A sensor detects (senses) changes in the ambient conditions or in the state of another device or a System & forwards or processes this information in certain manner.
A sensor which detects or measures a physical property and records indicates or otherwise responds to it : Oxford dictionary
Types of Sensor’s:
Sensor classification schemes range from very simple to the complex. Depending on the classification purpose, different classification criteria may be selected. Here are several practical ways to look at sensors.
All sensors may be of two kinds: passive and active. A passive sensor does not need any additional energy source. It generates an electric signal in response to an external stimulus. The active sensors require external power for their operation, which is called an excitation signal. That signal is modified (modulated) by the sensor to produce the output signal.
Depending on the selected reference, sensors can be classified into absolute and relative. An absolute sensor detects a stimulus in reference to an absolute physical scale that is independent on the measurement conditions, whereas a relative sensor produces a signal that relates to some special case. An example of an absolute sensor is a thermistor—a temperature-sensitive resistor. Its electrical resistance directly relates to the absolute temperature scale of Kelvin. Another very popular temperature sensor—a thermocouple—is a relative sensor.
Sensor’s Sensing Element’s:
Wireless sensor networks are composed of individual embedded systems that are capable of (1) interacting with their environment through various sensors, (2) processing information locally, and (3) communicating this information wirelessly with their neighbors. A sensor node typically consists of three components and can be either an individual board or embedded into a single system.
Wireless modules are the key components of the sensor network as they possess the communication capabilities and the programmable memory where the application code resides. A wireless modules usually consists of a microcontroller, transceiver, power source, memory unit, and may contain a few sensors.
A sensor board is mounted on the wireless modules and is embedded with multiple types of sensors. The sensor board may also include a prototyping area, which is used to connect additional custom made sensors.
A programming board, also known as the gateway board, provides multiple interfaces including Ethernet, Wi-Fi, USB, or serial ports for connecting different motes to an enterprise or industrial network or locally to a PC/laptop. These boards are used either to program the wireless modules or gather data from them.
Sensor Network Protocol Stack:
The physical layer is responsible for frequency selection, carrier frequency generation, signal detection, modulation, and data encryption. Frequency generation and signal detection have more to do with the underlying hardware and transceiver design and hence are beyond the scope of our book. More specifically, we focus on signal propagation effects, power efficiency, and modulation schemes for sensor networks.
The data link layer is responsible for the multiplexing of data streams, data frame detection, and medium access and error control. It ensures reliable point-to-point and point-to-multipoint connections in a communication network. More specifically, we discuss the medium access and error control strategies for sensor networks.
Sensor nodes are scattered densely in a field either close to or inside the phenomenon The information collected relating to the phenomenon should be transmitted to the sink, which may be located far from the sensor field. However, the limited communication range of the sensor nodes prevents direct communication between each sensor node and the sink node. This requires efficient multi-hop wireless routing protocols between the sensor nodes and the sink node using intermediate sensor nodes as relays. The existing routing techniques, which have been developed for wireless ad hoc networks, do not usually fit the requirements of the sensor networks. The networking layer of sensor networks is usually designed according to the following principles:
•Power efficiency is always an important consideration.
• Sensor networks are mostly data-centric.
• In addition to routing, relay nodes can aggregate the data from multiple neighbors through local processing.
• Due to the large number of nodes in a WSN, unique IDs for each node may not be provided and the nodes may need to be addressed based on their data or location.
IOT Main & Sub Application list with Sensor’s details:
Reference: Handbook of Modern Sensors Physics, Design & Application By Jacob Fraden, Wireless sensor networks by Ian F. Akyildiz, Georgia Institute of Technology, USA, libelium.com