Measures 3" x 1/2" x 1 1/2" overall. About the size of a bluetooth earpiece and lightweight, less than an ounce, the Alertme is the latest and safest lifesaver electronic device that helps prevent accidents caused by falling asleep behind the wheel. Its ergonomic design makes it very comfortable to wear and improves your vigilance on the road. The device works by detecting when the user's head nods while driving or working, indicating that they may be drowsy. Alarm mode: sound alert with a 15/20 angle. The sound alert last 30,000 nods in total. Mercury-Free product. Operates on 3 button cell (LR41) batteries (included).