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# conductivity tester LM3914

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#### geologie

##### New Member
Hi! I'm a CHEM teacher that want to built a LED bargraph conductivity tester. I want to use the LM3914 driver for this task. The tester will be use for ionic solution conductivity appreciation. I don't want metered tester nor those with only one LED that say yes or no. The 10 LED will give me a semi quantitative feedback on solution conductivity that is all i need for 15 years old students. I read about the LM3914 but i'm not an electronic designer but i will be able to built the tester i only someone can design me the schematic!

Thanks

Michel

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What is the conductivity range you wish to measure?

Zero to infinite conductivity. Like a sugar solution doesn't conduct and a satured salt solution is very low in resistance. Is is possible with this wide range in conductivity?

Michel

I made a conductivity measurement circuit for my former employer. We used carbon electrodes, and just set a threshold and used the output to activate a water valve. There was no display, but what I wanted to say was that we had to use an AC signal to get an accurate measurement, as the measured conductivity would diminish over a shot time period with DC, as the ions plated out.

Just a hint....

The LM3914 has a range of measurement with 10% between its 10 steps.
So it has a huge step between no conductivity and the first step which is 10% conductivity.
It has very little difference between its last step which is 100% conductivity and its 9th step which is 90% conductivity.

You probably would want to use the LM3915 which has logarithmic steps rather than the linear steps of the LM3914 since that would give you more resolution at the lower resistance levels where you likely want it.

You will need an amp to detect the resistance and convert it to the voltage required by the LM3915. Zero to infinity is a nice large range but hard to design to. I really need an idea of what the minimum to maximum resistances you want to detect are i.e what resistance causes the first LED to light and what resistance would cause the last LED to light.

How about changing to audible conductivity test? You only need a timer IC (NE555) wired as astable.

The output signal will be in the range of 1Hz (no conductance) to high pitch sound (5KHz +) at short circuit.

I made hundreds of those circuits for schools in Germany.

Most important is the so called "Aha-effect" which is very distinctive using an audible tester.

Boncuk

Hi Michel,

just in case you go along with my idea here is a circuit for it. R3 is shorted by the probes dipped in a liquid. The higher the conductivity the higher is the pitch of the sound produced.

The PCB design is single sided with extra wide traces, rugged enough for school service. Board size is 1.6250X1.1inches.

PM me if you are interested. I'll send you the Eagle files via email.

Boncuk

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