Elenco Circuit Maker Sound Plus 200 - Manual (Page 67)

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Project #171
Project #170 Water Detector
Saltwater Detector
Build the circuit at left and connect the two jumpers to it, but leave
the loose ends of the jumpers lying on the table initially. The LED
(D1) will be dark because the air separating the jumpers has very
high resistance. Touch the loose jumper ends to each other and
the LED will be bright, because with a direct connection there is
no resistance separating the jumpers.
Now take the loose ends of the jumpers and place them in a cup
of water, without letting them touch each other. The LED should
be dimly lit, indicating you have detected water!
For this experiment, your LED brightness may vary depending
upon your local water supply. Pure water (like distilled water) has
very high resistance, but drinking water has impurities mixed in
that increase electrical conduction.
Place the jumpers in a cup of water as in the preceding project;
the LED (D1) should be dimly lit. Slowly add salt to the water and
see how the LED brightness changes, mix it a little so it dissolves.
It will slowly become very bright as you add more salt. You can
use this bright LED condition as a saltwater detector! You can then
reduce the LED brightness by adding more water to dilute the salt.
Take another cup of water and try adding other household
substances like sugar to see if they increase the LED brightness
as the salt did.
CM-200_Manual_031914.qxp_CM-200_Manual_031914 4/2/14 12:05 PM Page 67