Smart Restroom Radar Solution: WT41 mmWave Radar for Occupancy Detection

Introduction

As smart cities evolve, public restrooms still rely on inefficient trial-and-error methods to check stall availability. Achieving accurate detection without compromising privacy has become a key challenge.

The WT41 mmWave radar sensor addresses this with non-contact sensing and micro-motion detection, offering reliable occupancy detection along with low power consumption and strong interference resistance. This article explores its use in smart restrooms and its broader potential in security, healthcare, and IoT.

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Many people have had this experience: in public restrooms at malls or tourist sites, you end up opening doors one by one to check which stalls are free—only to run into someone inside. Or you assume a closed door means “occupied,” wait for quite a while, and then realize no one is actually there.

This kind of “trial-and-error” restroom experience feels increasingly out of place in an era when smart cities are rapidly taking shape.

So the question is: can we know which stalls are available before walking in?
The answer is yes.

In this article, we introduce the “invisible guardian” behind smart restrooms — the WT41 mmWave radar sensor.

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01 The Limits of Traditional Stall Detection Solutions

Before getting into WT41, let’s take a look at the common approaches used today—and where they fall short.

Infrared sensing

This is probably the most widely used method. It works like motion-sensing lights at home, detecting body heat through infrared radiation.

In theory, it sounds reliable. In practice, not so much:

• Restroom environments have fluctuating temperatures and complex lighting conditions
• Sensors often struggle with accuracy
• If someone stays still (for example, using their phone), the system may incorrectly assume the stall is empty

Mechanical contact detection

Another approach is to install sensors in door locks:

• Door locked = occupied
• Door unlocked = vacant

It’s simple, but not ideal:

• Requires physical contact
• Mechanical parts wear out easily in humid, high-use environments
• Maintenance and repair costs add up

Camera-based solutions

Cameras can deliver high accuracy—but they raise obvious concerns:

• Serious privacy risks
• Legal and ethical barriers

In reality, this approach is not viable in restrooms.

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Clearly, each traditional solution has its own weak point.
So is there a way to achieve accurate detection without compromising privacy or reliability?

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02 An Invisible “Electronic Eye” — The WT41 Series

This is where mmWave radar comes in.

If a smart restroom needs “eyes,” the WT41 sensor acts as an invisible one—capable of precise detection without collecting any visual data.

WT41 is based on FMCW (frequency-modulated continuous wave) technology.

A simple way to understand it is to think of how bats navigate:

• They emit signals
• Receive reflections
• Determine what’s in front of them

WT41 works in a similar way, using high-frequency electromagnetic waves:

• It detects whether someone is inside a stall
• It does so without any physical contact
• It does not capture images, so privacy is fully protected

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What makes WT41 stand out?

Accuracy

• Distance accuracy up to ±25 cm
• Supports micro-motion detection
  • Even if a person is sitting still, subtle movements like breathing can still be detected

Environmental resilience

• Performs reliably in humid, low-light, or high-odor environments
• Can operate behind non-metal materials such as acrylic, glass, or thin plastic
• Can be installed discreetly inside doors or ceilings
• Operating temperature range: -20°C to 85°C

Low power consumption

• Average current: 19 μA
• Peak current: 120 mA
• Can run for extended periods on battery power
• Supports a wide voltage range (2.5V–5.5V)

Compact size

• Only 23 × 15.6 × 1.0 mm
• Easy to integrate into existing restroom structures
• Supports multiple interfaces (IO, UART, IIC) for system integration

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03 How It Works in Practice

A typical system consists of three parts:

• Sensor terminals
• Data transmission module
• Management platform

Here’s how it works in a real scenario:

When a user enters a stall, the WT41 sensor continuously emits 24 GHz electromagnetic waves.

• If the stall is empty, the signal reflects back in a stable pattern
• If someone is present, the reflected signal changes

The sensor analyzes these changes—particularly frequency and phase—to determine occupancy almost instantly.

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What happens next?

The sensor performs two tasks simultaneously:

1. Sends occupancy data to the management system via UART or IIC
2. Controls an indicator light outside the stall:
   • Occupied / Vacant status clearly displayed

This means users no longer have to guess—they can simply look for available stalls.

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A small but thoughtful detail

To avoid false signals, WT41 includes a short delay:

• When a person leaves, the system waits about 3 seconds before marking the stall as vacant

This prevents situations where someone briefly steps out and the system immediately signals “available,” which could lead to awkward encounters.

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04 More Than Detection — Enabling Smarter Functions

The system doesn’t stop at telling you whether a stall is occupied.

Usage analytics

• Tracks how long each stall is used
• Identifies peak usage periods
• Helps optimize cleaning schedules

Smarter cleaning workflows

• Staff can clean only when stalls are unoccupied
• Reduces unnecessary interruptions

Timeout alerts

• If someone stays too long (e.g., over 30 minutes), the system sends an alert
• Especially useful in hospitals or care facilities

System integration
The sensor can also work with other systems:

• When a stall is vacant → activate ventilation or deodorization
• When occupied → turn on lighting

A small sensor becomes part of a larger, connected system.

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05 Broader Potential in Smart City Applications

The use of WT41 in restrooms is just one example of what mmWave radar can do.

Its key strengths—non-contact sensing, resistance to interference, privacy protection, and low power consumption—make it suitable for a wide range of applications.

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Other application scenarios

Smart security

• Detects human presence without cameras
• Suitable for server rooms, warehouses, and restricted areas

Healthcare and elderly care

• Monitors subtle human movement
• Detects prolonged inactivity
• Enables early warnings in emergencies

Smart homes and IoT

• More accurate presence detection
• Enables automatic control of lighting, HVAC, and other devices

Commercial spaces

• Tracks occupancy and usage patterns
• Supports better space management and operations

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Conclusion

The WT41 mmWave radar sensor demonstrates how sensing technology can quietly improve everyday experiences.

From eliminating guesswork in restrooms to enabling data-driven facility management, it contributes to more efficient operations and better user experiences.

And this is only the beginning.

As part of the broader smart city ecosystem, technologies like WT41 are helping build environments that are more responsive, efficient, and user-friendly—without compromising privacy.