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Topic: Problem interfacing LM555 to arduino  (Read 418 times) previous topic - next topic

nathan_22

hello,

I'm looking for appropriate sensor that can fit to my project. my project is to measure the sugar content in water. before, i used the concept of conductivity but every time I test in high sugar  content it get errors from my oscilloscope. I used LM555 to oscillate the voltage in astable mode.  I put probes between discharge pin and threshold control of 555.


thanks in advance for your suggestions it would be a great help to my project


here's my circuit

allanhurst

#1
Sep 06, 2017, 01:22 am Last Edit: Sep 06, 2017, 03:26 am by allanhurst
Sugar content is normally measured optically with a refractometer.

Allan

edit : and pure sugar in pure water is an almost perfect insulator - you'd just be measuring the  (unknown) concentration of  polar molecules.

My profile doesn't include a first degree in chemistry.

nedit.

nathan_22

thanks Allan, but the thing is this I want to reverse engineer the concept of conductivity, instead of voltage I want to measure Resistance. since sugar is non-conductive solution. Is there any way to measure its resistance using the circuit I made?

thanks in advance....

allanhurst

#3
Sep 11, 2017, 09:40 am Last Edit: Sep 11, 2017, 09:51 am by allanhurst
Your circuit shows a bog-standard 555 astable oscillator...  it'll give you an asymmetrical pulse train at some frequency.

I dare say you could use it to drive one of the plates of a  platinum plate conductivity cell and sense the other with a precision rectifier ( 3 opamps) to drive the a/d of an arduino - this would give you a sum of the dielectric constant and conductivity of your solution.

Your idea of using your electrodes between discharge and threshold would not work with a 'perfect' insulator - no current would flow. And would not alter the frequency anyway if a finite resistor was provided.

I've no idea of how the dielectric constant of water varies with sugar in it - perhaps you could look it up - somebody may have done it. You could calibrate with pure water and solutions of known sugar concentration.

The tiniest amout of polar contaminate would ruin your measurement unless you used a much higher frequency than a 555 can provide. Soil moisture meters use 100MHz for this reason.

I think it's a daft idea.

What's wrong with a refractometer? Cheap and  accurate.

Or weigh before and after evaporating a sample to dryness.....

Allan

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