Budget smartphone owners often face a situation where the screen of the device goes out during a conversation or, conversely, does not turn off when the phone is held to the ear. This is a classic symptom of incorrect operation of the proximity sensor. On the Xiaomi Redmi 9A model, this component plays a critical role in the comfort of operation, preventing accidental cheek presses and saving battery power. Understanding where it is located will help you avoid common errors when sticking protective glasses.
In this article, we will discuss in detail the physical location of the sensor, methods of its software calibration and ways to eliminate hardware failures. You will learn why the protective film can block the device and how to independently conduct diagnostics without going to the service center. On the Redmi 9A, the sensor is combined with the front camera in the notch of the screen, which makes it especially sensitive to contamination and improper installation of accessories.
A design review will show that Xiaomi engineers used modular assembly, where the sensor is integrated into the display plume, which means that mechanical damage to the screen often causes failure of the sensor, but in most cases the problem is software-based or external factors that can be easily eliminated at home with basic knowledge.
Physical location of the sensor on the body
To start with, we'll determine the exact location of the component. Unlike the old, wide-frame models, where the sensor was visible as a separate black window, in the Redmi 9A, it's hidden under the display. Physically, it's at the top of the screen, right next to the front camera module. If you look at the display turned off in bright light, you can see a small circular hole or area in the upper central or slightly displaced part of the bangs.
The infrared sensor design involves a emitter and a receiver. The emitter emits an invisible beam that bounces off objects (like your ear) and returns to the receiver. The distance between these elements is minimal, so they are often combined into one compact module, which is why the area at the top of the screen is critical for the normal operation of the phone during calls.
It's important to understand that any obstacle in this area can disrupt the system, and even a microscopic layer of glue from a poor-quality protective glass can scatter the infrared beam, making the phone "think" it's pressed against the ear, so when you replace the glass or film, you need to carefully clean the area around the front camera and the sensor itself.
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Use only high-quality protective glass with camera cutouts and sensors, or make sure that the adhesive base of the glass does not overlap the upper central part of the display.
The location of the component also dictates the rules of wearing the smartphone. If you hold the phone by the upper end, your finger can block the sensor area, in which case the screen will go out immediately after the call begins, the habit of holding the Redmi 9A by the bottom or middle of the body will help avoid false positives.
Design features and principle of work
Technically, the proximity sensor in Xiaomi Redmi 9A is an optoelectronic device that operates in the infrared spectrum, making it invisible to the human eye. The principle of operation is based on measuring the time of return of the reflected signal or the intensity of reflection. When an object approaches closer than 2-3 centimeters, the system receives a signal to lock the screen.
Modern builds, such as this one, often use virtual sensor technology or function combinations, but Redmi 9A most often uses a classic hardware module connected to the motherboard via a plume, which provides high accuracy, but makes the system vulnerable to software driver failures.
- 📡 Emitter: generates an IR ray of a certain frequency, safe for the eyes.
- 📡 Receiver: captures reflected light and converts it into an electrical signal.
- 📡 Controller: Processes data and sends a command to the system to turn the display on or off.
The MIUI shell feature is software-based filtering of readings, which ignores short-term light changes so that the screen doesn't blink when a fly passes or when the angle of incidence changes, but if the calibration is downed, this algorithm may not work correctly, mistaking the normal position of the phone for locking.
Why can't the sensor see through thick glass?
Software calibration through the engineering menu
If the sensor is physically functional but it's acting strange, it needs software setup. In Xiaomi smartphones, there's a hidden engineering menu, it's accessed through a standard dialing interface, you don't need root rights or a computer connection, it's done by the system itself.
To enter the test mode, you need to dial a special command. Open the Phone app and enter the code ##6484##. Immediately after entering the last digit, the CIT menu will open. Here are the tests of all components of the device.
☑️ Algorithm of sensor calibration
In the window that opens, you'll see two values: current distance and status (Near/Far). Cover the top of the screen with your palm - the value should change by 0.0, and the status on Near. Remove your hand - the value should rise (usually 5.0), and the status should change to Far. If the readings do not change, press the Calibrate button and repeat the procedure with the sensor closed.
After successful calibration, be sure to fully restart the device, so that the new thresholds are written to the controller's nonvolatile memory. Without rebooting, the system can continue to use old, erroneous data, and the problem persists.
Diagnostics and elimination of hardware problems
When software methods don't work, you might suspect a physical malfunction. On the Redmi 9A, a common cause is the plume's drifting away after falling or getting moisture. Even if the phone looks whole, a micro-crack in the contact group can interrupt the signal, requiring disassembly of the device and visual inspection of the internal components.
It's also worth checking for foreign objects in the upper speaker and camera area. Dust, pocket pile, or glue residue from an old film can create a permanent shadow for the sensor. Careful cleaning with a soft brush and compressed air often returns performance without replacing parts.
| Symptoms. | Probable cause | Method of decision |
|---|---|---|
| The screen doesn't go out when you ring. | Sensor contamination or software failure | Cleaning the top of the screen, calibration |
| The screen goes out immediately after the set | Permanent locking of the sensor | Checking of protective glass, calibration |
| Sensor doesn't respond to arm | Breakage or failure of the plume | Replacement of sensor or plume module |
If you find that the sensor doesn't respond even after calibration and cleaning, the module itself may have failed. At Xiaomi, it often comes assembled with a charging or display plume, depending on the specific revision of the board. Replacement requires fine electronics skills and heating tools.
Influence of accessories and protective glasses
Statistics from service centers show that more than 60 percent of sensor problems on the Redmi 9A are caused by improperly selected security accessories. Cheap glass often has a solid black frame or a layer of glue that overlaps the sensor's work area. Visually, this may be invisible, but for an IR beam it is an insurmountable barrier.
When choosing protection, pay attention to the presence of a special cutout or transparent zone in the area of the front camera and speaker. If you have already glued the glass and encountered problems, try to carefully remove it and check the sensor without it. If the problem disappears, the cause is found.
⚠️ Attention: Some users try to cut the hole in the protective glass themselves.This can lead to chipping at the edges and a decrease in protective properties.
High-sided cases can also interfere if they cover the upper edge of the body. Make sure the case does not press on the screen area and does not create shadow when you hold the phone to your head. Sometimes just move your finger a little or change the grip of the device.
Resetting and system update
In rare cases, the cause of a failure is a conflict of system files or an error after the MIUI update. If the calibration does not help, you can try resetting to factory settings. Before you do this, be sure to back up important data, as all files will be deleted.
Go to Settings → About Phone → Reset. Select “Erase All Data.” Once reset, the phone will return to its original state and the sensor drivers will be reinstalled, often addressing problems that have arisen after a failed firmware update.
Also check for system updates. Xiaomi engineers regularly release patches that fix sensors. Go to Settings → About Phone → MIUI version and click “Check Updates.” If a new version is available, install it.
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Integrated approach: cleaning, proper protective glass and calibration solve 90% of the problems with the proximity sensor without the need for repair.