A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
A sensor is a device that detects something in the environment — light, temperature, sound, motion, pressure, or proximity — and converts it into an electrical signal that a circuit can use. Sensors are like a circuit's eyes, ears, and fingertips: they let electronic devices respond to the world around them. A night light uses a light sensor to detect darkness. A thermostat uses a temperature sensor to detect cold. A motion-sensor light uses a motion detector to detect movement. Engineers use sensors to make devices "smart" — able to react automatically to changing conditions instead of requiring a human to flip a switch.
Build circuits that respond to the environment. Start with a light-dependent resistor (LDR): wire it into a circuit with an LED, and the LED brightness changes when you cover the LDR with your hand (blocking light). Then try a tilt switch — a small tube with a metal ball that completes a circuit when tilted one way and breaks it when tilted the other. Have students brainstorm everyday devices that use sensors (automatic doors, phone screens that dim, car backup sensors) and identify what each sensor detects and how the device responds.
You have built circuits with switches — devices that let you control whether current flows. But what if the circuit could control itself? What if it could detect light, or temperature, or motion, and respond automatically? That is what sensors do.
A sensor is a component that detects something in the physical world and converts it into an electrical signal. Think of it as a translator: the sensor experiences light, heat, sound, or pressure and translates that experience into a change in current or voltage that the circuit can understand and act on.
One of the simplest sensors is a light-dependent resistor (LDR). In the dark, the LDR has high resistance and barely lets current through. In bright light, its resistance drops and current flows freely. Wire an LDR into a circuit with an LED, and you have a device that automatically adjusts brightness based on ambient light — dim in daylight, bright in darkness. This is the same principle behind the night lights that turn on automatically when a room gets dark.
A tilt switch is even simpler — a small tube with a metal ball inside. When the tube is upright, the ball rests on two contacts at the bottom, completing the circuit. When the tube is tilted, the ball rolls away from the contacts, breaking the circuit. This simple sensor can detect orientation and is used in devices like game controllers and construction levels.
Here is the important distinction: sensors do not make decisions. A temperature sensor reports "it is 60 degrees" as an electrical signal, but it does not decide to turn on the heater — the thermostat circuit makes that decision. A motion sensor reports "something moved," but the automatic door circuit decides to open the motor. The sensor is the input (what does the world look like?), and the circuit is the processor (what should we do about it?). This separation of detection and decision is a fundamental engineering concept that scales all the way up to complex systems like self-driving cars, which have dozens of sensors all feeding information to a central computer that makes driving decisions.
Once you understand sensors, you start seeing them everywhere. Your phone has a light sensor (auto-brightness), an accelerometer (detects tilting for screen rotation), a proximity sensor (turns off the screen when you hold the phone to your ear), a microphone (sound sensor), and a camera (light sensor array). Each sensor gives the phone's computer information about the world, and the computer decides what to do with it. Every "smart" device is really just a combination of sensors, circuits, and decision logic — and understanding that combination starts right here.