In this video I am going to tear down a Bose 700 headphone to show the internal
construction and to study the touch sensor
Before I start, let me talk a bit about projected capacitive touch sensor
which is the type used the Bose headphone.
Projected capacitive sensors project electric field from the transmitter electrode,
and then measure at the receiver, the electric field reflected by the finger on top,
hence the name projected capacitive sensor.
This method of sensing when implemented with rows
and columns of scanning, can detect multiple touch with very high resolution,
and therefore it has become the standard technology for the 2D touchscreen and
trackpad.
Here I have a Bose 700 headphone and the touch interface is
also implemented with a projected capacitive sensor.
The main user interface of this headphone is swapping up and down
for the volume control.
Whether the swipe is fast or slow, the sensor can detect the gesture correctly.
This might not seem like a big deal but engineers would appreciate that it is
the high-resolution detection that make this possible.
Another user interface is swiping left and right, and to applications such as an audio player,
this can be used for the skip-forward backward function.
Also there is a double tap function for toggling between pause and play,
and a press and hold function for reading the battery life.
Now let me tear down this 300 USD headphone so that you don't have to tear down yours,
or even to buy one to do so.
This is the view of the compartment inside the headphone.
On the touch area we can see a FPC pasted on the casing.
With only traces and pads on the FPC, it is
obvious that this is the electrodes for sensing the touch.
The sensor IC is hidden underneath the main PCB so let me
remove the PCB and see what is beneath it.
okay it is a 28 pin QFP IC with a topmark IQS 5 7 2. From the datasheet on
the website IQS 572 is an 8-row by 9-column projected capacitive
trackpad controller, capable of sensing 2048 points by 1792 points.
Based on the application circuits and description, all that is needed is rows and
columns of diamond shape electrodes plus some passive components and the
IC will detect and report the XY coordinates of multiple touch to the host MCU.
From the actual implementation of this headphone, we see 4 rows and 9 columns of
electrode being used and the resolution is about two thousands by nine hundreds
which is more than enough for gesture detection.
At the back of the touch electrode we can is a mesh ground plane.
Ground plane is normally used for shielding purpose but excessive ground plane can also
reduce the sensitivity, so mesh structure is used instead of solid plane to provide
balance between shielding and sensitivity. Another interesting observation is that
the touch surface is the three-dimensional curved surface, and the
FPC is able to conform to the three-dimensional shape of the plastic
The length of the traces to the rows and columns are different and so is the
area of the diamond electrodes.
This will result in some differences in the raw signal when the same touch is applied to different locations.
Luckily as I understood it, the controller can adjust the threshold and the gain for each node
so that the overall sensitivity is consistent over the entire touch area.
While we are at it, let's take a look at other components in the headphone.
Here we see three MEMS microphones on this side of the
headphone and another three on the other side.
The microphone points towards the front and the back to pick up the ambient noise for the noise cancellation.
With six mics working together the array structure has a strong directivity
picking up the voice signal from the front and suppressing the ambient noise
from other direction.
The sampling and pre-processing of the audio signal is performed by the IC AD71158.
The main processor is a Qualcomm CSRA 68105 Bluetooth audio SOC.
This IC does the system control, reads the touch gesture, performs the signal processing,
sends and receive audio data through its built-in Bluetooth.
The battery is located inside the left headphone.
Now back to the touch sensor.
Projected capacitive touch sensors have a surprising characteristic:
the capacitance decreases as the finger gets nearer to the electrodes
and this is totally opposite to our intuition.
Although theoretically complicated, the application of projector capacitive
touch sensors has never been easier, thanks to the competitions resulted in
many vendors making modules available.
Thank you for watching, and as always please like, share and subscribe.