Modern smartphones of the Xiaomi brand are equipped with advanced sensor systems that provide comfortable use of the gadget in everyday life. Particular attention deserves an ultrasonic proximity sensor, which is radically different from the usual infrared analogues installed in budget models.
Unlike infrared sensors that emit light, ultrasonic technology uses high-frequency sound waves that are invisible to the human eye. This engineering solution allows Mi and Redmi manufacturers to create devices with bezel-free displays without leaving cutouts at the top of the case.
Many users experience a screen that goes out too early or doesn't turn off during a call. Often, the reason lies in the way the ultrasonic sensor works or the pollution of the dynamic area. In this article, we'll go into detail about the physical basis of the technology, software calibration methods and how to fix hardware failures.
Physical foundations of ultrasonic technology
The ultrasound sensor uses echolocation similar to that used by bats or dolphins. At the top of the smartphone, often hidden under the display glass or in the body of the speaker, is a piezoelectric emitter that generates sound waves above 20 kHz that the human ear does not perceive.
When you hold the phone to your ear, the sound wave bounces off the surface of your head and returns to the receiver. The system analyzes the delay time of the reflected signal and calculates the distance to the object. If the distance is less than a given threshold (usually 2-3 cm), the MIUI or HyperOS operating system is instructed to turn off the touchscreen and display to save energy and prevent accidental cheekstrokes.
The key advantage of this technology is the ability to covertly edit. Infrared sensors require direct access to the surface, which creates a black dot on the screen, and Ultrazuc freely passes through thin layers of glass and glue, allowing the concept of True Full Screen to be realized without mechanical cutouts.
But the physics of the process imposes its limitations: Sound waves are sensitive to acoustic interference and dense materials that cover the speaker, and improperly fitted protective glass or thick film can significantly distort the signal, leading to errors in the proximity system.
Differences from infrared sensors
To better understand the specifics of the ultrasound module, it is necessary to compare it with the classic infrared sensor that has been used in mobile technology for decades: an IR sensor consists of an LED and a photodetector: it emits light and captures its reflection. It is a reliable but cumbersome technology.
Ultrazuk has many of the shortcomings of optical systems, and is independent of the lighting and color of the object (black clothing or light skin is equivalent to it), but it has its own Achilles heel, sensitivity to vibrations and airflows, and the need for clean acoustic channels of the speaker.
In Xiaomi devices, switching between sensor types often depends on the model's price segment.The flagship Mi and Xiaomi series more often use ultrasound or laser rangefinders, while the Redmi Note series can combine different solutions.
Here are the main differences in the form of the list:
- 🔊 Principle of operation: Ultrazuk uses sound waves, IR - light radiation.
- 📐 Location: Ultrazuk is hidden under the display, IR requires a hole or window in the case.
- 🌫️ Environmental influence: Ultrazuk is sensitive to dust in dynamics, IR - to glass contamination above the sensor.
- ⚡ Energy consumption: Ultrasonic modules often consume less power in standby mode.
Diagnosis and performance testing
The first step in case of a suspected malfunction is software diagnostics, and the MIUI shell has a powerful engineering menu that allows you to test all components of iron without installing third-party software, and you need to type a special code in the phone book to access it.
Use the command ##64663## to enter the CIT engineering menu. In the list that opens, look for the Proximity sensor or Approximation Sensor. When you click on it, you will run a test where you will be asked to cover the top of the phone with your hand.
If the sensor is in good condition, the screen value will change from 5.0 (or Near) to 0.0 (or Far) when the palm is approached, and it is important to check the response quickly and sharply, since the ultrasonic sensor may have little inertia compared to the optical sensor.
☑️ Testing the sensor in CIT
If the test values do not change or change randomly without touching, this indicates a software failure or physical damage to the plume. In some cases, a simple reboot of the device that resets the sensor drivers helps.
⚠️ Warning: Do not use powerful sound sources (columns, music) near your phone during the check. Acoustic vibrations can distort the readings of the ultrasound sensor, giving a false positive result.
Calibration of the proximity sensor
A common cause of malfunction is a downgraded calibration, which happens after a display change, firmware update, or software failure, and calibration allows the system to redetermine the "baseline" of the absence of obstacles and the level of the near object.
The procedure is also done through the CIT engineering menu, and you select the Proximity sensor and press the Calibration button, and you'll be asked to remove all items from the top of the phone, and then tightly close the sensor, and the system will record the boundary values.
In some versions of HyperOS, the path may be different. Try to go to Settings → About Phone → All settings and quickly press 7-10 times on the Kernel Version or MIUI Version to activate the hidden calibration menu if the standard one is not available.
For advanced users, a calibration method is available through ADB-It requires root rights and caution. Standard calibration through CIT 95% of the time, it solves the problem of a “perpetually burning” or “eternally extinguishing” screen.