MEMS and sensors

What is the principle of the optical sensor that measures heart rate at the wrist in smartwatches and similar devices?

  1. AIt measures changes in skin temperature
  2. BIt calculates from the acceleration of arm movement alone
  3. CIt shines LED light and detects with a photodiode the changes in reflected light caused by changes in blood flow
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Answer: C. It shines LED light and detects with a photodiode the changes in reflected light caused by changes in blood flow

An optical heart-rate sensor shines light, often green, from an LED onto the skin and uses a photodiode to detect changes in the reflected light caused by the blood volume in the vessels changing with each heartbeat. This method is called photoplethysmography (PPG). Noise from body movement is corrected using accelerometer data.

  • A. Skin temperature changes are hard to use for detecting the pulse.
  • B. The accelerometer is used to correct for motion, but the heartbeat itself is detected optically.
  • C. Correct. A photodiode detects changes in reflected light.
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  1. Which sensor does a smartphone use to automatically adjust screen brightness to its surroundings?
  2. What is the general principle of the sensor a smartphone uses to turn off the screen when your face approaches during a call, preventing accidental touches?
  3. What is the driving principle of the "comb-drive" structure used as a MEMS actuator?
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  5. Which sensor, widely used in motion sensors that turn on lights when a person approaches, uses a material that generates charge when its temperature changes?