A Q&A With a Maker of a Battery-Free Cellphone

IEEE Member Vamsi Talla helped design the device

3 min read
The battery-free phone, powered by ambient radio signals or light, can even actuate earphones.
Photo: Mark Stone/University of Washington

THE INSTITUTEIs the future for smartphones battery-free? That's one question The Institute asked IEEE Member Vamsi Talla, who recently finished building the first battery-free cellphone with fellow researchers at the University of Washington, in Seattle.

The project is supported by a Google Faculty Research award in addition to three U.S. National Science Foundation grants—which totaled more than US $2 million.

The team began working on the phone last year while Talla was earning his Ph.D. in electrical engineering, which he completed this year. He is now the CTO of Jeeva Wireless, a Wi-Fi technology company in Seattle.

Right now the phone only can make voice calls and dial an emergency operator. Talla says the phone could be especially useful for people living in developing countries who don't have a way to charge a phone.

In this interview, Talla shares why his team is building the battery-free phone and what the future is for the technology.

How does the battery-free phone work?

The phone receives power from sunlight or RF waves sent from a nearby base station, a fixed point of communication for customer cellular phones on a carrier network. With a technique called backscattering, the phone can make a voice call by modifying and reflecting the same waves back to the base station.

We also were able to make Skype voice calls, proving that the prototype—made of commercial, off-the-shelf components—can communicate with a base station and applications like Skype. The phone consumes only 3 microwatts of power—which is about 10,000 times less than what a current smartphone consumes.

What are the benefits to the phone being battery-free?

For one, users can dial the number for the emergency operator without any battery power. In fact, you can call anyone with zero power. This technology is especially useful in developing countries, where there aren't many reliable sources to charge a device.

We created our phone with a simplistic design and made it significantly cheaper than current smartphones on the market—which means it could be accessible to more people.

What we're working on next is enabling wireless and text communication in current smartphone when their battery dies.

Is the future for smartphones battery-free?

We believe so. Cellular towers wouldn't require any hardware changes. Networks would have to rely on different protocols and update their back-end software—which they do regularly anyhow.

The phone would also require a chip to provide the functionality of sending communications without battery power. The chip would be a special-purpose IC placed on a piece of silicon.

Then any phone manufacturer that wants to include a “battery-free cellphone mode" could buy and integrate that chip into its design.

How much would the phone cost?

The current prototype isn't being mass-produced, so we don't have a fair indication of the cost. However, when we produce them in large volumes, it has the potential to be extremely affordable, maybe even less than $1 to manufacture. It is very hard to forecast at this early stage.

Have there been any setbacks while building the phone?

Something we found challenging was implementing our own base station using an extensive software-defined radio called Gnu. It cost about $3,500. Software-defined networks decouple hardware from software and execute such software, not necessarily in the equipment but either in the cloud or in clusters of distributed IT servers.

It's not the easiest software to work with. I would say that was the biggest roadblock. Implementing the base station required most of our time.

We struggled because we're essentially taking the smart technology away from the phone and simplifying it—which puts all of the complexity on the base station.

What enhancements do you plan to add?

We want to improve the operating range and audio quality. Additionally, it's only capable of making voice calls—which we'd like to change.

We're working on adding a camera and an E Ink display—a high-visibility and high-contrast display technology with low-power requirements. These technologies would allow the phone to have smartphone capabilities.

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
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.

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