How do electronic touch pads and switches work?

The new soft drink dispenser at a nearby store has touch pads that release soda as long as you are pressing on them. I noticed that if I press a pad with something other than my fingers (like a straw or car key) nothing happens, no matter how hard I press. Yet with my fingers, I sometimes don’t even have to make actual contact — just very close proximity. What is happening here? — RLB

Those touch pads are sensing your presence electronically, not mechanically. More specifically, electric charge on the pad pushes or pulls on electric charge on your finger and the pad’s electronics can tell that you are there by how charge on the pad reacts to charge on your finger.

Because your finger and your body conduct electricity, the pad’s electric charge is actually interacting with the electric charge on your entire body. In contrast, a straw is insulating, so the pad can only interact with charge at its tip, and while your car keys are conducting, they are too small to have the effect that your body has on that pad.

There are at least two ways for a pad and its electronics to sense your body and its electric charges. The first way is for the electronics to apply a rapidly alternating electric charge to the pad and to watch for the pad’s charge to interact with charge outside the pad (i.e., on your body). When the pad is by itself, the electronics can easily reverse the pad’s electric charge because that charge doesn’t interact with anything. But when your hand is near the pad or touching it, it’s much harder for the electronics to reverse the pad’s electric charge. If you’re touch the pad, the electronics has to reverse your charge, too, so the electronics sense a new sluggishness in the pad’s response to charge changes. Even when you’re not quite touching the pad, the electronics has some add difficulty reversing the pad’s charge. That’s because the pad’s charge causes your finger and body to become electrically polarized: charges opposite to those on the pad are attracted onto your finger from your body so that your finger becomes electrically charged opposite to the charge of the pad. When the electronics then tries to withdraw the charge from the pad in order to reverse the pad’s charge, your finger’s charge acts to make that withdrawal difficult. The electronics finds that it must struggle to reverse the pad’s charge even though you’re not in direct contact with the pad. Overall, your finger complicates the charge reversals whenever it’s near or touching the pad.

The second way for the pad’s electronics to sense your presence is to let your body act as an antenna for electromagnetic influences in the environment. We are awash in electric and magnetic fields of all sorts and the electric charge on your body is in ceaseless motion as a result. You’ve probably noticed that touching certain input wires of a stereo amplifier produces lots of noise in the speakers; that’s partly a result of the electromagnetic noise in our environment showing up as moving charge on your body. The little pad on the soda dispenser picks up a little of this electromagnetic noise all by itself. When you approach or touch the pad, however, you dramatically increase the amount of electromagnetic noise in the pad. The pad’s electronics easily detect that new noise.

In short, soda dispenser pads are really detecting large electrically conducting objects. Their ability to sense your finger even before it makes contact is important because they need to work when people are wearing gloves. I first encountered electrical touch sensors in elevators when I was a child and I loved to experiment with them. Conveniently, they’d light up when they detected something and there was no need to clean up spilled soda. We’d try triggering them with elbows and noses, and a whole variety of inanimate objects. They were already pretty good, but modern electronics has made touch pads even better. The touch switches used by some lamps and other appliances function in essentially the same way.

Do brownouts or other power outages damage appliances?

If a home looses some of its power during a power outage and the lights shine dim, will it burn up the motor in the refrigerator? Will it damage other appliances (TV, VCR. stereo. etc)? Should the main disconnect be shut off? — J, Ohio

Power outages come in a variety of types, one of which involves a substantial decrease in the voltage supplied to your home. The most obvious effect of this voltage decrease is the dimming of the incandescent lights, which is why it’s called a “brownout.” The filament of a lightbulb is poor conductor of electricity, so keeping an electric charge moving through it steadily requires a forward force. That forward force is provided by the voltage difference between the two wires: the one that delivers charges to the filament and the one that collects them back from the filament. As the household voltage decreases, so does the force on each charge in the filament. The current passing through the filament decreases and the filament receives less electric power. It glows dimly.

At the risk of telling you more than you ever want to know, I’ll point out that the filament behaves approximately according to Ohm’s law: the current that flows through it is proportional to the voltage difference between its two ends. The larger that voltage difference, the bigger the forces and the more current that flows. This ohmic behavior allows incandescent lightbulbs to survive decreases in voltage unscathed. They don’t, however, do well with increases in voltage, since they’ll then carry too much current and receive so much power that they’ll overheat and break. Voltage surges, not voltage decreases, are what kill lightbulbs.

The other appliances you mention are not ohmic devices and the currents that flow through them are not simply proportional to the voltage supplied to your home. Motors are a particularly interesting case; the average current a motor carries is related in a complicated way to how fast and how easily it’s spinning. A motor that’s turning effortlessly carries little average current and receives little electric power. But a motor that is struggling to turn, either because it has a heavy burden or because it can’t obtain enough electric power to overcome starting effects, will carry a great deal of average current. An overburdened or non-starting motor can become very hot because it’s wiring deals inefficiently with the large average current, and it can burn out. While I’ve never heard of a refrigerator motor dying during a brownout, it wouldn’t surprise me. I suspect that most appliance motors are protected by thermal sensors that turn them off temporarily whenever they overheat.

Modern electronic devices are also interesting with respect to voltage supply issues. Electronic devices operate on specific internal voltage differences, all of which are DC — direct current. Your home is supplied with AC — alternating current. The power adapters that transfer electric power from the home’s AC power to the device’s DC circuitry have evolved over the years. During a brownout, the older types of power adapters simply provide less voltage to the electronic devices, which misbehave in various ways, most of which are benign. You just want to turn them off because they’re not working properly. It’s just as if their batteries are worn out.

But the most modern and sophisticated adapters are nearly oblivious to the supply voltage. Many of them can tolerate brownouts without a hitch and they’ll keep the electronics working anyway. The power units for laptops are a case in point: they can take a whole range of input AC voltages because they prepare their DC output voltages using switching circuitry that adjusts for input voltage. They make few assumptions about what they’ll be plugged into and do their best to produce the DC power required by the laptop.

In short, the motors in your home won’t like the brownout, but they’re probably protected against the potential overheating problem. The electronic appliances will either misbehave benignly or ride out the brownout unperturbed. Once in a while, something will fail during a brownout. But I think that most of the damage is down during the return to normal after the brownout. The voltages bounce around wildly for a second or so as power is restored and those fluctuations can be pretty hard some devices. It’s probably worth turning off sensitive electronics once the brownout is underway because you don’t know what will happen on the way back to normal.