ICStation team has built this DIY temperature & humidity & smoke alarm system based on Mega 2560.
The working voltage of this system is DC5V. It can measure the current temperature, humidity and smoke. It can display real-time data by the 1602 LCD and can realize the sound and light alarm when in the dangerous temperature and humidity. It is a simply and easily to operate monitoring alarm system about temperature humidity.
Head over to Instructables for the full guide: http://www.instructables.com/id/DIY-Temperature-Humidity-Smoke-Detector-Alarm-Syst/