The Arduino microcontroller is a popular open-source hardware platform used for building a variety of electronic projects. It's designed to be easy to use and versatile. A sensor is a device that detects and responds to some type of input from the physical environment. The input can be light, heat, motion, moisture, pressure or any number of other environmental phenomena.
I have done many kind of mini projects Such as
1.water level indicator
2.Automatic Fire Alarm
3.Autometic Vehicle counter system using 555 timer.