Use infrared proximity sensor to extend the playback time of true wireless headphones

The new generation of young consumers has changed their audiovisual usage habits for decades. Earphones were only worn when needed, but now the introduction of true wireless (TWS) earplugs has changed this habit: now, users can’t listen even if they don’t listen. Will wear earplugs all the time, just like people always wear watches, and TWS earplugs are very comfortable, convenient and unobtrusive.

The new generation of young consumers has changed their audiovisual usage habits for decades. Earphones were only worn when needed, but now the introduction of true wireless (TWS) earplugs has changed this habit: now, users can’t listen even if they don’t listen. Will wear earplugs all the time, just like people always wear watches, and TWS earplugs are very comfortable, convenient and unobtrusive.

Industry analysts predict that by 2023, the market will grow at a compound annual growth rate of 27%. By then, TWS is expected to surpass all other types of wireless and wired headsets in sales.

Faced with such rapid growth, earplug manufacturers are bound to face fierce competition, and the choice of consumer products will be affected by important parameters such as audio quality, comfort and reliability.

Another crucial factor will be battery life to maintain longer usage time. One way to reduce power consumption is to ensure that the earbuds automatically stop playing after being removed from the ear and turn on again when inserted into the ear.

This requires short-range proximity sensing. In mobile phones, an infrared (IR) proximity sensor module detects when the phone is held on the user’s face during a voice call, so that the Display can be turned off. The following article describes how this technology adapts to TWS earplugs in a small space, and how to reliably detect whether the earplug is inside or outside the ear.

The working principle of infrared proximity detection

The basic operation of the infrared proximity sensor is shown in Figure 1.

Use infrared proximity sensor to extend the playback time of true wireless headphones

Figure 1: IR proximity sensor detects light reflected by nearby objects

It contains two main components:

An invisible infrared emission source that emits modulated light pulses. Ideally, the emitted power should be concentrated in a narrow band.

A photodiode (light sensor) with peak sensitivity at a wavelength that matches the peak intensity of the emitter.

By strictly controlling the operating wavelength of the system and modulating the pulse, the sensor system can be protected from noise. Noise mainly includes interference from external infrared energy sources (such as sunlight) and internal reflection (crosstalk) from the module housing to the optical system. other parts. When the emitted infrared light hits a target within the range, it will be reflected on the photodiode, which converts the measured infrared energy into a digital value, which increases proportionally as the target approaches.

In TWS earplugs, the proximity sensor is usually configured to trigger a detection signal when an object (in this case, the user’s ears open) is within 3 mm, and to release the signal when the closest object is within 10 mm. . Reliable proximity detection requires sufficient signal-to-noise ratio (SNR). In order to determine the SNR, the manufacturer needs to calculate the difference between the detection threshold and the release threshold count divided by the baseline jitter value (when there is no object in the range):

(Average detection count value) C (Average release count value)
(Jitter count value)

Generally, when this ratio is> 4, the SNR is considered acceptable.

Why every mW is important

Proximity sensors can reduce power consumption by detecting when the earbuds are removed from the ears to enter standby mode, but the sensor itself consumes energy: most of the energy consumption of the sensor is attributed to the infrared transmitter. Fortunately, earplug designers can use one of two technologies to limit the power consumption of the sensor. The first is to control the transmission cycle. In the integrated proximity sensor module TMD2635 of ams, the duty cycle configuration is easy to control (see Figure 2).

Use infrared proximity sensor to extend the playback time of true wireless headphones

Figure 2: Pulse timing of a single proximity event in TMD2635

The number of times the emitter is pulsed (PPULSE) and the duration of the effective drive current of each pulse (PPULSE_LEN) can be adjusted, and the power consumption is proportional to the number of pulses and the pulse length. The total time of a proximity measurement (PRATE) can be extended or shortened, which is the main method of controlling the duty cycle. The system designer can also introduce a waiting time (PWTIME) between the proximity measurement cycles.

