If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content


It's estimated that there will be 35 billion devices connected to the Internet by 2021. The majority of those devices will be small sensors that are sending out streams of information about what they're sensing, such as temperature, moisture, sound, and imagery.
In fact, if you have a smartphone, you're the owner of multiple sensors: a camera, an accelerometer, and a GPS sensor. Besides our pockets, sensors are located in factories, city streets, robots, transportation, drones, oceans—sensors are everywhere!
The proliferation of connected sensors is often called the Internet of Things. Let's explore those "things" in more detail and see how they affect our daily lives.

Types of sensors

A sensor is a low-power device that can detect some aspect of its environment and send the data to another device.
A sensor device directly connected to a computer.
A connected sensor is a sensor that also has a way to send data to either a local network or the Internet.
A sensor device wirelessly connected to a network.
Sensors can sense the same things humans can, and often much more.

Acoustic sensors

Just as ears can sense vibrations (that the brain then turns into sound), sensors can record vibrations in the environment.
All mobile phones and personal computers come with an acoustic sensor, the microphone. Singers and online lecturers (like myself!) often invest in external microphones that are better at filtering out background noise.
Smart home speakers like Amazon Alexa and Google Home always come with an embedded microphone so they can understand your commands.
🤔 What are the privacy implications of acoustic sensors? Is there any place you wouldn't want a sensor that could record your conversations?

Visual sensors

Our eyes use rods and cones to sense the visuals in front of us (while our brain then makes sense of those visuals). Similarly, cameras use optical sensors to record the visual scene in their lens.
CMOS sensors are the most common type of optical sensor used in digital cameras, web cameras, and smartphones today.1
CMOS sensor. Image source: Bengt Nyman
Optical sensors also perceive infrared light, wavelengths that are outside of the usual visual spectrum for humans. When the goal of a camera is to take photos that capture the world as humans see it, the camera uses an infrared cut-off filter to remove the infrared waves.2 Security cameras can take advantage of those infrared waves at night, however, to capture images in low-light situations.

Weather sensors

One of the most often measured aspects of a system is temperature. The human body has an internal temperature of 98.6°F; a few degrees above or below is cause for concern. Similarly, electromechanical systems have ideal temperature ranges and may malfunction when they're too hot or too cold.
Temperature sensors can be built using metals with temperature-dependent properties. As the temperature increases, the metal has a lower electrical resistance. There's a known relationship between resistance and temperature for each type of metal, so a computer can translate the resistance into a temperature measurement.3
An NTC thermistor temperature sensor next to a chart graphing the relationship between resistance and temperature. Image sources: Soumyapatra13, Miroslav Cika
When a sensor detects a temperature outside the ideal temperature range, the system can issue an alert or even adjust itself. For example, when a Raspberry PI computer gets over 85°C, most often due to CPU-intensive operations, it automatically slows down its processor speed to do less operations per second.
Other types of weather-related sensors include carbon monoxide detectors, air pollution sensors, ozone monitors, and humidity detectors.

Positioning sensors

Thanks to battery power and wireless networking, devices are increasingly portable. Our mobile phones go wherever we go, cars can make cross-country road trips, and drones can fly miles into the air. That leaves all those devices with the same question: "Where am I?"
All those devices can pinpoint their physical location on the globe thanks to the Global Positioning System (GPS). The US government started the GPS project in the 1970s and now controls around 30 GPS satellites orbiting the earth. Other large countries such as Russia and China have launched their own similar satellites, so that they'll have a backup positioning mechanism if the US government ever restricts their access.4
GPS satellites orbiting the earth. Image source: NOAA
GPS receivers are tiny sensors with antennas that receive radio signals from the GPS satellites orbiting in the sky above. In most cases, if a sensor can receive signals from at least four satellites, then the device can calculate its position.5
GPS works best in an outdoor environment with a clear view of the sky. It doesn't work as well indoors due to the interference caused by roofs, walls, and other objects. If a shipping robot in an Amazon factory wants to know which aisle they're in, GPS is unlikely to be able to reliably provide a precise enough position.
Indoor positioning systems are typically based on technologies like RFID. The device carries an RFID reader and its surrounded by objects that each have their own RFID tags, broadcasting their existence over radio wavelengths.6 The device may not know its absolute position in the world, but it can deduce its position relative to the objects it cares about, and sometimes that's all that matters.

More sensors

This is just a sampling of the world of sensors. Industries are constantly inventing new sensors to detect whatever is important to their field, like leaf sensors for agriculture, parking sensors for cars, altimeters for airplanes, tactile sensors for video games, and motion detectors for security systems. For more sensors, you can browse this long list.
You could even experiment with sensors yourself, by combining a mini computer like an Arduino or Raspberry PI with sensors and a bit of computer programming. Check out Instructables for fun project ideas.
🙋🏽🙋🏻‍♀️🙋🏿‍♂️Do you have any questions about this topic? We'd love to answer—just ask in the questions area below!

Want to join the conversation?