564 Sensor Selection Decision Guide
Frameworks and Strategies for Optimal Sensor Choice
564.1 Learning Objectives
By completing this guide, you will be able to:
- Apply a systematic decision framework for sensor selection
- Evaluate trade-offs between accuracy, cost, power, and reliability
- Calculate total cost of ownership for sensor deployments
- Avoid common mistakes in sensor selection
Choosing the wrong sensor can:
- Waste money: Buying expensive sensors when cheap ones work fine
- Cause failures: Cheap sensors failing in harsh environments
- Drain batteries: High-power sensors killing battery-powered devices
- Miss requirements: Inaccurate sensors providing unusable data
The goal is to find the sensor that meets your requirements at the lowest total cost.
564.2 The Five-Step Decision Framework
564.2.1 Step 1: Define Requirements
Before looking at sensors, answer these questions:
| Question | Why It Matters | Example |
|---|---|---|
| What am I measuring? | Determines sensor category | Temperature, CO2, distance |
| What range do I need? | Must cover operating conditions | -20 to +50°C for outdoor |
| What accuracy is required? | Drives sensor quality/cost | ±1°C for comfort, ±0.1°C for industrial |
| What update rate? | Affects power and processing | 1 Hz for weather, 100 Hz for motion |
Define the minimum acceptable accuracy. Don’t say “as accurate as possible” - this leads to over-specifying and wasted budget.
564.2.2 Step 2: Consider Budget
Budget analysis must include Total Cost of Ownership (TCO), not just purchase price.
TCO Formula: \[ TCO = (Units + Replacements) \times UnitCost + (Maintenance \times Years) + (Calibration \times Years) \]
Example: 5-Year TCO Comparison
| Sensor | Unit Cost | Replacements | Maintenance | Calibration | 5-Year TCO |
|---|---|---|---|---|---|
| DHT22 | $5 | 1 ($5) | $0 | $0 | $10 |
| SHT85 | $25 | 0 | $0 | $0 | $25 |
| MQ-135 | $3 | 2 ($6) | $100 | $250 | $406 |
| SCD40 | $50 | 0 | $0 | $0 | $50 |
Key Insight: MQ-135 costs $3 initially but $406 over 5 years. SCD40 costs $50 initially but only $50 over 5 years. The “expensive” sensor is 8x cheaper long-term!
564.2.3 Step 3: Evaluate Power
Power requirements vary dramatically by application:
| Power Source | Max Average Current | Strategy |
|---|---|---|
| Coin cell (CR2032) | <100 µA | Ultra-low power, duty cycling |
| AA/AAA battery | <1 mA | Low power sensors, sleep modes |
| USB / 5V adapter | <50 mA | Most digital sensors work |
| Wired 24V industrial | Unlimited | Any sensor, including high-power |
Power Calculation Example:
A sensor draws 3 mA active and 10 µA sleep. With 1% duty cycle (on 1 second per 100 seconds):
\[ Average = (0.01 \times 3mA) + (0.99 \times 10\mu A) = 30\mu A + 9.9\mu A = 40\mu A \]
This works with coin cell battery!
564.2.4 Step 4: Check Compatibility
| Factor | What to Check | Common Issues |
|---|---|---|
| Interface | I2C, SPI, analog, digital | MCU must support the protocol |
| Voltage | 3.3V, 5V, or both | Level shifter may be needed |
| Libraries | Arduino, Python, vendor SDK | Check community support |
| Ecosystem | Availability, documentation | Obscure sensors lack resources |
564.2.5 Step 5: Plan Maintenance
| Sensor Type | Typical Maintenance | Planning Required |
|---|---|---|
| Digital temp/humidity | None (factory calibrated) | Replace at end of lifespan |
| Pressure sensors | None (MEMS stable) | Check annually |
| Gas sensors (MQ series) | Frequent calibration | Budget $50-100/year |
| NDIR CO2 sensors | Auto-calibration | Periodic baseline reset |
| IMU sensors | Gyro drift compensation | Software calibration |
564.3 Accuracy Requirements by Application
Matching accuracy to application is critical for cost optimization.
564.3.1 Temperature Applications
| Application | Accuracy Needed | Recommended Sensor |
|---|---|---|
| Home thermostat | ±1-2°C | DHT22 ($5) |
| Smart HVAC | ±0.5°C | SHT85 ($25) |
| Industrial process | ±0.2°C | SHT85 + calibration |
| Cleanroom | ±0.05°C | SHT85 with certified calibration |
| Laboratory | ±0.01°C | Platinum RTD (not covered here) |
564.3.2 Gas Detection Applications
| Application | Accuracy Needed | Recommended Sensor |
|---|---|---|
| Air quality indication | ±50% (trend only) | MQ-135 ($3) |
| Smart ventilation | ±100 ppm | CCS811 ($12) |
| HVAC optimization | ±50 ppm | SCD40 ($50) |
| Safety-critical | ±10 ppm | Industrial sensors (not covered) |
564.3.3 Distance Applications
| Application | Accuracy Needed | Recommended Sensor |
|---|---|---|
| Parking occupancy | ±5 cm | HC-SR04 ($2) |
| Tank level (liquid) | ±1 cm | HC-SR04 with temp compensation |
| Obstacle avoidance | ±5 cm | VL53L1X ($15) |
| Precision measurement | ±1 mm | VL53L1X or industrial ToF |
564.4 Budget Strategies
564.4.1 Very Low Budget (<$10/sensor)
Strategy: Optimize for initial cost, accept higher TCO if deployment is short-term.
