Combining theory and practice to solve a common problem: a simple circuit for indoor plants watering

We introduce here an activity for high school students to get them more acquainted with simple electronics circuits. The aim is to build a simple circuit that lights up a LED when a plant needs watering. The only prerequisites are the knowledge of Ohm's law and Kirchhoff's laws. The circuit that we designed is extremely simple, so students can build it during one regular class period (60 min). The soil is treated as a variable resistor in series with two regular resistors. The reference value is obtained using a potentiometer. The two branches of the circuit are connected in parallel. This approach can emphasize the understanding of the cited laws and of the difference between series and parallel circuits.