I have a plethora of plants in my house. They all have different watering schedules, but like many people, I'm a creature of habit.
The plants that need watering daily or every other day are taken care of, but those that need water on a more varied schedule can often get overlooked. Chirp sets out to remedy this with a simple alarm.
The Chirp alarm is a small pcb that is inserted into the soil at its driest desired point. From that moment forward it monitors the soil and begins to sound an alarm when the soil reaches the preset dry point. It begins with a quiet chirp, getting more urgent over time. However, it is nice enough to silence itself at night by detecting light levels. The developers note that the battery, a single button cell, can last roughly a year depending on how often it chirps and the light sensors are used. If you keep your plants watered, the battery lasts longer.
One interesting aspect of the chirp is that it is uses capacitive humidity sensing. A capacitive sensor doesn't require physical contact so it can be coated to help prevent corrosion. If you would like to read details about how exactly the capacitive sensing works, the developer has written a nicely detailed article on it here. The rest of the hardware is pretty simple. The device uses a ATTiny44a with a standard AVR 6 pin ISP programming header, allowing for customizing your modifications. This ISP header is also there so that another device can be used to read the data, should a user want to incorporate the Chirp into a larger project.
This project is entirely open-source and even comes with downloadable code. A gallery of images is posted on the website, showing the various prototypes the designers iterated on before landing on final design. One amusing thing to point out: Chirp actually began as a joke. Some people say there is a grain of truth to the best jokes, so with that in mind, the Chirp was born.