Xiaomi smartphone owners often face a situation where the screen does not go out during a phone conversation or, conversely, turns off when the device is held to the ear. In the Xiaomi Redmi 9T model, this problem is solved not by finding a separate hole on the front panel, but by understanding the technology of the virtual sensor. The company's engineers have implemented a solution based on ultrasonic waves, which allows you to abandon the extra hole in the display and increase the moisture protection of the case.
Physically, you won't find a classic infrared emitter above the screen or next to a conversational speaker, as you did in the old models. The whole system is hidden under the display glass at the top of the smartphone. It's this design feature that often leads users to look for non-existent black dots on the screen frame. Understanding the way an ultrasonic sensor works is the first step to successful setup.
Despite the lack of visible elements, the system works fairly accurately if it is properly calibrated. In most cases, the failures are caused by software errors or contamination of the top of the screen, rather than a physical breakdown. Next, we will discuss in detail how this technology works, where it is located, and how to return it to work correctly without going to the service center.
⚠️ Warning: Do not try to find the sensor with a flashlight or a magnifying glass on the off screen – in the Redmi model 9T It has no visible appearance in the form of a separate window.
The principle of operation of the ultrasonic sensor in the Redmi 9T
The technology used in the Xiaomi Redmi 9T is very different from the usual infrared analogues, it uses ultrasonic echolocation. A special emitter hidden under the display generates high-frequency sound waves that are not audible to the human ear. When you hold the phone to your head or hand, the waves bounce off the object and return to the receiver.
The system analyzes the signal's time of return and intensity by calculating the distance to the obstacle. If the object is closer to a certain threshold, the software tells the screen to go out. This prevents accidental cheek pressing during a conversation, the advantage of this approach is that it can completely seal the top of the body.
But ultrasonic technology has its own peculiarities: it's sensitive to the thickness of the protective glass and the quality of the stickers; if you use a cheap, poorly fit glass at the top, it can distort the signal; and it can also be affected by strong vibrations or extreme temperature changes that change the density of air inside the case.
Why Xiaomi has abandoned the IR sensors?
Exact location of hidden sensor
Although the sensor is invisible, its maximum sensitivity is clearly defined by the design of the smartphone. In the Xiaomi Redmi 9T, the active area where the radiation and reception of ultrasound occurs is located at the top of the display. It is located directly above the Mi logo or in the area of the conversational speaker, if you look at the front panel.
It's important to understand that the sensor is not a pinhead-sized point element, it's a complex system that covers a specific area under the glass. The most critical area is the central top of the front camera, and that's the area that needs to be as free as possible from foreign objects to work properly.
If you use a case with high sideways or a protective glass that goes too far on the curved edges (although the 9T screen is flat, some glass has a black frame), this can cause interference.
- 📍 Area of responsibility: upper third of the screen in the center.
- 📍 Neighboring elements: front camera and talk speaker grid.
- 📍 Accessories: Thick protective glass can reduce sensitivity.
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The proximity sensor in Redmi 9T is not a separate detail on the surface, but a module integrated under the display at the top of the central part of the smartphone.
Diagnostics of performance through the engineering menu
Before you can carry a phone for repair or try to disassemble it, you need to run a software diagnostic, and there's a special engineering menu in the MIUI shell that allows you to check all the sensors, and it's the most reliable way to tell if the phone is seeing objects coming or if the problem is deeper.
You don't need root rights or a computer connection to enter the test mode, just type in a special code in the Phone app. Once you open the menu, select the item that checks the sensors. In the list of parameters, look for the Proximity Sensor value.
In the test, the system will ask you to cover the top of the screen with your hand, and if the numbers change (usually from 5.0 to 0.0 or vice versa, depending on the firmware version), then the physical part is in good shape. If the values do not change even when you press your hand tightly, there may be a software failure or a plume has gone away inside the case.
##6484##Type this code in the call app to open the CIT menu. Select "Proximity sensor" and follow the instructions on the screen. If the test fails, the system will prompt the recalibrate sensor itself, a step that often solves the problem of false positives.
☑️ Sensor diagnostics
Calibration of the sensor: step-by-step instructions
If the diagnostics show that the sensor is responding but not correctly (for example, the screen only goes out when the finger is pressed close, not at a distance of 2 cm), calibration is required. This is the process of adjusting sensitivity thresholds. In Xiaomi Redmi 9T, this can be done through the same CIT engineering menu.
When you're in the proximity sensor test, press Calibration. Put your phone on a flat surface away from foreign objects. Press Start or Do it. At that point, you can't touch the screen or shake the device. The system counts the reference values "far" and "close."
Once the procedure is complete, the phone can automatically restart. If the problem persists after calibration, try to clear the phone and Google Play apps, as they also affect the processing of sensor signals, sometimes resetting all settings to factory settings helps, but this is a last resort.
⚠️ Attention: During the calibration process, it is strictly forbidden to touch the screen or move the phone, otherwise the read data will be incorrect, and the problem will worsen.
Influence of protective glasses and accessories
One of the most common causes of poor ultrasonic sensor performance in the Redmi 9T is poor quality protective films or glass. Ultrasonic waves do not pass through the air layers. If the protective glass at the top does not fit tightly, an air cushion is formed that reflects the signal ahead of time.
Also, cases with high sides that physically overlap the top of the screen can create a problem. Even if the side is transparent, it can create an acoustic shadow or reflect waves at the wrong angle. Owners of this model are advised to use cases with a notch under the speaker or those that do not go to the front panel.
If you recently put on a new security glass and the sensor stopped working immediately after, the reason is almost certainly this: Try to carefully remove the glass and check the sensor's performance. If without the glass everything works perfectly, replace the accessory with a better one, with full adhesion throughout the area.
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Use full-glue protective glass to avoid air pockets in the sensor area.
Table of the main symptoms and solutions
To navigate quickly, it's convenient to use a pivot table to match the observed behavior of the smartphone with the most likely cause and method of troubleshooting, and not all symptoms require complex repairs.
| Symptoms. | Probable cause | Method of decision |
|---|---|---|
| The screen doesn't go out when you talk. | Upper contamination or software failure | Wipe the screen, perform calibration in CIT |
| The screen goes out in your pocket | High sensitivity or cover | Remove the case, check the settings "Pocket mode" |
| The sensor doesn't respond at all. | Trail or module failure | Reconnect the plume, replace the sensor |
| It only works in the call app. | Phone app error | Clear the app cache, update Google Phone |
Hardware repair and replacement of the module
If software methods, calibration and replacement of accessories have not helped, there is likely a hardware malfunction.In the Xiaomi Redmi 9T, the proximity sensor is often integrated into the charging port plume or into a separate small module under the display, depending on the board revision. Self-replacement requires skills with small electronics.
To access the insides, you need to remove the back cover (it is glued), unscrew the frame screws and gently disconnect the display and battery plumes. The main difficulty is that the ultrasonic sensor can be soldered on the main board or be part of a complex plume. In the Redmi 9T, the sensor is often part of the bottom plume with a charging connector, which makes it easier to replace.
When replacing, it is important to use original components or quality analogues. Cheap Chinese plumes often have incorrect characteristics of the ultrasonic emitter, which leads to a recurrence of the problem after a week of use.
How to check if the sensor plume has moved away?
Can I close the sensor if it interferes?
Does the reboot affect the sensor?
To sum up, finding the proximity sensor in the Xiaomi Redmi 9T is about finding the invisible. Understanding that it's hidden under the screen and works at ultrasound helps you get the problem right. In most cases, it's enough to calibrate and replace the poor-quality protective glass to make the device work like new again.