## 4 Sensors in Smart Home
### Nonwearable Sensors  
Nonwearable sensors are usually installed in fixed objects or locations of a house or a room and can detect occupants and their movements. Nonwearable sensors have the capability to identify the usage of objects, measure water flow, room temperature, or door openings/closings. Table 5 is a summary of different types of nonwearable sensors which were used for monitoring the occupants' daily activities. * Infrared (IR)--IR sensors are widely used in most of activities of daily living (ADL) classification studies and projects [55; 56; 57; 58; 59; 60]. They are employed to find out the occupants' presence, motion detection in particular areas, and finding the occupants' location within the home. [PERSON] et al. [61] deployed a passive IR (PIR) sensor to detect the usage of stoves and ovens. * Ultrasonic sensors--Based on their capability for distance measurement, these sensors are usually used for occupant localization and presence detection. In some studies like [62; 63; 64], a combination of ultrasonic sensors together with other sensors were employed for monitoring the occupants daily behaviour. In other studies, ultrasonic sensors were used to obtain precise pacing trajectories and to be able to recognize the ones that were abnormal [65; 66]. * Photoelectric sensors--These sensors have the capability to detect a light. When the intensity of lighting becomes greater or less than a threshold value, the device would be triggered and generate a signal. This type of sensor is not widely used; however, in some projects, they are used as a presence detection sensor [46; 67; 68]. * Video-based sensors--In some monitoring approaches a camera is mounted in a particular location of a house for movement and activity detection. It should be noted that the performance of \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sensor** & **Detected Parameter** & **Usage Example** \\ \hline Passive/Active IR & Motion & Occupant presence detection \\ \hline Ultrasonic & Motion & Occupant presence detection \\ \hline Photoelectric & Motion & Occupant presence detection \\ \hline Video/Thermal & Activity & Occupant presence detection \\ \hline Vibration & Vibration & Occupant presence detection \\ \hline Pressure & Pressure & Fall detection \\ \hline Magnetic switches & Door opening/Closing & Cupboard opening \\ \hline RFID & Object-interaction & Watching TV \\ \hline Audio & Activity & Showering \\ \hline Wattmeter & Usage information & Electric kettle usage \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Nonwearable sensors used for activity classification. video-based sensors can be affected in an environment with low lights [69]. However, a video camera-based approach can violate the privacy of the residents. In this regard, low-resolution thermal sensors have been proposed to be used instead of a video camera to mitigate the privacy concerns [70; 71]. * Vibration sensors--These sensors have the capability to detect a person falling [61; 72], identifying interaction with various objects [73], flushing toilets, or detecting water flows [74; 75]. * Pressure sensors--These sensors are mostly used for steps and fall detection. They are usually deployed in the floor of the monitoring space [46; 61]. [PERSON] et al. [76] installed pressure sensors not only in floors but also in furniture to find useful activity information. * Magnetic switches--They are usually employed to find whether doors or cupboards are opened or closed. These sensors have the ability to provide information on users entering specific rooms and opening dressers, refrigerators, or trash cans [33]. * Audio sensors--They are employed to detect sounds in houses and distinguish various types of sounds. In [63; 77] microphones were installed for sounds classification and identifying speech, phone ringing, dish clanging. [PERSON] et al. [78] installed a series of acoustic sensors to detect a person falling.