Never work on an AC-powered appliance when it has power applied. Simply turning the appliance off isn’t enough to be safe. If the appliance has a power cord, unplug it before you work on it. If it doesn’t have a power cord, turn off the power to the appliance by throwing the circuit breaker on your home’s electrical panel.
Take extra precautions when you’re working with your own AC circuits. In Book 4, I tell you more about working with AC circuits; I say more about AC safety then.
Even relatively small voltages can hurt you
Most of the projects in this book work with AA batteries, usually two or four of them tied together to produce a total of three or six volts. That’s not enough voltage to do serious harm. Even if you do get a shock with three or six volts, you will probably barely feel it.
However, it’s possible to injure yourself with voltages even as low as three or six volts. If you accidentally create a short circuit between the two poles of a battery, a lot of current will flow very fast. This will very likely cause the wire connecting the two ends of the battery to get very hot, and the battery itself may also heat up. The heat may be enough to inflict a nasty burn.
As a result of this danger, you should take the following precautions when working with the battery-powered circuits described in this book:
Don’t connect power to the circuit until the circuit is completely finished and you’ve reviewed your work to ensure that everything is connected properly.
Don’t leave your circuits unattended when they’re connected to power. Always remove the batteries before you walk away from your workbench.
Periodically touch the batteries with your finger to make sure they aren’t hot. If they’re getting warm, remove the batteries and recheck your circuit to make sure you haven’t made a wiring mistake.
If you smell anything burning, remove the batteries and recheck your circuit.STAYING SAFE BY STAYING DRYWe’ve all seen murders committed on TV crime dramas by throwing a plugged-in electrical appliance such as a hair dryer into a bathtub while the victim was taking a bath. I’ve always wondered how often that really happens, and how likely it’s fatal. For example, how quickly would the circuit breaker kick in and cut power to the hair dryer? Would the special GFCI-protection devices required in all modern bathrooms work as designed and cut power to the hair dryer in time?I’ve never wanted to conduct an experiment to actually find out — nor should you, under any circumstances. Water and electricity are a very bad combination because water is an excellent conductor of electricity, and it flows everywhere.Strictly speaking, pure uncontaminated water is actually an insulator. But pure water is very rare. Most water is filled with contaminates, and those contaminants turn the water into an excellent conductor. Thus, it’s true that you should avoid water when working with electrical current. Here are a few tips for staying safe by staying dry:Make sure the floor is dry. Don’t work on electronic or electrical devices in an area where the floor is wet.Beware of high humidity, especially if it condenses into moisture on your projects.Dry your hands before working with electrical current. Even a small amount of sweat on your hands can lower your body’s natural resistance and accentuate the danger of electrical shocks from lower voltages.
Always wear protective eyewear to protect yourself against exploding batteries. (Under the right circumstances, other components can explode as well!)
Sometimes voltage hides in unexpected places
One of the biggest shock risks in electronics comes from voltages that you didn’t expect to be present. It’s easy enough to keep your eye on the voltages that you know about, such as in your power supply or batteries, but some electronic circuits are designed to amplify voltages. So even though your circuit runs on 6-volt batteries, there may be much larger voltages at specific points within your circuit.
In addition, some electrical devices can actually store an electric charge long after the power from your circuit has been disconnected. The most notorious device with this characteristic is the capacitor, which alternately builds up and then releases electrical charges. Thus, you should be wary of any circuit that contains capacitors — especially if the capacitors are large. Common ceramic-disk capacitors, which are typically smaller than a tiddlywink, don’t store much charge. However, if your circuit has capacitors the size of batteries, you should be very careful when working around them. Such capacitors can hold large charges long after the power has been cut off.
Here are some safety points concerning capacitors:
One of the most common places to find large capacitors is in the power-supply circuit. Any electronic device that plugs into a household electrical outlet has a power-supply circuit that may contain a large capacitor. Be very careful around these capacitors. In fact, if the power-supply circuit is inside its own enclosed box, don’t open the box. Instead, replace the entire power supply if you suspect it’s bad.
Another common place to find high-voltage capacitors is in a flash camera. Even though the battery may be just 1.5 V, the capacitor that drives the flash unit may well be holding a charge of 300 V or more.
Before working on a circuit that contains a capacitor, always discharge the capacitor first. You can discharge small capacitors by shorting out their leads with the blade of a screwdriver. Make sure you touch only the insulated handle of the screwdriver while you short out the leads, and don’t touch any other part of the circuit with your free hand.
Larger capacitors should be discharged by connecting their leads to a lamp or a large resistor. The easiest way to do this is to wire up a lamp holder to a pair of alligator clips, screw a lamp into the lamp holder, then carefully connect the clips to the capacitor leads. If the capacitor is holding a charge, the lamp will glow for a moment as the capacitor discharges through the lamp.
If you don’t feel completely confident in what you’re doing where large capacitors are concerned, walk away from the project.
Other Ways to Stay Safe
Electric shock isn’t the only danger you’ll encounter when you work with electronics. The following paragraphs summarize a few of the other risks you may be exposed to and describes the precautions you should take to minimize those risks:
Soldering poses an obvious fire hazard. If your soldering iron is hot enough to melt solder, it’s also hot enough to ignite combustible materials such as paper, wire insulation, and so on. Therefore:Always be aware of when your soldering iron is on. Don’t plug it in until you need it, and unplug it when you’re finished soldering.Never set a hot soldering iron down directly on your workbench. Instead, get a soldering iron holder to safely hold the soldering iron while it’s hot. Figure 4-1 shows a soldering iron resting in a simple stand. As you can see, this stand keeps the business end of the soldering iron safely elevated away from the work surface.FIGURE 4-1: A soldering iron resting on a stand.Give your soldered joints a few minutes to cool down before you handle them.Watch out for the soldering iron’s electrical cord. Obviously, you want to avoid burning the cord with the soldering iron. As ridiculous as it sounds, I did this