This alarm is activated when the pin attached to the keychain is pulled. So, if you find yourself fleeing from an assailant all you have to do is grab your keys and yank the device off of your keychain. However, you can also activate the alarm by pressing a button on top of the alarm. The built in flashlight works without the alarm as well, and is perfect for lighting up anything in the dark. The dimensions are: 2 3/4" x 1 3/4" x 1" and 1 12 volt battery is included.