Sensors & Measurement · Study deck

Sensing with Existing Infrastructure

Imagine a school wants a rough room count without fitting a new sensor above each door.

Physics Phoebe is your guide for this deck.

sensortypesinfrastructure

After studying this chapter

Learning objectives

You will be able to:

Explain: This paradigm shift—leveraging existing infrastructure instead of deploying dedicated sensors—can reduce costs while providing area-wide coverage instead of point measurements.
Explain: By watching the power meter, a computer can figure out which devices are turned on -- like identifying people by their footsteps!".
Explain: If software traffic or a device change breaks the relationship, mark the estimate uncertain and return to a direct check.
Explain: A smart computer can analyze those ripples and figure out that someone walked by, WITHOUT any cameras or motion sensors!".

Major section

Test the Wi-Fi Occupancy Clue

The team must decide whether signs from its Wi-Fi system are good enough for that job.

The school's Wi-Fi system may provide a clue about room use without a new door counter.
That clue is a proxy: a measured sign that may move with occupancy but does not directly count people.
If software traffic or a device change breaks the relationship, mark the estimate uncertain and return to a direct check.

Major section

Use Existing Infrastructure

This paradigm shift—leveraging existing infrastructure instead of deploying dedicated sensors—can reduce costs while providing area-wide coverage instead of point measurements.

Traditional IoT thinking: "We need temperature data -> Deploy temperature sensors.".
Infrastructure-leveraging thinking: "We need occupancy data -> Use existing Wi-Fi routers.".
The Wi-Fi router becomes a "free" presence sensor.

Major section

Design Guidelines

"What do you mean?" asked Temperature Terry.

"Think about your Wi-Fi router at home," Max explained. "It sends out radio waves all the time.
A smart computer can analyze those ripples and figure out that someone walked by, WITHOUT any cameras or motion sensors!".
The infrastructure does double duty!".

Major section

Design Guidelines (continued)

the LED was amazed: "So the Wi-Fi router IS a sensor?".

And your electricity meter is a sensor too," Max continued. "Every appliance in your house uses electricity differently.
A hair dryer uses 1500 watts, a fridge uses 100 watts, a phone charger uses 5-25 watts.
By watching the power meter, a computer can figure out which devices are turned on -- like identifying people by their footsteps!".

Major section

Campus Occupancy Monitoring

Installing dedicated occupancy sensors in every room (2,400 rooms) would cost over $360,000.

Each Cisco AP reports associated client count every 60 seconds via SNMP.
Key correction factor: Students carry 1.2-1.8 devices on average (phone + laptop).
The hybrid approach (Wi-Fi infrastructure + targeted dedicated sensors) gives the best cost-accuracy tradeoff.

Major section

Common Pitfalls

A single strain gauge only measures strain at its exact location.

Infrastructure monitoring requires sensor arrays with spacing matched to the expected damage scale.
Infrastructure sensors are often installed for 10-20 year lifespans.
IP67 ratings are tested in clean water but real deployments involve condensation cycles and chemical exposure.

Deck summary

Key takeaways

The team must decide whether signs from its Wi-Fi system are good enough for that job.

This paradigm shift—leveraging existing infrastructure instead of deploying dedicated sensors—can reduce costs while providing area-wide coverage instead of point measurements.
"What do you mean?" asked Temperature Terry.
the LED was amazed: "So the Wi-Fi router IS a sensor?".
Installing dedicated occupancy sensors in every room (2,400 rooms) would cost over $360,000.

Retrieval practice

Recall check 1 of 4

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

Q1In this chapter’s illustrative 200-room cost comparison, each room already has an access point, an approximate occupied/empty proxy is acceptable, and privacy controls are in place. Using the listed upfront figures ($50 for Wi-Fi analysis software and no new Wi-Fi hardware), which approach has the lowest stated upfront cost?

AInstall PIR sensors in every room ($15 x 200 = $3,000 + installation)

BInstall cameras with computer vision ($50 x 200 = $10,000)

CAnalyze existing Wi-Fi access point data ($0 hardware + $50 software)

DDeploy CO2 sensors in every room ($25 x 200 = $5,000)

Show answer

Answer: C For this illustrative upfront comparison, existing Wi-Fi access points and the stated $50 software figure give the smallest listed new-hardware cost.

Retrieval practice

Recall check 2 of 4

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

Q2A university has Wi-Fi access points in all 50 buildings and needs an approximate building-level occupancy proxy for HVAC scheduling, with privacy controls and existing association data available. In the chapter’s new-hardware comparison, which approach is most cost-effective?

AInstall PIR motion sensors in every room of every building

BAnalyze existing Wi-Fi access point data to count connected devices per building

CPlace cameras at every entrance and use computer vision

DDeploy CO2 sensors in every room to estimate occupancy from air quality

Show answer

Answer: B Wi-Fi access point analysis leverages infrastructure that already exists in every building.

Retrieval practice

Recall check 3 of 4

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

Q3Non-Intrusive Load Monitoring (NILM) identifies appliances by analyzing power consumption. Which appliance signature would be easiest to detect?

AA 5 W phone charger with a small, steady power draw.

BA sleeping laptop drawing about 2 W with little variation.

CA dimmed LED bulb drawing 3 W with a weak signature.

DA 1500 W hair dryer with a sharp switch-on and switch-off signature.

Show answer

Answer: D A 1500W hair dryer creates a large, unmistakable power signature -- a sudden 1500W spike when turned on, and a 1500W drop when turned off.

Retrieval practice

Recall check 4 of 4

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

Q4What existing wireless channel changes can indicate indoor motion or occupancy?

ACellular Positioning

BPower Line Monitoring

CWiFi CSI (Occupancy)

DAmbient RF Harvesting

Show answer

Answer: C Channel-state changes can proxy for motion or occupancy around an existing Wi-Fi link.

Q5Complete the WiFi RSSI-based proximity sensing:

Awlan = network.WLAN(network.STA_IF)

Bwlan = network.WiFi()

Cwlan = network.WLAN(network.AP_IF)

Dwlan = network.connect()

Show answer

Answer: A WiFi RSSI can estimate distance using the log-distance path loss model.

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Answers 1 of 2

Answer key.

C · For this illustrative upfront comparison, existing Wi-Fi access points and the stated $50 software figure give the smallest listed new-hardware cost.
B · Wi-Fi access point analysis leverages infrastructure that already exists in every building.
D · A 1500W hair dryer creates a large, unmistakable power signature -- a sudden 1500W spike when turned on, and a 1500W drop when turned off.
C · Channel-state changes can proxy for motion or occupancy around an existing Wi-Fi link.

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Answers 2 of 2

Answer key.

A · WiFi RSSI can estimate distance using the log-distance path loss model.