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Apple Watch's Wristful of Sensors and MEMS

Break out the champagne, MEMS and sensor makers, Apple just arrived at your party

4 min read
Apple Watch's Wristful of Sensors and MEMS
Photo: Justin Sullivan/Getty Images

Today was Apple’s big launch, as everybody who follows technology in anyway already knows (and probably tried to watch on the troubled live feed). Phones got bigger and the smart watch got smaller and finally comes in more than one size—including one tiny enough to fit on my too-small-for-the-Galaxy-Gear wrist.

But it’s what’s inside that little package that is really interesting—sensors, lots of them, along with a microelectromechanical system (MEMS) actuator that lets the watch reach out and touch you.

“This is an exciting day for people in the MEMS industry,” says Matthew Crowley, CEO of Vesper, a company that makes MEMS microphones. “By getting involved in the development of sensors, Apple validates that this is an important industry.”

The Apple Watch incorporates a heart monitor that uses two types of optical sensing systems, one type looking at visible light and one at infrared. This is an intriguing approach, Crowley said; many sports wearables use electrical conductivity to measure heart rate; Apple didn’t include this technology at all. And the optical technology Apple chose instead appears to be innovative; typical optical heart rate monitors look at infrared light only. These typical monitors, however, tend to have reliability issues, and work best when placed under the wrist, not on top, where you’ll most likely be wearing the watch, so it's not surprising that Apple was looking to improve on them. Crowley theorized that Apple has found a way of using visible light to improve the reliability of the sensor. Apps have been developed that use the normal iPhone camera to calculate heart rate through the changes in facial skin color, so it’s possible that this technology is coming into play as part of this system.

This means a whole new class of devices will use MEMS microphones. The volumes will be huge, driving prices down. It will change the industry.

The microphone in the Apple watch is certainly a MEMS device, Crowley says. He’s of course particularly excited about Apples' audio choices because he runs a microphone company. “This means a whole new class of devices will use microphones,” Crowley enthused. For microphones, along with all the other sensors Apple is putting into its watches and phones, Crowley said, “the volumes will be huge, driving prices down. It will change the industry.” While the Apple Watch includes just one microphone, future versions, Crowley said, will likely move to arrays of microphones. And companies will be in a race to make existing devices smaller, he said; the move of sensors into watches will make size a much more important factor.

The MEMS industry has even more to be excited about because it doesn't just make sensors; it also makes tiny actuators. And that’s the kind of device that’s going to give the Apple Watch its ability to “touch” the wearer, a technology that until now has been called haptics, but going forward will likely be tagged with Apple’s newly-coined “taptics.” (Haptics, a word taken from Greek, was just never going to catch on, it's about time someone came up with something better.) Crowley says that unlike the vibration systems on most of today's phones, which use a rotating mass, Apple’s watch appears to contain a linear resonant actuator, or perhaps a piezoelectric actuator. “There are a lot of new components on this watch,” he says, “many of which Apple seems to have invented itself.”

Apple also added one more sensor to the iPhone (not of its own invention)—a pressure sensor that can determine when someone changes elevation, running up a hill, or climbing a flight of stairs. Until now, Samsung was the only major mobile device maker to include a pressure sensor in some of its products. And while Apple’s presentation made it seem like the only need for a pressure sensor is to better track exercise, the indoor navigation community has got to be thrilled: It’s really hard to keep track of a map user inside a shopping mall if you don’t know what floor they’re on.

Reaction from a healthcare innovator

The Apple Watch introduction was also being closely followed by people looking to improve healthcare. “We’re suffering a tsunami of chronic disease,” says Joseph Smith, chief medical and science officer for West Health. “We spend 80 percent of every healthcare dollar managing people with chronic conditions. It would be great if Apple would get into that system and make it work better; with their amazing sense of what works for the customer they could clearly do that.”

The Apple Watch will be a highly versatile platform which may hold promise for applications to help as we transform healthcare to be more automatic, coordinated, and connected.

Smith appears to be cautiously optimistic about the Apple watch. On one hand, he says, “it doesn’t look like their transformational entry into the serious part of healthcare, it’s more of a best of breed of gadget rather than a new breed.”

However, he says, Apple today made it clear that the Apple Watch “will be a highly versatile platform which may hold promise for applications to help as we transform healthcare to be more automatic, coordinated, and connected.”

With all the sensors on the watch and smart developers bringing together information from multiple sensors, he says, we might get some surprises, say, monitoring tissue hydration using the available IR and natural light sensors on the watch.

Smith continues, “It remains to be seen whether Apple will transcend from the wearable gadget space to becoming a serious participant in healthcare.”

The Apple Watch is due to sell in 2015 for $350.

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
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A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay
Blue

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.

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