Electronics - Water Level Sensing Circuit

Why do we need it?
If the entire filter system was built without the circuit, the water circulation pump would keep on going until either the battery ran out of power or the pump broke. There needs to be a way to control how much water should be running through the filtering/piping system. That is what the water-sensing circuit is for.

General Operation:
To control when to pump in water through the filter, an Arduino was used to detect if water was in the system. If there was water in the system, the motor would be pumping water through. If somehow there was no more water left to be pumped, the Arduino would be able to detect this, and shut off the motor until there is some more water in the system.

Setup and Operation:
The sensor that was used to give input to the Arduino about if there was water or not is in this circuit below:

There is an infrared LED (L1) shining on a phototransistor (Q1). Physically, the transparent tubing would be placed in between the infrared LED and phototransistor, and there would be a certain amount of infrared light reaching the phototransistor. In general, a phototransistor produces current proportional to the amount of light shining on it. In this case, the phototransistor that was chosen was more sensitive to the infrared spectrum, so it will produce more current when there is more infrared light reaching it and less current when less infrared light is shining on it.

The transparent tubing will either have water running through it or not. When it has some water going through the tube, the water absorbs more of the infrared light between the LED and the phototransistor, so the reading on the voltage should be much lower than when there is no water going through the tubing.

The Arduino Fio is connected at 1 of the outputs of the phototransistor to read the voltage at that point. The Arduino Fio is programmed to cut power to the water circulation pump if there is no water in the system. A voltage threshold is set within the program to determine what voltage read at the phototransistor output is considered to have water or no water in the system.

To ensure that the circuit is protected from the water, the entire circuit was housed in Corning pipette box to contain it from water. Holes were drilled to move the infrared LED and phototransistor through the box and to connect the motor with the circuit.