Wearable Tech Could Help Track Gun Violence

Accelerometers like those used in fitness trackers can tell when the wearer has fired a gun, and could be used to report incidents to law enforcement

3 min read
Hand holding a gun
Wearable accelerometers can pinpoint when a user fires a gun
Image: Randi Klett

Some convicted criminals released on parole or probation are required to wear electronic monitoring devices so that police officers and court officials can track their movements. Despite these precautions, individuals serving their sentences in the community are still responsible for almost half of the incidents of gun violence prosecuted in the United States, says University of Pennsylvania criminology professor Charles Loeffler. Adding existing technology to current monitoring devices, though, could help deter these shooters by recording and reporting when they fire a gun.

In a paper released last week in the online journal PLOS ONE, Loeffler reported that wearable accelerometers, like those commonly used to track the distance logged by joggers, could also be used to track when someone fired a gun. Shooting a handgun, it turns out, forms a hard to miss pattern on accelerometer readouts.

“A gunshot is pretty distinctive,” Loeffler tells IEEE Spectrum. “You're typically at rest because you’re trying to aim, and in a split second, your hand, wrist, and arm experience an impulsive transfer of energy.”

To learn how the accelerometer readings of a handgun being fired differ from those associated with other activities, Loeffler recruited ten officers from the University of Pennsylvania police department. The officer tested a variety of brands and calibers of firearms, from a .22 caliber revolver to a semi-automatic Colt .45, while wearing accelerometers on their wrists. Loeffler then compared that test data against samples from people wearing the same device while going about their normal days, and to others using it while performing heavy construction work, including operating a .22 caliber gunpowder-driven nailgun.

Despite being driven by the same charge as a small handgun, the nailgun is a good example of how firearms can be easily recognized by an accelerometer. “In a nailgun, the .22 charge is not used to propel a bullet, but a piston,” Loeffler says. “A piston hitting the nail looks a lot different in the data because of the mechanics of how those forces are transmitted.”

Loeffler’s tests showed that gun use was easily identifiable by a few factors. First off, the accelerometer can detect the muzzle blast from a firearm produced when the rapidly expanding gases inside a gun barrel meet the air outside of it. Accelerometers were also good for detecting the recoil of that released energy traveling up the subject’s arm, as well as the lift produced when the bullet leaves the barrel. These three factors, taken together, comprise the signature of a gunshot in an accelerometer reading, no matter the make or caliber of the weapon.

Loeffler’s data shows that signature is hard to mistake. Of 357 gunshots tested in the study, just three were mistakenly identified as something else. The technique also gives very few false positives—just three of the 693 other instances of accelerometer activity tracked in the study were misidentified as gunshots. Further fine-tuning of the technique even has the potential to distinguish between the accelerometer signatures created by different calibers of firearms.

Combined with sensors that track parolees via GPS, accelerometers could be used to alert police departments immediately when a person wearing a wrist monitor fired a gun. That would save authorities the time and energy involved in cross-referencing the location of a reported gun shot with the whereabouts of the many monitors they track. That would give police a clearer picture of whether or not a person being monitored was at the scene of a gun crime, and could potentially deter people being monitored from firing in the first place, cutting down on gun crime.

Loeffler is working with colleagues in the Penn engineering department to explore the feasibility of working accelerometer technology into existing monitoring devices. They don’t foresee much trouble getting a test version designed, as both monitors and accelerometers are already designed to be small, wearable, and easy on power consumption. The bigger challenge, he says, may be getting courts and police departments to adopt the technology if it is made available to them.

“Getting departments to adopt [this technology] would really depend on how much value they perceive from this offering,” Loeffler says. “It will be more expensive than doing business as usual. The most likely places to deploy something like this are those that are dealing with a more pronounced gun violence problem, and where there is good integration between the agencies involved, the courts and police.”

This post has been updated to reflect differences the number of gun crimes reported and the number prosecuted.

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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

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.