RFID + Camera + Lock = Smart Mailbox

New mailbox hardware applies to become your home's watchdog

3 min read
Interior view of smart mailbox with hardware components
The picture of the door shows the RFID reader (on the left) and the solenoid lock (on the right).
Photo: Jonathan Ross Tew

A self-locking mailbox could someday flag down delivery drones and intelligently screen your driveway for intruders.

Columbus State University computer scientist Lydia Ray presented the technology, called the ADDSMART project, during a 20 October session at the annual IEEE Ubiquitous Computing, Electronics, and Mobile Communication Conference in New York City.

The project aims to achieve two goals: clearly marking addresses for autonomous vehicles, and reducing the energy and data storage costs of home surveillance systems. An early prototype mailbox attachment suggests that the trick, in both cases, may be radio-frequency identification.

Powered by an Arduino Yun processor, one component of the ADDSMART device controls a high-frequency 13.56-MHz RFID reader, USB camera, passive-infrared motion sensor, solenoid lock, and an onboard Wi-Fi module. The second component is an RFID tag. 

Ray came up with the idea when she saw an Amazon ad for drones delivering packages. She wondered how that would be possible, as some of her regular mail still arrives at the wrong address.

In the United States, Amazon and Google and startups such as the Reno, Nev.–based Flirtey, are trying delivery via drones. One of a drone’s challenges is to home in on its destination. But accurately identifying addresses with standard GPS alone is really difficult, Ray says, because GPS uses latitude and longitude. The GPS sensor is good for identifying a location—but an additional system is needed for pinpointing a precise address.

Some approaches for tackling the location problem include computer vision techniques with cameras. Ray points out that even identifying addresses with human vision can be hard. At her house, the address is written on the pavement and “is not easily identifiable.” Then, Google Street View, which updates infrequently at best, doesn’t show that her neighbor’s house recently changed colors; and it wouldn’t even work so well for finding an address at night.

With an RFID tag on a home’s mailbox and an RFID reader on a drone or car, Ray believes that the delivery process could become relatively easy. The drone would use GPS to navigate to an address and then confirm that the address is correct by checking the RFID tag.

Once Ray decided to attach an RFID tag to a mailbox, she realized that RFID can do more than flag down drones: it offers security, too. An RFID-reader-equipped system could store a list of “safe” RFID tags whose possessors would be able to pass by a home or open the mailbox unimpeded. 

Instead of a home surveillance system continuously checking for intruders, a video camera could save energy by starting to record only when an unrecognized vehicle or person passes the mailbox. The mailbox could also unlock when authorized users—such as a homeowner or mail carrier—arrive.

After soldering and wiring the necessary hardware for the smart mailbox and writing computer scripts for running commands, Ray and her student, Jonathan Ross Tew, tested the sensors indoors and outdoors. 

When the motion sensor detected a change in passive infrared radiation—a type of electromagnetic radiation given off by anything warmer than about -270°C—the USB camera took a picture. Computer scripts sent the picture via email to a recipient and uploaded it to Dropbox.

Also, when an RFID tag was in the RFID reader’s limited detection range, the system checked whether the tag was marked with the homeowner ID or postman ID. In either case, it would open the solenoid lock, but the postman tag also triggered an email alert.

Ray said the passive-infrared motion sensor the team used pretty much failed outdoors—there were 931 false positives out of 937 tries. Using a more expensive sensor could help, she said.

Future work includes testing the system under various conditions and investigating the total area of surveillance coverage, technical interference with nearby smart mailboxes (like this Kickstarter project, Mr. Postman), security, and privacy.

So how will smart mailboxes flag down drones in apartment and condo complexes with cluster mailboxes? Ray told IEEE Spectrum that each individual mailbox could have its own RFID tag with an apartment number or post box number for flagging down drones. As for the surveillance function, Ray says a monitoring system similar to the one used in the prototype could monitor the door of each apartment.

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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.