If you like growing plants at home, then you know that they are restricted in space and climate. There are some things that you can do and there are that you can’t. You cant control the temperature and light much especially in areas where you live or work. It can be put near the window and temperature is mostly a room temperature. But there is one parameter that can and must be kept in mind – moisture. It is vital to almost any plant to keep soil at some moisture level and this is what you cannot ensure properly unless you keep checking it constantly. So why bother when this work can be done by putting a simple circuit.
Frank Donald built a really simple setup that works on Atmega16 based circuit. Microcontroller here is to read soil moisture sensor that comes as cheap module which includes comparator. So it gives high signal when soil is dry and low when wet. But to make it a bit smarter, he is reading analog signal from sensor, so microcontroller could read this signal and according to its level control a relay to water pump. By setting some thresholds he made simple hysteresis to prevent relay from chattering.
In other hand similar result can be achieved much simpler – you can connect sensor digital output directly to relay driving transistor and have similar result.