A device, such as a wearable device, can include one or more light emitters and one or more light sensors. The one or more light emitters and one or more light sensors can be used to detect signals. For example, a photoplethysmogram (PPG) signal can be obtained by measuring the perfusion of blood within the skin of a user. The signals detected by the one or more light sensors can be used to detect whether the device is properly secured to a user's skin.