Make: Electronics
Page 2
Fuses
Automotive-style, mini-blade type, 3 amps. Quantity: 3. RadioShack part number 270-1089, or Bussmann part ATM-3, available from automotive parts suppliers such as AutoZone (Figure 1-12).
Figure 1-12. A 3-amp fuse intended primarily for automotive use, shown here larger than actual size.
Or similar. A blade-type fuse is easier to grip with alligator clips than a round cartridge fuse.
Potentiometers
Panel-mount, single-turn, 2K linear, 0.1 watt minimum. Quantity: 2. Alpha part RV170F-10-15R1-B23 or BI Technologies part P160KNPD-2QC25B2K, from Mouser.com or other component suppliers (Figure 1-13).
Figure 1-13. Potentiometers come in many shapes and sizes, with different lengths of shafts intended for different types of knobs. For our purposes, any style will do, but the larger-sized ones are easier to play with.
Or similar. The “watt” rating tells you how much power this component can handle. You don’t need more than 0.5 watts.
Resistors
Assortment 1/4-watt minimum, various values but must include 470 ohms, 1K, and 2K or 2.2K. Quantity: at least 100. RadioShack part number 271-312.
Or search eBay for “resistor assorted.”
Light-emitting diodes (LEDs)
Any size or color (Figures 1-14 and 1-15). Quantity: 10. RadioShack part number 276-1622 or All Spectrum Electronics part K/LED1 from Mouser.com.
Or similar. Just about any LEDs will do for these first experiments.
Figure 1-14. Typical 5-mm diameter light-emitting diode (LED).
Figure 1-15. Jumbo-sized LED (1 cm diameter) is not necessarily brighter or more expensive. For most of the experiments in this book, buy whatever LEDs you like the look of.
Experiment 1: Taste the Power!
Can you taste electricity? Maybe not, but it feels as if you can.
You will need:
9-volt battery
Snap connector for battery terminals
Multimeter No More Than 9 Volts
A 9-volt battery won’t hurt you. But do not try this experiment with a higher-voltage battery or a larger battery that can deliver more current. Also, if you have metal braces on your teeth, be very careful not to touch them with the battery.
Procedure
Moisten your tongue and touch the tip of it to the metal terminals of a 9-volt battery. The sudden sharp tingle that you feel is caused by electricity flowing from one terminal of the battery (Figure 1-16), through the moisture on and in your tongue, to the other terminal. Because the skin of your tongue is very thin (it’s actually a mucus membrane) and the nerves are close to the surface, you can feel the electricity very easily.
Now stick out your tongue, dry the tip of it very thoroughly with a tissue, and repeat the experiment without allowing your tongue to become moist again. You should feel less of a tingle.
Figure 1-16. Step 1 in the process of learning by discovery: the 9-volt tongue test.
What’s happening here? We’re going to need a meter to find out.
Tools
Setting up your meter
Check the instructions that came with the meter to find out whether you have to install a battery in it, or whether a battery is preinstalled.
Most meters have removable wires, known as leads (pronounced “leeds”). Most meters also have three sockets on the front, the leftmost one usually being reserved to measure high electrical currents (flows of electricity). We can ignore that one for now.
The leads will probably be black and red. The black wire plugs into a socket labeled “COM” or “Common.” Plug the red one into the socket labeled “V” or “volts.” See Figures 1-17 through 1-20.
Figure 1-17. The black lead plugs into the Common (COM) socket, and the red lead plugs into the red socket that’s almost always on the righthand side of a multimeter.
The other ends of the leads terminate in metal spikes known as probes, which you will be touching to components when you want to make electrical measurements. The probes detect electricity; they don’t emit it in significant quantities. Therefore, they cannot hurt you unless you poke yourself with their sharp ends.
If your meter doesn’t do autoranging, each position on the dial will have a number beside it. This number means “no higher than.” For instance if you want to check a 6-volt battery, and one position on the voltage section of the dial is numbered 2 and the next position is numbered 20, position 2 means “no higher than 2 volts.” You have to go to the next position, which means “no higher than 20 volts.”
If you make a mistake and try to measure something inappropriate, the meter will show you an error message such as “E” or “L.” Turn the dial and try again.
Figure 1-18.
Figure 1-19.
Figure 1-20. To measure resistance and voltage, plug the black lead into the Common socket and the red lead into the Volts socket. Almost all meters have a separate socket where you must plug the red lead when you measure large currents in amps, but we’ll be dealing with this later.
Fundamentals
Ohms
We measure distance in miles or kilometers, weight in pounds or kilograms, temperature in Fahrenheit or Centigrade—and electrical resistance in ohms. The ohm is an international unit.
