There are a multitude of sensors currently available for robotics, and in addition to that, new sensors are always in development and the ones that are already available are always being improved. Here’s a list of the most common sensors with some additional information about each one:

Switches:

There’s a huge variety of switches available for robotics application, such as:

Push buttons – Used to turn on / off devices, select options or control a programmed robot.

Limit switches – Used as a safety switch to tell a machine that a device on a machine has reached the limit on a track or shaft

Shaft encoder switches – These are pressed down every time a shaft makes one revolution.

Contact / bumper switches. – These are useful as an obstacle or physical object sensor for robots

These are all classified as passive sensors (meaning they are a type of detector and measure a physical property of the environment), and they are simplest passive sensors available (meaning they don’t require a lot of processing capability). Switches operate by allowing voltage to flow through them when they are closed, and preventing voltage from passing if they are open. They are usually normally open (N/O) but they can also be normally closed and they typically have a spring that would allow them to return to their “Normal” state once they are no longer being pressed by a physical object.

Light sensors:

These are used to detect light and have a wide range of uses. This doesn’t just include visible light either. They can also be tuned to detect infrared, ultraviolet, specific wavelengths of visible light, polarized light and even detect the intensity of the light. They can be either passive sensors or active (this means that they consist of both an emitter and a detector and they operate by detecting the changes that occur from the emitted signal interacting with the physical environment). They are usually referred to as photocells. Photocells operate by changing their resistance based on how much light is present; low light increases resistance and if there is lots of light the resistance decreases. Because of the properties of light, some light sensors, such as laser sensors can be useful at long distances. There are even Infrared thermometers which can be used to measure the temperature based off of the infrared light received. Depending on the type of light received, in astronomy they can be even used to determine the types of elements a star or planet is made of. Some other useful features of photocells are:

Reflectance optosensors: These active sensors typically use an LED or other light source to emit light and the reflectance optosenser senses the light that is received after it bounces back off of physical objects. These sensors usually require calibration for proper operation.

Breakbeam optosensor: These active sensors are similar to reflectance sensors except that they rely on a physical object, such as a spinning motor shaft, to break the beam of light between the emitter and detector to measure things like revolutions per minute of the shaft.

Laser range sensors: These are active sensors and operate similar to the reflectance sensors noted above but they are somewhat special so I thought they were worth a separate mention. They can be very useful for long ranges (to measure distance) or wide ranges (useful to model the physical structure of land or sculptures etc.). These sensors are expensive, big and require lots of power but provide the best resolution and are not as susceptible to specular reflection.

Cameras – These are a type of light sensor as well and are the most complex sensor available. This means that they require the most processing capability and use lots of power to do so. They have many uses, such as tracking (color blob tracking or movement), recording, security etc. Two cameras can be used in a stereo setup to even determine distance to an object, similar to how our eyes work in humans and other carnivore animals to increase hand-eye coordination (apparently most herbivores have eyes on the sides of their heads to scan more of the horizon in search of predators, so their vision operates a bit differently).

Sound sensors:

These sensors detect sound waves and are either passive, such as a microphone, where it is relying on sound waves from the physical world (such as a humans voice or a musical instrument), or active like an Ultrasound device, which produces a frequency on an emitter in a range higher then human hearing is capable of (usually between 40 to 60kHz, humans can typically hear between 2 and 20kHz), and if the sound wave is bounced back in the acceptable range of the sensor, the detector receives the signal back in order to measure distance. Other names for these sensors are Sonar, and Ultrasonic sensors. They are used to convert a sound wave into a digital signal for recording or to just amplify that signal to make it louder. Ultrasound can be used for imaging as well, for example ultrasound can be used to provide real time images of a baby in a mother's womb, or it can be used to create a topographical map of the sea floor. Another use (which only really requires the emitter) is to deter animals from a certain area by emitting a soundwave at a frequency that is unpleasant to that particular creature. Ultrasound sensors usually require lots of power to produce the chirps or pulses needed for operation. They are relatively cheap and widely available, but they do suffer from the negative effects of specular reflection depending on the properties of the surfaces they are bouncing signals off of.

Temperature sensors:

There are a several types of temperature sensors available for robotics. The most common ones are semi-conductor based temp sensors (transistors), Thermocouples and Thermistors. Usually by using means of detecting the resistance or voltage of the sensor, you can use a lookup table or electronic device capable of measuring these changes in these values. That corresponds to changes in the sensors properties when exposed to different temperatures, which is now known and understood all thanks to the scientific research from the past. These sensors are passive sensors are rely on changes in their surrounding environment. There are infrared sensors available, that are technically a light sensor, but this invisible light can be used as well to measure temperature.

Other sensors:

There are a wide variety of other, less common sensors available out there. Some of these are: accelerometers, gyroscopes, magnetic sensors, torque sensors and resistive position sensors (Bend sensors). Other common sensor types that I’m not going to go into much detail on are: current, voltage, and power sensors and even potentiometers.