Make: Electronics

by Charles Platt

  Anyway, now we know what an LED wants, we can figure out how to supply it. If you have any difficulties dealing with decimals, check the Fundamentals section “Decimals,” on the next page, before continuing.

  Background

  The origins of wattage

  James Watt (Figure 1-70) is known as the inventor of the steam engine. Born in 1736 in Scotland, he set up a small workshop in the University of Glasgow, where he struggled to perfect an efficient design for using steam to move a piston in a cylinder. Financial problems and the primitive state of the art of metal working delayed practical applications until 1776.

  Despite difficulties in obtaining patents (which could only be granted by an act of parliament in those times), Watt and his business partner eventually made a lot of money from his innovations. Although he predated the pioneers in electricity, in 1889 (70 years after his death), his name was assigned to the basic unit of electric power that can be defined by multiplying amperes by volts. See the Fundamentals section, “Watt Basics,” on page 31.

  Figure 1-70. James Watt’s development of steam power enabled the industrial revolution. After his death, he was honored by having his name applied to the basic unit of power in electricity.

  How Big a Resistor Does an LED Need?

  Suppose that we use the Vishay LED. Remember its requirements from the data sheet? Maximum of 3 volts, and a safe current of 20mA.

  I’m going to limit it to 2.5 volts, to be on the safe side. We have 6 volts of battery power. Subtract 2.5 from 6 and we get 3.5. So we need a resistor that will take 3.5 volts from the circuit, leaving 2.5 for the LED.

  The current flow is the same at all places in a simple circuit. If we want a maximum of 20mA to flow through the LED, the same amount of current will be flowing through the resistor.

  Now we can write down what we know about the resistor in the circuit. Note that we have to convert all units to volts, amps, and ohms, so that 20mA should be written as 0.02 amps:

  V = 3.5 (the potential drop across the resistor)

  I = 0.02 (the current flowing through the resistor)

  We want to know R, the resistance. So, we use the version of Ohm’s Law that puts R on the left side:

  R= V/I

  Now plug in the values:

  R = 3.5/0.02

  Run this through your pocket calculator if you find decimals confusing. The answer is:

  R = 175Ω

  It so happens that 175Ω isn’t a standard value. You may have to settle for 180 or 220Ω, but that’s close enough.

  Evidently the 470Ω resistor that you used in Experiment 3 was a very conservative choice. I suggested it because I said originally that you could use any LED at all. I figured that no matter which one you picked, it should be safe with 470Ω to protect it.

  Cleanup and Recycling

  The dead LED can be thrown away. Everything else is reusable.

  Fundamentals

  Decimals

  Legendary British politician Sir Winston Churchill is famous for complaining about “those damned dots.” He was referring to decimal points. Because Churchill was Chancellor of the Exchequer at the time, and thus in charge of all government expenditures, his difficulty with decimals was a bit of a problem. Still, he muddled through in time-honored British fashion, and so can you.

  You can also use a pocket calculator—or follow two basic rules.

  Doing multiplication: move the decimal points

  Suppose you want to multiply 0.03 by 0.002:

  1. Move the decimal points to the ends of both the numbers. In this case, you have to move the decimal points by a total of 5 places to get 3 and 2.

  2. Do the multiplication of the whole numbers you have created and note the result. In this case, 3 x 2 = 6.

  3. Move the decimal point back again by the same number of places you counted in step 1. In this case, you get 0.00006.

  Doing division: cancel the zeros

  Suppose you need to divide 0.006 by 0.0002:

  1. Shift the decimal points to the right, in both the numbers, by the same number of steps, until both the numbers are greater than 1. In this case, shift the point four steps in each number, so you get 60 divided by 2.

  2. Do the division. The result in this case is 30.

  Theory

  Doing the math on your tongue

  I’m going to go back to the question I asked in the previous experiment: why didn’t your tongue get hot?

  Now that you know Ohm’s Law, you can figure out the answer in numbers. Let’s suppose the battery delivered its rated 9 volts, and your tongue had a resistance of 50K, which is 50,000 ohms. Write down what you know:

  V = 9

  R = 50,000

  We want to know the current, I, so we use the version of Ohm’s Law that puts this on the left:

  I = V/R

  Plug in the numbers:

  I = 9/50,000 = 0.00018 amps

  Move the decimal point three places to convert to milliamps:

  I = 0.18 mA

  That’s a tiny current that will not produce much heat at 9 volts.

  What about when you shorted out the battery? How much current made the wires get hot? Well, suppose the wires had a resistance of 0.1 ohms (probably it’s less, but I’ll start with 0.1 as a guess). Write down what we know:

  V = 1.5

  R = 0.1

  Once again we’re trying to find I, the current, so we use:

  I = V/R

  Plug in the numbers:

  I = 1.5/0.1 = 15 amps

  That’s 100,000 times the current that may have passed through your tongue, which would have generated much more heat, even though the voltage was lower.

