Building a Tubidity meter

[Gustavohbo]

Well, actually I didn't mean anything that fancy. I just meant a 1/2" diameter glass beaker with various percentage'
mixtures of some powder that blocks light so you can vary the light intensity by varying the mixture percentage.
I'm not a chemist or artist so I wouldn't have a clue what powder particles would work best. It only needs to be 1"
high so size is not an issue. Most of the work would be experimenting to find a suitable powder.

I already did some test with wheat flour, coffee, dirt and paint. But it is very unstable, I could realize the same test with the same sample with another prototype or even the same prototype but a few days later and have a different output, maybe the powder decanted or reacted with something in the water... there are many variables.
I can not repeat the test with another prototype and expect the same output because I can not trust that the sample is the same that it was in the other protype/test.
But for simple tests, as just know if the sensor is working as expected coffee and flour are great.

[raschemmel]

I meant powder:water ratio
or milk:water
or powder:some other clear liquid

[johnerrington]

Ok, but why? what does the result of this subtraction mean?

COme on Gustav, its the difference between the photosensor with led on & LED off - so its measuring the light coming from the LED.

You have a pint of beer. You need to know how much you have drunk.
Amount drunk =amount originally - amount left.

You NEED to exclude outside light from your system. Thats why your "dark" reading is high as compared to this

When the LED is ON the output is about 1000 and when it is OFF the output is about 30.
I noticed that if I cover the phototransitor with my hand while the LED is OFF the readings decrease more, to about 3 a few times even 0.

[Gustavohbo]

You NEED to exclude outside light from your system. Thats why your "dark" reading is high as compared to this

I fixed this issue. Now the LED and the Phototransitor are inside a dark container that day light can not reach.
And I weld some wire to the LED and phototransistor to wire it to the breadboard.
Now the output is very stable:
ON: 989   OFF: 0   Subtraction: 989
ON: 989   OFF: 0   Subtraction: 989
ON: 989   OFF: 0   Subtraction: 989

The output from other phototransitor that is exposed to day light at the same time is:
ON: 973    OFF: 13
ON: 973    OFF: 13
ON: 973    OFF: 13

[johnerrington]

Thats great. The subtraction is to exclude lots of other effects - stray light, temperature changes etc.
Time now to try it with your sample tube.