Digital systems use an accelerometer to detect acceleration or orientation, an ultrasonic sensor to determine distance, a photoresistor to detect light intensity, and a hygrometer to measure humidity. Each acts as an input device by converting a physical condition into an electrical signal that a processor can interpret.
How Each Sensor Works
A sensor is an input transducer: it converts physical energy or a physical property into an electrical output. That signal may require amplification, calibration or analogue-to-digital conversion before a microcontroller can process it.
| Sensor | Mechanism and measured variable | Typical application |
|---|---|---|
| Accelerometer | Usually uses microscopic moving structures to measure acceleration along one or more axes. It can detect movement, vibration and orientation relative to gravity. | Screen rotation, fitness tracking or vehicle stability systems |
| Ultrasonic sensor | Emits a high-frequency sound pulse and detects its echo. Distance is calculated from the pulse's travel time and the speed of sound. | Parking assistance, obstacle avoidance or tank-level monitoring |
| Photoresistor | Changes electrical resistance with incident light intensity. Its resistance normally decreases as light intensity increases. | Automatic lighting or display brightness control |
| Hygrometer | Uses a humidity-sensitive material whose capacitance or resistance changes with atmospheric moisture. It commonly measures relative humidity. | Greenhouse control, weather monitoring or storage systems |
Selecting a Sensor
Selection depends on the required range, sensitivity, resolution, response time, accuracy, reliability, operating environment and cost. Designers must also consider whether the output is analogue or digital and whether additional signal-processing components are required.
A common misconception is that every sensor used in a digital system produces a digital signal. A photoresistor, for example, produces a continuous resistance change, so its signal normally needs an analogue-to-digital converter.
Exam Technique
For IB Design Technology HL B3.4, identify the sensor, state the physical variable measured, explain the conversion mechanism and justify its suitability for the application. Do not merely name a sensor: link its characteristics to the design specification and operating environment.
