas?

 

[schip666!]

You need to figure out the actual range of conductance in which you are interested, then null out anything below that range and amplify the rest. Some one else on this list is asking about wheatstone bridges and strain gauges, and your application is very similar. Maybe you can help each other...