The main thing you need to do to keep your plants alive is to water them, and while there are some DIY solutions out there that remind you to water, sometimes you need a more human approach. So, Instructables user dr.mcc built a moisture sensor that alerts you when you need to water a plant by using an emoticon.
Using an Arduino, a moisture sensor, an LED matrix, and a few other parts, dr.mcc’s build monitors the moisture level in a pot of soil and then lets you know when the plant needs water by displaying a sad face on the LED matrix. Reminding yourself to water your plants is never really that fun, but this way adds that extra special little bit of guilt to help you really remember.
More Human Moisture sensor | Instructables