The Greek omega symbol (Ω) is used to indicate ohms, as shown in Figures 1-21 and 1-22. Letter K (or alternatively, KΩ) means a kilohm, which is 1,000 ohms. Letter M (or MΩ) means a megohm, which is 1,000,000 ohms.
Number of ohms
Usually
expressed as
Abbreviated as
1,000 ohms
1 kilohm
1KΩ or 1K
10,000 ohms
10 kilohms
10KΩ or 10K
100,000 ohms
100 kilohms
100KΩ or 100K
1,000,000 ohms
1 megohm
1MΩ or 1M
10,000,000 ohms
10 megohms
10MΩ or 10M
A material that has very high resistance to electricity is known as an insulator. Most plastics, including the colored sheaths around wires, are insulators.
A material with very low resistance is a conductor. Metals such as copper, aluminum, silver, and gold are excellent conductors.
Figure 1-21. The omega symbol is used internationally to indicate resistance on ohms.
Figure 1-22. You’ll find it printed or written in a wide variety of styles.
Procedure
We’re going to use the meter to discover the electrical resistance of your tongue. First, set your meter to measure resistance. If it has autoranging, look to see whether it is displaying a K, meaning kilohms, or M, meaning megohms. If you have to set the range manually, begin with no less than 100,000 ohms (100K). See Figures 1-23 through 1-25.
Figure 1-23.
Figure 1-24.
Figure 1-25. To measure ohms, turn the dial to the ohm (omega) symbol. On an autoranging meter, you can then press the Range button repeatedly to display different ranges of resistance, or simply touch the probes to a resistance and wait for the meter to choose a range automatically. A manual meter requires you to select the range with the dial (you should set it to 100K or higher, to measure skin resista
nce). If you don’t get a meaningful reading, try a different range.
Touch the probes to your tongue, about an inch apart. Note the reading, which should be around 50K. Now put aside the probes, stick out your tongue, and use a tissue to dry it very carefully and thoroughly. Without allowing your tongue to become moist again, repeat the test, and the reading should be higher. Finally, press the probes against the skin of your hand or arm: you may get no reading at all, until you moisten your skin.
When your skin is moist (for instance, if you perspire), its electrical resistance decreases. This principle is used in lie detectors, because someone who knowingly tells a lie, under conditions of stress, tends to perspire.
A 9-volt battery contains chemicals that liberate electrons (particles of electricity), which want to flow from one terminal to the other as a result of a chemical reaction inside it. Think of the cells inside a battery as being like two water tanks—one of them full, the other empty. If they are connected with a pipe, water flows between them until their levels are equal. Figure 1-26 may help you visualize this. Similarly, when you open up an electrical pathway between the two sides of a battery, electrons flow between them, even if the pathway consists only of the moisture on your tongue.
Electrons flow more easily through some substances (such as a moist tongue) than others (such as a dry tongue).
Figure 1-26. Think of the cells in a battery as being like two cylinders: one full of water, the other empty. Open a connection between the cylinders, and the water will flow until the levels are equal on both sides. The less resistance in the connection, the faster the flow will be.
Background
The man who discovered resistance
Georg Simon Ohm, pictured in Figure 1-27, was born in Bavaria in 1787 and worked in obscurity for much of his life, studying the nature of electricity using metal wire that he had to make for himself (you couldn’t truck on down to Home Depot for a spool of hookup wire back in the early 1800s).
Despite his limited resources and inadequate mathematical abilities, Ohm was able to demonstrate in 1827 that the electrical resistance of a conductor such as copper varied in inverse proportion with its area of cross-section, and the current flowing through it is proportional to the voltage applied to it, as long as temperature is held constant. Fourteen years later, the Royal Society in London finally recognized the significance of his contribution and awarded him the Copley Medal. Today, his discovery is known as Ohm’s Law.
Figure 1-27. Georg Simon Ohm, after being honored for his pioneering work, most of which he pursued in relative obscurity.
Further Investigation
Attach the snap-on terminal cap (shown earlier in Figure 1-8) to the 9-volt battery. Take the two wires that are attached to the cap and hold them so that the bare ends are just a few millimeters apart. Touch them to your tongue. Now separate the ends of the wires by a couple of inches, and touch them to your tongue again. (See Figure 1-28.) Notice any difference?
Figure 1-28. Modifying the tongue test to show that a shorter distance, with lower resistance, allows greater flow of electricity, and a bigger zap.
Use your meter to measure the electrical resistance of your tongue, this time varying the distance between the two probes. When electricity travels through a shorter distance, it encounters less total resistance. As a result, the current (the flow of electricity per second) increases. You can try a similar experiment on your arm, as shown in Figure 1-29.
Figure 1-29. Moisten your skin before trying to measure its resistance. You should find that the resistance goes up as you move the meter probes farther apart. The resistance is proportional to the distance.