Recommended Sensors:
- DHT22 ($5) - Temperature/humidity
- BH1750 ($2) - Light
- HC-SR04 ($2) - Distance
- MPU-6050 ($5) - Motion
When to Use: Hobbyist projects, prototypes, short-term deployments (<2 years)
564.4.2 Moderate Budget ($10-30/sensor)
Strategy: Balance cost and quality, aim for 5+ year lifespan.
Recommended Sensors:
- SHT85 ($25) - Temperature/humidity
- BMP388 ($8) - Pressure
- CCS811 ($12) - Air quality
- VL53L1X ($15) - Distance
When to Use: Production devices, outdoor deployments, 3-5 year lifespan
564.4.3 High Budget (>$30/sensor)
Strategy: Optimize for accuracy and reliability, minimize maintenance.
Recommended Sensors:
- SCD40 ($50) - True CO2
- BNO055 ($30) - 9-axis motion
- Industrial-grade variants
When to Use: Industrial applications, regulatory compliance, 10+ year deployments
564.5 Environmental Considerations
564.5.1 Indoor (Stable Conditions)
- Temperature: 15-30°C, stable
- Humidity: 30-70% RH
- Vibration: Minimal
- Interference: Low
Acceptable Sensors: Consumer-grade (DHT22, MPU-6050, HC-SR04)
564.5.2 Outdoor (Variable Conditions)
- Temperature: -20 to +50°C (or extreme: -40 to +60°C)
- Humidity: 0-100% RH, rain/snow
- Vibration: Wind, traffic
- Interference: EMI from vehicles, lightning
Required Sensors: Industrial-grade (SHT85, BMP388, VL53L1X), weatherproof enclosures
564.5.3 Industrial (Harsh Conditions)
- Temperature: Varies widely
- Vibration: High (motors, machinery)
- Interference: Strong EMI from motors, welders
- Dust: Metal particles, oil mist
Required Sensors: Industrial-rated (BNO055, SHT85), conformal coating, IP67+ enclosures
564.6 Common Mistakes to Avoid
564.6.1 Mistake 1: Over-Specifying Accuracy
Problem: Buying expensive sensors when cheap ones meet requirements.
Example: Using SHT85 (±0.1°C, $25) for a home thermostat that only needs ±1°C accuracy.
Solution: Always define minimum acceptable accuracy before selecting sensors.
564.6.2 Mistake 2: Ignoring TCO
Problem: Choosing lowest initial cost without considering long-term expenses.
Example: MQ-135 costs $3 but has $406 TCO vs SCD40 at $50 with $50 TCO.
Solution: Calculate 5-year TCO including replacements, calibration, and maintenance.
564.6.3 Mistake 3: Forgetting Power Constraints
Problem: Selecting high-power sensors for battery applications.
Example: Using MQ-135 (150 mA heater) with coin cell battery.
Solution: Calculate average power consumption with duty cycling before selection.
564.6.4 Mistake 4: Wrong Environment Rating
Problem: Using indoor-rated sensors outdoors.
Example: DHT22 fails in -30°C winter or condensing humidity.
Solution: Check temperature range and IP rating for deployment environment.
564.6.5 Mistake 5: Interface Mismatch
Problem: Selecting sensors incompatible with microcontroller.
Example: SPI-only sensor with I2C-only MCU, or 5V sensor with 3.3V MCU.
Solution: Verify interface and voltage compatibility before purchase.
564.6.6 Mistake 6: No Library Support
Problem: Selecting obscure sensors without community support.
Example: Unusual sensor with no Arduino/Python library.
Solution: Check for existing libraries and documentation before selection.
564.7 Decision Checklist
Before finalizing sensor selection, verify:
564.8 Summary
Effective sensor selection requires balancing multiple competing factors:
564.8.1 Key Principles
- Match accuracy to requirements - Don’t buy precision you won’t use
- Calculate TCO, not just price - Include maintenance and replacements
- Power budget first - Battery devices need ultra-low power sensors
- Environment determines quality - Indoor vs outdoor vs industrial
- Check ecosystem - Availability, libraries, documentation matter
564.8.2 The Selection Framework
1. Define Requirements → What, range, accuracy, rate
2. Consider Budget → TCO over deployment lifetime
3. Evaluate Power → Battery or wired, duty cycle
4. Check Compatibility → Interface, voltage, libraries
5. Plan Maintenance → Calibration, replacement schedule564.8.3 Final Rule
The “best” sensor is the one that meets your requirements at the lowest total cost, not the most accurate or expensive option.
Engineering is about making smart trade-offs. Use this framework to make informed decisions.
564.9 What’s Next
- Sensor Selection Challenge Game - Practice sensor selection with real scenarios
- Sensor Selection Reference Guide - Complete specifications for 12 sensors
- Sensor Calibration Challenge - Learn calibration methods