By Steve Schuler, Science 2.0
In my previous article, you learned how to build a hot liquid level indicator for the vision impaired to help them fill cups with hot liquids such as tea or coffee. The 555 test circuit was used to generate a tone when the liquid in the cup reached the desired level--when the liquid level reached the red and black snaps of the Jumper Wires inside the cup.
The liquid (my coffee) conducts electricity because tapwater contains small amounts of contaminants. Some of the contaminants include anions (these have a negative charge) such as chloride (250 ppm), nitrate (10 ppm), and sulphate (250 ppm) as well as cations (these have a positive charge) such as iron (0.3 ppm) and aluminum (0.2 ppm).
My coffee, as a conductor, acted like a switch to close the circuit somewhat like a single pole single throw (SPST) switch. You can think of it like the Slide Switch (S1). An SPST switch is like a light switch, when you switch it on it will stay on until you switch it off. Similarly the 555 test circuit will continue to play the tone until you remove the snaps from the coffee (or turn the Slide Switch S1 off to cut the power from the batteries).