What is it?

This is a capacitive soil moisture sensor with a green and red LED. When a houseplant's soil is sufficiently wet, the green LED will flash at a rate of about 1Hz. However, when the soil gets too dry, the red LED will start flashing instead, indicating that the plant needs to be watered.

Why did you make it?

I made this plant because I have the memory of a goldfish and repeatedly forgot to water several houseplants in a timely manner. Other soil moisture sensors on the market were either too bulky, didn't give precise/accurate enough moisture readings, or had poor battery life.

What makes it special?

This soil moisture meter has a battery life that is far longer than other electronic meters I've seen available (at least 5 years on a single CR2032 coin cell). Additionally, it features a potentiometer that you can turn with a screwdriver to calibrate the sensor so that it goes from green to red exactly at the soil moisture level you want for a specific plant.

For example, if you have a thirsty plant that needs to be watered every 4 days, simply insert the sensor, water the plant and after four days adjust the potentiometer so that the sensor is just barely flashing red. Now the sensor will start flashing red if the plant goes more than 4 days without watering!

Lastly, the final thing that makes this circuit unique is it's relatively small size and simplicity. Only about 5 cm of the sensor actually extends above the soil and the sensor is very narrow (only about as wide as a coin cell battery at the widest). The indicator on the sensor also couldn't be simpler, if its flashing red it needs to be watered, if its flashing green it doesn't!