Teardown Tuesday: WorkTunes Hearing Protection HeadphonesMay 09, 2017 by Nick Davis
In this teardown, we examine the guts of 3M's hearing protection device which comes with a built-in AM/FM radio and is capable of playing music from your MP3 player.
These headphones are designed to protect hearing against sounds up to 22 decibels. They also double as an AM/FM radio and have voice-assist technology. This Teardown Tuesday takes a look inside!
At a Glance
This hearing protection headset, offered by 3M, helps reduce exposure to loud sounds produced when using power tools, lawn mowers, garden equipment, and other gas-powered tools. The image below, taken from 3M's User Guide, shows a summary of what this hearing protection offers.
Summary of 3M's WorkTunes Hearing Protector. Image courtesy of 3M.
Disassembling the Outer Components
This hearing protection device can be taken apart relatively quickly in less than five minutes—assuming you have the correct tool. The only special tool required is a torx bit, size T8.
The first disassembly step involves unclipping the over-the-head attachment from the ear protection "cups". A moderate opposing force between the two pieces forces them to separate. The two red boxes, in the figure below, shows the disconnection.
Over-the-head attachment unclipped.
Next, remove the soft ear padding. A gentle pull on the padding releases it.
Once the padding is removed, take away the plastic retaining fixture which the ear padding attaches to, and then finally remove the two foam pieces. After the foam is removed, the speaker, itself, along with two torx screws, are visible. A light pull on the speaker releases it. See the two figures below.
Foam ear padding, retaining fixture, and foam padding removed, revealing speaker and torx screws.
Speaker dislodged and torx screws removed.
Small PCB Located Inside the Battery-Side Enclosure
Once the torx screws are removed, the black outer pieces can be separated from the yellow pieces. After removing the battery-side enclosure, a small PCB is revealed. See figures below.
A small PCB is contained within the battery-side enclosure.
As noted in the figure above, there's a small foil strip which serves as a countermeasure to EMI (electromagnetic interference).
Also, although not noticeable in these pictures, the PCB is covered with a conformal coating which protects it from water, humidity, dust, and other debris.
After disconnecting the two cables from the PCB and removing two more torx screws, the PCB can be separated from the enclosure assembly. The figure below includes views of both the top side and bottom side as well as various components.
Small PCB components.
The voltage regulator is a fixed-voltage switching regulator that utilizes the inductor and capacitor shown in the picture above. A switching voltage regulator, as opposed to an LDO (low-dropout) regulator, is ideal in this type of battery-operated application because switching regulators are much more efficient than LDO regulators.
Larger PCB Located Inside the Radio-Electronics Enclosure
The focus now turns to the enclosure containing the radio electronics.
Like before, with the battery enclosure, after removing the two torx screws the black enclosure can be removed which contains a larger PCB. Besides the PCB itself, the other major components are the two antennas: one for AM and one for FM. The FM antenna is a whip-type antenna. The AM antenna, in contrast, is the type of a ferrite rod with a thin wire wound around it. The two antennas are required because AM and FM broadcast on different carrier frequencies: AM uses a carrier frequency range from 540 to 1600 kHz, while FM uses 88.1 to 108.1 MHz.
As with the other PCB, this PCB has a conformal coating applied to it for protection.
The PCB containing the radio electronics.
Antennas connected to PCB.
The PCB can be lifted from the enclosure after removing three more torx screws, the two cables, and the two control knobs (on the opposite side of the enclosure)—they simply pull off their metal shafts.
The two control knobs have been pulled off their shafts.
The two figures below show the major components of the design, including:
- AM/FM receiver IC (IC marking KT092304)—neither a datasheet nor a manufacturer could be located for this part
- Low voltage mono/stereo power amplifier (IC marking TDA7050T)
- This low voltage audio amplifier is used to strengthen the low-power signal—from the AM/FM Receiver IC—prior to it being sent to the headphones.
- Two-wire serial EEPROM (IC marking FT24C02A)
- This I2C EEPROM (electrically erasable programmable read-only memory) is where the manufacture's propriety code resides. It is connected to/communicates with the AM/FM receiver.
- Volume control (10k-ohm potentiometer: CTR A10K)
- The volume is controlled by adjusting the potentiometer in a voltage-divider circuit.
- Two 32.768 kHz crystals
- Note: 32.768 kHz frequency is often used in RTC (real-time clocks) circuits because it is a power of 2 value (215). In otherwords, the frequency is exactly 215 cycles per second. Therefore, this frequency can provide a precise 1-second period (1 Hz frequency) by using a 15 stage binary counter (1000 0000 0000 0000 binary = 32768 decimal).
- Two push buttons
- Radio mechanical tuner
PCB containing radio electronics (top side).
PCB containing radio electronics (bottom side).
This hearing protection from 3M, with a built-in radio and MP3-compatible, looks to be a well-built design with simplicity in mind. I'm a big fan of the conformal coating on both PCBs, but this may be the norm for similar radio-enabled hearing protection devices. The two knobs and three buttons (the radio tuning knob serves as a push button as well) work easily and smoothly. The potting (epoxy) appears to serve its purpose well—I believe the potted part(s) and/or PCB would be damaged, if not destroyed, if the removal of the potting was attempted.
The EMI foil looks to be a cheap band-aid fix for EMI problems after the design was completed, but this is a guess.
Side note: Surprisingly, and unfortunately, many experienced engineers (and far many more project managers) seem to overlook the addressing of EMI concerns until their devices fail EMI testing, which can prove to be hugely expensive or simply a show-stopper. In my personal experience, the project managers who insist on the consideration of EMI countermeasures at the beginning of a project most likely learned this lesson the hard way—by being burned in the past.
Also, it should be noted that the headphones do not function as regular headphones when the batteries go dead. In other words, when the batteries are exhausted your externally connected MP3 player will not power the headphones.
All in all, this device looks to be a nice design, especially for its low-price.
Next Teardown: Qi-Compatible Samsung Wireless Charger