Here is a handy little science and technology hobby, it is an alert system for plants. Simply build it up and install, when the plant is dry the LED will flash and that is the sign for when you must provide it with a bit of water, until the next time.
This is how the kit works and how long it takes build;
The Thirsty Plant kit takes about 1 hour to make. It is important that you download the handy how–to manual from the Technology Will Save Us website as it has step by step instructions and helpful tips. You will cast your own moisture sensor with plaster of paris, and use this to detect the state of your soil. When your plant is dry or thirsty, a flashing LED is triggered to remind you to water it!
This cool kit is going to cost around $35 (£23) plus a few tools and other bits and pieces.
Source: Technology Will Save Us