Following are most common kind of chemical sensors in use:

Chemical field-effect transistor

Chemiresistor

Electrochemical gas sensor

Fluorescent chloride sensor

Hydrogen sulfide sensor

Nondispersive infrared sensor

pH glass electrode

Potentiometric sensor

Zinc oxide nanorod sensor

Following are some common Gas sensors:

Carbon dioxide sensor

Breathalyzer

Carbon monoxide detector

Catalytic bead sensor

Hydrogen sensor

Air pollution sensor

Nitrogen oxide sensor

Oxygen sensor

Ozone monitor

Electrochemical gas sensor

Gas detector

Hygrometer

Common Type of Smoke Sensors

Smoke sensors detect the presence of Smoke, Gases and Flame surrounding their field. It can be detected either optically or by the physical process or by the use of both the methods.

Optical smoke sensor (Photoelectric): Optical smoke sensor used the light scatter principle trigger to occupants.

Ionization smoke sensor: Ionization smoke sensor works on the principle of ionization, kind of chemistry to detect molecules causing a trigger alarm.

IR sensors

An infrared sensor is a sensor which is used to sense certain characteristics of its surroundings by either emitting or detecting infrared radiation. It is also capable of measuring the heat being emitted by the objects.

Level sensors

A sensor which is used to determine the level or amount of fluids, liquids or other substances that flow in an open or closed system is called Level sensor.

There are two basic level measurement types:

Point level sensors: Point level sensors usually detect the particular specific level and respond to the user if the sensing object is above or below that level. It is integrated into single device to get an alarm or trigger

Continuous level Sensor: Continuous level sensors measure liquid or dry material levels within a specified range and provide outputs which continuously indicate the level.

Image sensors

Image sensors are instruments which are used to convert optical images into electronic signals for displaying or storing files electronically.

Two main types of sensors are used in:

CCD (charge-coupled device), and

CMOS (complementary metal-oxide semiconductor) imagers.

Motion detection sensors

A motion detector is an electronic device which is used to detect the physical movement (motion) in a given 

Following are key motion sensor types widely used:

Passive Infrared (PIR): It Detects body heat (infrared energy) and the most widely used motion sensor in home security systems.

Ultrasonic: Sends out pulses of ultrasonic waves and measures the reflection off a moving object by tracking the speed of sound waves.

Microwave: Sends out radio wave pulses and measures the reflection off a moving object. They cover a larger area than infrared & ultrasonic sensors, but they are vulnerable to electrical interference and more expensive.

Accelerometer sensors

Accelerometer is a transducer that is used to measure the physical or measurable acceleration experienced by an object due to inertial forces and converts the mechanical motion into an electrical output. It is defined as rate of change of velocity with respect to time

There are various kinds of accelerometers and following are few mainly used in IoT projects:

Hall-effect accelerometers: Hall-effect accelerometers are using Hall principle to measure the acceleration, it measures the voltage variations caused by changes in a magnetic field around them.

Capacitive accelerometers: Capacitive accelerometers sensing output voltage dependents on the distance between two planar surfaces. Capacitive accelerometers are also less prone to noise and variation with temperature.

Piezoelectric accelerometers: Piezoelectric sensing principle is working on the piezoelectric effect. Piezo-film based accelerometers are best used to measure vibration, shock, and pressure.

Gyroscope sensors

A sensor or device which is used to measure the angular rate or angular velocity is known as Gyro sensors, Angular velocity is simply defined as a measurement of speed of rotation around an axis. It is a device used primarily for navigation and measurement of angular and rotational velocity in 3-axis directions. The most important application is monitoring the orientation of an object.

Humidity sensors

Humidity is defined as the amount of water vapour in an atmosphere of air or other gases. The most commonly used terms are “Relative Humidity (RH)

Following are key type of optical sensors:

Photodetector: It uses light sensitive semiconductor materials like photocells,photodiodes or phototransistors to work as photodetector

Fiber Optics: Fibers optics carry no current, So its immune to electrical & electromagnetics interference and even in damaged condition no sparking or shock hazard happens.

Pyrometer: It estimates the temperature of an object by sensing the color of the light and Objects radiate light according to their temperature and produce same colors at same temperature.

Proximity & Infrared: Proximity use light to sense objects nearby and Infrared are used where visible light would be inconvenient.

Acoustic, sound, vibration

Geophone

Hydrophone

Lace Sensor a guitar pickup

Microphone

Seismometer

Sound locator

Automotive

Air flow meter

Air–fuel ratio meter

AFR sensor

Blind spot monitor

Crankshaft position sensor

Engine coolant temperature sensor

Hall effect sensor

Wheel speed sensor

Airbag sensors

Automatic transmission speed sensor

Brake fluid pressure sensor

Camshaft position sensor

Cylinder Head Temperature gauge

Crankshaft position sensor

Engine crankcase pressure sensor

Exhaust gas temperature sensor

Fuel level sensor

Fuel pressure sensor

Knock sensor

Light sensor

MAP sensor

Mass airflow sensor

Oil level sensor

Oil pressure sensor

Oxygen sensor (O2)

Parking sensor

Radar sensor

Speed sensor

Throttle position sensor

Tire pressure sensor

Torque sensor

Transmission fluid temperature sensor

Turbine speed sensor

Variable reluctance sensor

Vehicle speed sensor

Water-in-fuel sensor

Wheel speed sensor

ABS Sensors

Chemical

Breathalyzer

Carbon dioxide sensor

Carbon monoxide detector

Catalytic bead sensor

Chemical field-effect transistor

Chemiresistor

Electrochemical gas sensor

Electronic nose

Electrolyte–insulator–semiconductor sensor

Energy-dispersive X-ray spectroscopy

Fluorescent chloride sensors

Holographic sensor

Hydrocarbon dew point analyzer

Hydrogen sensor

Hydrogen sulfide sensor

Infrared point sensor

Ion-selective electrode

Nondispersive infrared sensor

Microwave chemistry sensor

Nitrogen oxide sensor

Olfactometer

Optode

Oxygen sensor

Ozone monitor

Pellistor

pH glass electrode

Potentiometric sensor

Redox electrode

Smoke detector

Zinc oxide nanorod sensor

Electric current, electric potential, magnetic, radio

Current sensor

Daly detector

Electroscope

Electron multiplier

Faraday cup

Galvanometer

Hall effect sensor

Hall probe

Magnetic anomaly detector

Magnetometer

Magnetoresistance

MEMS magnetic field sensor

Metal detector

Planar Hall sensor

Radio direction finder

Voltage detector