The second method of controlling the duty cycle is through a signal generated at the application software level. Here, the host processor can be programmed to cycle the valid/invalid state of the sensor in a polling or interrupt-driven manner. The polling method allows the host MCU to accurately control the system timing. Here, the proximity sensor is usually in a static low-power state. The host microcontroller will periodically issue a command to wake up, perform a proximity measurement, and then return to a static state. In this polling mode, the designer can configure the optimal duty cycle that uses the least power while providing acceptable waiting time, that is, the delay between the user inserting/removing the earbud and the sensor detection event .

In the interrupt-driven method, the MCU wakes up the sensor, reads its previous samples, and then allows it to run freely. When the next data event occurs, the sensor sends an interrupt signal to the host, and then automatically enters the sleep state. The advantage of this interrupt-driven method is that the designer can choose which type of event will generate an interrupt signal. This allows the system to offload many tasks from the host firmware to the sensor. Since the CPU in the host consumes power, uninstalling can save power. Therefore, when TMD2635 performs its “sleep after interruption” function, it will automatically disable its internal oscillator and enter a low-power state.

The programmable threshold function of TMD2635 is particularly useful for triggering an interrupt when the adjacent data event falls outside the preset range between the high count threshold and the low count threshold. It can be set to only after the count repeatedly exceeds the threshold window multiple times Trigger, this function and other interrupt filtering functions are implemented in the hardware of TMD2635, thereby reducing the burden on the host processor.

It is worth noting that, compared with polling, the timing of the interrupt-driven mode is less deterministic, and the event-driven duty cycle will vary with the response time of the host processor and the number of adjacent events. Unless simplified presets are made, this variability makes accurate power calculations difficult, and benchmark testing is usually the best way to determine power consumption under dynamic operating conditions.

In interrupt-driven mode, the sensor spends most of its time in free-running idle mode, which usually consumes an average current of 30 µA, which consumes more power than polling, which usually only consumes 0.7 µA when the sensor is in sleep mode的current.

In the proximity detection system based on the module-based transmitter TMD2635, a low-power vertical cavity surface emitting laser (VCSEL) can bring further advantages. Most infrared proximity sensors have an LED emitter, but VCSELs can provide higher electro-optical conversion efficiency, which is usually ten times higher than that of LEDs. In addition, since the beam is very narrow and the viewing angle is only 1° to 5°, the light energy of all the emitters can be aimed at the target. As a result, compared with the equivalent LED-based sensor system, its total power consumption is significantly reduced, crosstalk interference is reduced, and SNR is higher.

save space

Compared with earlier devices, the latest IR proximity sensor module integrates VCSEL technology and has a substantial improvement in power consumption. Sensor manufacturers are also adjusting their product designs to fit the narrow spaces inside TWS earplugs while maintaining a high level of optical performance.

Figure 3 shows that the proximity sensor can take up very little space in the TWS earplug reference design developed by ams. The package size of the TMD2635 used in this design is 1mm x 2mm x 0.5mm (see Figure 4).

Use infrared proximity sensor to extend the playback time of true wireless headphones
Figure 3: TWS headset reference design based on the TMD2635 module

Use infrared proximity sensor to extend the playback time of true wireless headphones
Figure 4: TMD2635 proximity detection module is very small, occupying only 1mm3 volume

The biggest difficulty in manufacturing such a small device is the optical design: to ensure that the emitted and reflected beam has a clear path to and from the target, while limiting the impact of crosstalk on the photodiode measurement. In TMD2635, ams achieves this goal by combining component miniaturization, precise assembly and high-performance optical stacking (see Figure 5).

Use infrared proximity sensor to extend the playback time of true wireless headphones
Figure 5: Side view of TMD2635

The holes above the emitter and photodiode are covered by a polycarbonate material that is highly transparent to infrared light. The via can be round (1.5 mm in diameter) or oval (1 mm x 2 mm), allowing designers more flexibility when placing the sensor in the earplug housing.

in conclusion

The TMD2635 module combines configurable power management technology, efficient VCSEL transmitter and optical components, and now provides designers with a way to more easily integrate proximity sensing in a small space inside the earbuds, while providing Reliable detection of earphone position. The high optical efficiency and low sleep mode current of the module’s laser transmitter help keep the average power consumption at a very low level, thereby helping earplug manufacturers to extend the life of their products, even with batteries as small as 25 mAh.

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