  Could that tiny little battery really pump out 15 amps? Remember that the battery got hot, as well as the wire. This tells us that the electrons may have met some resistance inside the battery, as well as in the wire. (Otherwise, where else did the heat come from?) Normally we can forget about the internal resistance of a battery, because it’s so low. But at high currents, it becomes a factor.

  I was reluctant to short-circuit the battery through a meter, to try to measure the current. My meter will fry if the current is greater than 10A. However I did try putting other fuses into the circuit, to see whether they would blow. When I tried a 10A fuse, it did not melt. Therefore, for the brand of battery I used, I’m fairly sure that the current in the short circuit was under 10A, but I know it was over 3A, because the 3A fuse blew right away.

  The internal resistance of the 1.5-volt battery prevented the current in the short circuit from getting too high. This is why I cautioned against using a larger battery (especially a car battery). Larger batteries have a much lower internal resistance, allowing dangerously high currents which generate explosive amounts of heat. A car battery is designed to deliver literally hundreds of amps when it turns a starter motor. That’s quite enough current to melt wires and cause nasty burns. In fact, you can weld metal using a car battery.

  Lithium batteries also have low internal resistance, making them very dangerous when they’re shorted out. High current can be just as dangerous as high voltage.

  Fundamentals

  Watt basics

  So far I haven’t mentioned a unit that everyone is familiar with: watts.

  A watt is a unit of work. Engineers have their own definition of work—they say that work is done when a person, an animal, or a machine pushes something to overcome mechanical resistance. Examples would be a steam engine pulling a train on a level track (overcoming friction and air resistance) or a person walking upstairs (overcoming the force of gravity).

  When electrons push their way through a circuit, they are overcoming a ki
nd of resistance, and so they are doing work, which can be measured in watts. The definition is easy:

  watts = volts × amps

  Or, using the symbols customarily assigned, these three formulas all mean the same thing:

  W = V × I

  V = W/I

  I = W/V

  Watts can be preceded with an “m,” for “milli,” just like volts:

  Number of watts

  Usually expressed as

  Abbreviated as

  0.001 watts

  1 milliwatt

  1mW

  0.01 watts

  10 milliwatts

  10 mW

  0.1 watts

  100 milliwatts

  100 mW

  1 watt

  1,000 milliwatts

  1W

  Because power stations, solar installations, and wind farms deal with much larger numbers, you may also see references to kilowatts (using letter K) and megawatts (with a capital M, not to be confused with the lowercase m used to define milliwatts):

  Number of watts

  Usually expressed as

  Abbreviated as

  1,000 watts

  1 kilowatt

  1 KW

  1,000,000 watts

  1 megawatt

  1 MW

  Lightbulbs are calibrated in watts. So are stereo systems. The watt is named after James Watt, inventor of the steam engine. Incidentally, watts can be converted to horsepower, and vice versa.

  Theory

  Power assessments

  I mentioned earlier that resistors are commonly rated as being capable of dealing with 1/4 watt, 1/2 watt, 1 watt, and so on. I suggested that you should buy resistors of 1/4 watt or higher. How did I know this?

  Go back to the LED circuit. Remember we wanted the resistor to drop the voltage by 3.5 volts, at a current of 20 mA. How many watts of power would this impose on the resistor?

  Write down what you know:

  V = 3.5 (the voltage drop

  imposed by the resistor)

  I = 20mA = 0.02 amps

  (the current flowing through the resistor)

  We want to know W, so we use this version of the formula:

  W = V × I

  Plug in the values:

  W = 3.5 × 0.02 = 0.07 watts (the power being dissipated by the resistor)

  Because 1/4 watt is 0.25 watts, obviously a 1/4 watt resistor will have about four times the necessary capacity. In fact you could have used a 1/8 watt resistor, but in future experiments we may need resistors that can handle 1/4 watt, and there’s no penalty for using a resistor that is rated for more watts than will actually pass through it.

  Experiment 5: Let’s Make a Battery

  Long ago, before web surfing, file sharing, or cell phones, kids were so horribly deprived that they tried to amuse themselves with kitchen-table experiments such as making a primitive battery by pushing a nail and a penny into a lemon. Hard to believe, perhaps, but true!

  This is seriously old-school—but I want you to try it anyway, because anyone who wants to get a feel for electricity should see how easy it is to extract it from everyday objects around us. Plus, if you use enough lemons, you just might generate enough voltage to power an LED.

  The basic components of a battery are two metal electrodes immersed in an electrolyte. I won’t define these terms here (they’re explained in the following section “Theory: The nature of electricity”). Right now all you need to know is that lemon juice will be your electrolyte, and copper and zinc will be your electrodes. A penny provides the necessary copper, as long as it is fairly new and shiny. Pennies aren’t solid copper anymore, but they are still copper-plated, which is good enough.

