Like stand alone motion detectors, these can activate when you’re away from home or whenever you choose. For example, you may keep the window sensors always activated but deactivate the door sensors when you’re at home. These sensors can function as part of your security system or on their own, and they vary in size and design from tiny, sleek sensors to bulky and obvious. Sensors also vary in sensitivity. Some may trigger when the door or window vibrates versus actually opening. A carbon monoxide detector triggers when the level of carbon monoxide in your home reaches a dangerous level.