We will now build a very basic water detector circuit using a transistor.
We know that we can use a transistor as a switch and we also know that water tends to conduct electricity. Water can thus be used as a trigger to switch ON a transistor.
This is what the circuit would look like. An NPN transistor controls an LED and we have two jumper wires which will be used as probe points. One wire is connected to the base of the transistor by using a current limiting resistor and the other one is connected to the positive supply voltage e using a current limiting resistor as well.
Ideally, we would want to connect the two terminals to a mesh-like this which can increase its overall sensitivity. This is a PCB that is used as a touch sensor in BBox 2, but it can also be used to detect a conductive substance like water.
Let’s build this circuit and quickly test it using a water droplet.
As can be seen, the transistor switches ON when the two jumper wires come in contact with the water droplet. This is a very simple project which goes to show that you don’t have to design something very complicated for it to be useful.
One thing to keep in mind is that the main reason for R3 is to act as a current limiting resistor in case the jumper wire is accidentally connected directly to the negative terminal. The 10K value will limit the current to a maximum of 0.3mA which is a very low value. In the next post, we will extend this principle to build a water level indicator.