  To find some metallic zinc, you will have to make a trip to a hardware store, where you should ask for roofing nails. The nails are zinc-plated to prevent them from rusting. Small metal brackets or mending plates also are usually zinc-plated. They should have a slightly dull, silvery look. If they have a mirror-bright finish, they’re more likely to be nickel-plated.

  Cut a lemon in half, set your multimeter so that it can measure up to 2 volts DC, and hold one probe against a penny while you hold the other probe against a roofing nail (or other zinc-plated object). Now force the penny and the nail into the exposed juicy interior of the lemon, as close to each other as possible, but not actually touching. You should find that your meter detects between 0.8 volts and 1 volt.

  You can experiment with different items and liquids to see which works best. Immersing your nail and penny in lemon juice that you have squeezed into a shot glass or egg cup may enhance the efficiency of your battery, although you’ll have a harder time holding everything in place. Grapefruit juice and vinegar will work as substitutes for lemon juice.

  To drive a typical LED, you need more than 1 volt. How to generate the extra electrical pressure? By putting batteries in series, of course. In other words, more lemons! (Or more shot glasses or egg cups.) You’ll also need lengths of wire to connect multiple electrodes, and this may entail skipping ahead to Chapter 2, where I describe how to strip insulation from hookup wire. Figures 1-71 and 1-72 show the configuration.

  Figure 1-71. A three-lemon battery. Don’t be too disappointed if the LED fails to light up. The lemons have a high electrical resistance, so they can’t deliver much current, especially through the relatively small surface area of the nails and the pennies. However, the lemon battery does generate voltage that you can measure with your meter.

  Figure 1-72. Bottled lemon juice seems to work just as well as fresh lemon juice. I cut the bottoms off three paper cups, inserted a galvanized bracket into each, and used heavyweight stranded copper wire to make the positive electrodes

  If you set things up carefully, making sure than none of the electrodes are touching, you may be able to illuminate your LED with two or three lemon-juice batteries in series. (Some LEDs are more sensitive to very low currents than others. Later in the book I’ll be talking about very-low-current LEDs. If you want your lemon-juice battery to have the best chance of working, you can search online for low-current LEDs and buy a couple.)

  Theory

  The nature of electricity

  To understand electricity, you have to start with some basic information about atoms. Each atom consists of a nucleus at the center, containing protons, which have a positive charge. The nucleus is surrounded by electrons, which carry a negative charge.

  Breaking up the nucleus of an atom requires a lot of energy, and can also liberate a lot of energy—as happens in a nuclear explosion. But persuading a couple of electrons to leave an atom (or join an atom) takes very little energy. For instance, when zinc reacts chemically with an acid, it can liberate electrons. This is what happens at the zinc electrode of the chemical battery in Experiment 5.

  The reaction soon stops, as electrons accumulate on the zinc electrode. They feel a mutual force of repulsion, yet they have nowhere to go. You can imagine them like a crowd of hostile people, each one wanting the others to leave, and refusing to allow new ones to join them, as shown in Figure 1-73.

  Figure 1-73. Electrons on an electrode have a bad attitude known as mutual repulsion.

  No
w consider what happens when a wire connects the zinc electrode, which has a surplus of electrons, to another electrode, made from a different material, that has a shortage of electrons. The electrons can pass through the wire very easily by jumping from one atom to the next, so they escape from the zinc electrode and run through the wire, propelled by their great desire to get away from each other. See Figure 1-74. This mutual force of propulsion is what creates an electrical current.

  Now that the population of electrons on the zinc electrode has been reduced, the zinc-acid reaction can continue, replacing the missing electrons with new ones—which promptly imitate their predecessors and try to get away from each other by running away down the wire. The process continues until the zinc-acid reaction grinds to a halt, usually because it creates a layer of a compound such as zinc oxide, which won’t react with acid and prevents the acid from reacting with the zinc underneath. (This is why your zinc electrode may have looked sooty when you pulled it out of the acidic electrolyte.)

  Figure 1-74. As soon as we open up a pathway from a zinc electrode crowded with electrons to a copper electrode, which contains “holes” for the electrons, their mutual repulsion makes them try to escape from each other to their new home as quickly as possible.

  This description applies to a “primary battery,” meaning one that is ready to generate electricity as soon as a connection between its terminals allows electrons to transfer from one electrode to the other. The amount of current that a primary battery can generate is determined by the speed at which chemical reactions inside the battery can liberate electrons. When the raw metal in the electrodes has all been used up in chemical reactions, the battery can’t generate any more electricity and is dead. It cannot easily be recharged, because the chemical reactions are not easily reversible, and the electrodes may have oxidized.