Use your meter to test the electrical resistance of water. Dissolve some salt in the water, and test it again. Now try measuring the resistance of distilled water (in a clean glass).
The world around you is full of materials that conduct electricity with varying amounts of resistance.
Cleanup and Recycling
Your battery should not have been damaged or significantly discharged by this experiment. You’ll be able to use it again.
Remember to switch off your meter before putting it away.
Experiment 2: Let’s Abuse a Battery!
To get a better feeling for electrical power, you’re going to do what most books tell you not to do. You’re going to short out a battery. A short circuit is a direct connection between the two sides of a power source.
Short Circuits
Short circuits can be dangerous. Do not short out a power outlet in your home: there’ll be a loud bang, a bright flash, and the wire or tool that you use will be partially melted, while flying particles of melted metal can burn you or blind you.
If you short out a car battery, the flow of current is so huge that the battery might even explode, drenching you in acid (Figure 1-30).
Lithium batteries are also dangerous. Never short-circuit a lithium battery: it can catch fire and burn you (Figure 1-31).
Use only an alkaline battery in this experiment, and only a single AA cell (Figure 1-32). You should also wear safety glasses in case you happen to have a defective battery.
You will need:
1.5-volt AA battery
Single-battery carrier
3-amp fuse
Safety glasses (regular eyeglasses or sunglasses will do)
Alligator clip (small or large)
Figure 1-30. Anyone who has dropped an adjustable wrench across the bare terminals of a car battery will tell you that short circuits can be dramatic at a “mere” 12 volts, if the battery is big enough.
Figure 1-31. The low internal resistance of lithium batteries (which are often used in laptop computers) allows high currents to flow, with unexpected results. Never fool around with lithium batteries!
Figure 1-32. Shorting out an alkaline battery can be safe if you follow the directions precisely. Even so, the battery is liable to become too hot to touch comfortably. Don’t try this with any type of rechargeable battery.
Procedure
Use an alkaline battery. Do not use any kind of rechargeable battery.
Put the battery into a battery holder that’s designed for a single battery and has two thin insulated wires emerging from it, as shown in Figure 1-32. Do not use any other kind of battery holder.
Use an alligator clip to connect the bare ends of the wires, as shown in Figure 1-32. There will be no spark, because you are using only 1.5 volts. Wait one minute, and you’ll find that the wires are getting hot. Wait another minute, and the battery, too, will be hot.
The heat is caused by electricity flowing through the wires and through the electrolyte (the conductive fluid) inside the battery. If you’ve ever used a hand pump to force air into a bicycle tire, you know that the pump gets warm. Electricity behaves in much the same way. You can imagine the electricity being composed of particles (electrons) that make the wire hot as they push through it. This isn’t a perfect analogy, but it’s close enough for our purposes.
Chemical reactions inside the battery create electrical pressure. The correct name for this pressure is voltage, which is measured in volts and is named after Alessandro Volta, an electrical pioneer.
Going back to the water analogy: the height of the water in a tank is proportionate to the pressure of the water, and comparable to voltage. Figure 1-33 may help you to visualize this.
Figure 1-33. Think of voltage as pressure, and amperes as flow.
But volts are only half of the story. When electrons flow through a wire, the flow is known as amperage, named after yet another electrical pioneer, André-Marie Ampère. The flow is also generally known as current. It’s the current—the amperage—that generates the heat.
Background
Why didn’t your tongue get hot?
&
nbsp; When you touched the 9-volt battery to your tongue, you felt a tingle, but no perceptible heat. When you shorted out a battery, you generated a noticeable amount of heat, even though you used a lower voltage. How can we explain this?
The electrical resistance of your tongue is very high, which reduces the flow of electrons. The resistance of a wire is very low, so if there’s only a wire connecting the two terminals of the battery, more current will pass through it, creating more heat. If all other factors remain constant:
Lower resistance allows more current to flow (Figure 1-34).
The heat generated by electricity is proportional to the amount of electricity (the current) that flows.
Here are some other basic concepts:
The flow of electricity per second is measured in amperes, or amps.
The pressure of electricity, measured in volts, causes the flow.
The resistance to the flow is measured in ohms.
A higher resistance restricts the current.
A higher voltage overcomes resistance and increases the current.
Figure 1-34. Larger resistance results in smaller flow—but if you increase the pressure, it may overcome the resistance and increase the flow.
If you’re wondering exactly how much current flows between the terminals of a battery when you short it out, that’s a difficult question to answer. If you try to use your multimeter to measure it, you’re liable to blow the fuse inside the meter. Still, you can use your very own 3-amp fuse, which we can sacrifice because it didn’t cost very much.