An ultrasonic sensor (超音波センサー) sends out sound at a frequency too high for human ears (ultrasound) and works out distance from the time it takes to hit an object and come back.
Emit ultrasound
Bounces off an object
Receive the echo
Calculate distance from time
Multiplying the speed of sound by the round-trip time and dividing by 2 gives the distance to the object.
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Sound travels through air at a nearly constant speed (about 340 m per second at room temperature), so multiplying the round-trip time by that speed and halving it gives the distance. Because it does not rely on light, it can easily detect transparent things such as glass, or objects in the dark. It is used for parking assistance in cars and for measuring the liquid level inside tanks.
AnalogyIt is like shouting at a mountain and judging how far away it is by how long the echo takes to come back.
How to remember"Ultra" means beyond the range of sound people can hear. Bats dodge walls while flying in the dark using the same echo principle.
Exam points
Mix-upAn ultrasonic sensor (超音波センサー) measures distance from reflected sound, while an optical distance sensor uses reflected light. Tell them apart by which kind of wave is sent and received.
Mix-upThe measured time covers both the trip out and the trip back, so distance = speed of sound × time ÷ 2. Forgetting to divide by 2 gives twice the real distance.
A temperature sensor (温度センサー) measures the temperature of its surroundings or of an object and outputs the value as an electrical signal.
A thermistor (サーミスタ), for example, is a component whose electrical resistance changes with temperature, and reading that change as a voltage reveals the temperature. Besides controlling the temperature of air conditioners and fridges, it is built into IoT devices that monitor room temperature as their input part.
AnalogyIt plays the role of a thermometer, turning the hot and cold you feel on your skin into numbers a machine can read.
How to rememberThe "thermo" in thermostat and thermography means heat. Thermistor shares the same root, so remember it as a part whose properties change with heat.
Exam points
ExamA temperature sensor (温度センサー) is the part that turns a measured temperature into an electrical signal (電気信号) and feeds it as input into an IoT device or other equipment.
Mix-upA sensor (センサー) is the input side that turns a physical quantity into an electrical signal, and an actuator (アクチュエーター) is the output side that receives an electrical signal and turns it into motion. The direction of conversion is reversed.
A humidity sensor (湿度センサー) measures a value tied to how much water vapor the air holds (humidity) and turns it into an electrical signal.
Most output relative humidity (%), the share of water vapor compared with the most the air can hold at that temperature. Since relative humidity shifts with temperature, it is often combined with a temperature sensor in a single temperature-humidity sensor. It is used for automatic humidifier and dehumidifier operation and to help prevent condensation and mold in greenhouses and warehouses.
AnalogyWe sense how damp the air is from how fast the laundry dries; this device does the same job with numbers.
How to rememberThe "60% humidity" in a weather forecast is relative humidity: it shows not the amount of water itself but how full the container called air is.
Exam points
Mix-upRelative humidity (相対湿度) is the ratio to the maximum the air can hold at that temperature. Even with the same amount of water vapor, the value drops as the air gets warmer.
A pressure sensor (圧力センサー) detects the pressure of a gas or liquid, or a force applied to a surface, and turns it into an electrical signal.
One method, for instance, reads the strain of a thin membrane (a diaphragm) that flexes slightly under pressure as a change in electrical resistance or capacitance. It is used to monitor tire pressure, in smartphone barometers that estimate altitude from changes in air pressure, and in blood pressure monitors.
AnalogyPress a balloon with your finger and you feel how taut it is; this device tells you that tautness as a number.
How to rememberA bag of chips puffs up on a mountaintop because the air pressure outside drops. Altitude and air pressure go hand in hand, so pressure tells you height.
Exam points
Mix-upA pressure sensor (圧力センサー) measures how hard something presses or the level of air pressure, while an acceleration sensor (加速度センサー) measures changes in speed and the direction of gravity (movement, tilt, vibration and so on). They measure different quantities.
A smoke sensor (煙センサー) detects smoke particles in the air so that a fire can be reported early.
Smoke enters
Particles scatter light
Receiver catches light
Sound the alarm
Once the light reaching it passes a set amount, the sensor judges that smoke is present and raises the alarm.
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The photoelectric type (光電式), for example, shines light inside and detects smoke when particles that drift in scatter that light onto a receiver. It is built into home smoke alarms and building fire-safety equipment, and sounds an alarm once the smoke passes a certain density.
AnalogyOn a foggy night you can see the beams of car headlights because tiny droplets scatter the light. The sensor recreates that effect inside a small box to notice smoke.
How to rememberPut the saying "where there's smoke, there's fire" to work: the sensor is a lookout that suspects fire as soon as it sees smoke.
Exam points
Mix-upA smoke sensor (煙センサー) catches the particles given off by burning, while a temperature sensor (温度センサー) catches the rise in temperature from heat. For a fire that smolders before the heat climbs, smoke is usually the earlier sign.
Temperature sensor vs. Smoke sensor
Aspect
Temperature sensor
Smoke sensor
Key differenceWhat it catches in a fire
The rise in temperature from heat
Smoke particles given off by burning
Where it suits fire detection
Kitchens, where cooking makes smoke and steam
Bedrooms and stairways
Main uses
Controlling air conditioning and refrigeration, detecting fire by heat
Detecting fire by smoke
Key points
An ultrasonic sensor treats the time until its ultrasound returns as a round trip, multiplies it by the speed of sound and divides by 2 to get the distance.
Sensors that measure the environment turn physical quantities such as temperature, the share of water vapor and pressure into electrical signals that serve as input to IoT devices.
One component used in temperature sensors is the thermistor (サーミスタ), which relies on its electrical resistance changing with temperature.
A pressure sensor measures the pressure of a gas or liquid or an applied force, and can also estimate altitude from air pressure.
A smoke sensor catches the particles given off by burning and, built into a fire alarm, triggers the warning.