Sensors & Measurement · Study deck

Choosing the Right Sensor

Picture a greenhouse team deciding when to water one bed.

Physics Phoebe is your guide for this deck.

sensortypesselection
Physics Phoebe, the module guide, in a scene from this chapter.
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After studying this chapter

Learning objectives

You will be able to:

  • Explain: The mathematical gist.: The chapter's CR2477 exercise uses 0.070 mAh/day, or 2.92 µA, and a 1,000 mAh cell, giving 14,286 days before other losses.
  • Explain: Temperature Terry (DS18B20) raised his hand: "I am waterproof, I cost $3, I have +/-0.5 degree accuracy, and I sleep at only 1 microamp!".
  • Explain: This demonstrates how your priorities directly determine the "best" sensor -- there is no objectively best sensor, only the best sensor for your requirements.
  • Explain: The useful question is not which sensor has the longest feature list.
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Major section

Start With the Measurement Story

A precise catalog number is not installed proof.

  • The useful question is not which sensor has the longest feature list.
  • A gateway is the local device or service that joins sensor data to a wider system.
  • This opening does not name one best sensor or score.
  • The winning part is the one that still supports the decision.
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Major section

Field Conditions Drive Selection

The mathematical gist.: The chapter's CR2477 exercise uses 0.070 mAh/day, or 2.92 µA, and a 1,000 mAh cell, giving 14,286 days before other losses.

  • The total becomes 4.06 µA, so the same division gives 10,267 days, or 28.1 years.
Soil-moisture sensor field cross-section showing dry topsoil, wet saline soil, probe corrosion, battery service route, gateway distance, and requirement cards.
Soil-moisture sensor field cross-section showing dry topsoil, wet saline soil, probe corrosion, battery service route, gateway distance, and requirement cards.
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Major section

Putting Numbers to It

For DS18B20, the project weights are accuracy 9, waterproofing 10, sleep current 7, cost 5, and humidity 3.

  • Weights encode your priorities, so changing a weight changes the ranking.
  • Adjust the requirement weights below to see how they change the sensor ranking.
  • This demonstrates how your priorities directly determine the "best" sensor -- there is no objectively best sensor, only the best sensor for your requirements.
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Major section

For Kids: Meet the Sensor Squad!

But my temperature accuracy is only +/-1 degree.".

  • But there were SO many choices!
  • Temperature Terry (DS18B20) raised his hand: "I am waterproof, I cost $3, I have +/-0.5 degree accuracy, and I sleep at only 1 microamp!".
  • For an indoor project, I might choose differently!".

Why it matters

Max made a comparison chart and checked off each requirement. "DS18B20 wins for this project because it is waterproof, accurate enough, cheap, and super low-power.

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Major section

Nest Protect Sensor Selection

Sensor selection decisions that work at prototype scale can fail catastrophically at production volumes.

  • The Nest Protect smoke and CO detector illustrates how production constraints reshape sensor choices.
  • The MQ-2 performed well in lab tests -- detecting smoke, CO, and natural gas.
  • Total cost of ownership was 40% lower despite higher sensor unit cost.

Numbers to remember

40%Total cost of ownership was 40% lower despite higher sensor unit cost.
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Deck summary

Key takeaways

A precise catalog number is not installed proof.

  • The mathematical gist.: The chapter's CR2477 exercise uses 0.070 mAh/day, or 2.92 µA, and a 1,000 mAh cell, giving 14,286 days before other losses.
  • For DS18B20, the project weights are accuracy 9, waterproofing 10, sleep current 7, cost 5, and humidity 3.
  • But my temperature accuracy is only +/-1 degree.".
  • Sensor selection decisions that work at prototype scale can fail catastrophically at production volumes.
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Retrieval practice

Recall check 1 of 2

Physics Phoebe says: answer from memory, then check your reasoning.

Q1When comparing sensors, should you design based on "typical" or "maximum" accuracy specifications?

ATypical -- since most sensors will achieve this performance
BMaximum -- design to the guaranteed worst-case accuracy.
CThe average of typical and maximum
DIt does not matter -- both values are close enough
Show answer

Answer: B Always design for maximum (worst-case) specifications.

Q2You are designing a battery-powered outdoor weather station that needs temperature accuracy of +/-0.5C and must be waterproof. Which sensor is the best choice?

ADHT22 -- it has +/-0.5C accuracy and measures humidity too
BBME280 -- it measures temperature, humidity, and pressure
CSHT31 -- it has the best accuracy at +/-0.3C
DDS18B20 -- it has +/-0.5C accuracy, waterproof probe option.
Show answer

Answer: D

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Retrieval practice

Recall check 2 of 2

Physics Phoebe says: answer from memory, then check your reasoning.

Q3Place each selection decision where it lives so you can defend a sensor choice against the measurement, integration, and deployment constraints.

ADefine Measurand & Range
BEvaluate Accuracy Needs
CCheck Interface Compatibility
DVerify Constraints (Cost/Environment)
Show answer

Answer: A Place each selection decision where it lives so you can defend a sensor choice against the measurement, integration, and deployment constraints.

Q4Complete the sensor auto-detection and configuration:

Afound = i2c.scan()
Bfound = i2c.detect()
Cfound = i2c.list_devices()
Dfound = i2c.enumerate()
Show answer

Answer: A A lookup dictionary maps I2C addresses to sensor names.

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Print reference

Answers

Answer key.

  1. B · Always design for maximum (worst-case) specifications.
  2. D
  3. A · Place each selection decision where it lives so you can defend a sensor choice against the measurement, integration, and deployment constraints.
  4. A · A lookup dictionary maps I2C addresses to sensor names.
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