Potentiometer
1 megohm linear potentiometer, Part number 271-211 from RadioShack, part number 24N-1M-15R-R from Jameco, or similar.
Transistors
NPN transistor, general-purpose, such as 2N2222 by STMicroelectronics, part PN2222 by Fairchild, or part 2N2222 from RadioShack. Quantity: 4. See Figure 2-13.
Figure 2-13. Transistors are commonly sold either in little metal cans or sealed into little lumps of plastic. For our purposes, the packaging makes no difference.
2N6027 programmable unijunction transistor manufactured by On Semiconductor or Motorola. Quantity: 4 (allowing for 2 spares in case of damage).
Capacitors
Electrolytic capacitors, assorted. Must be rated for a minimum of 25 volts and include at least one capacitor of 1,000 μF (microfarads) and two capacitors of 2.2 μF. If you search on eBay, make sure you find electrolytic capacitors. If they’re rated for higher voltages, that’s OK, although they will be physically larger than you need. See Figure 2-14.
Figure 2-14. An assortment of electrolytic capacitors.
Ceramic capacitors, assorted. Make sure you get at least one rated at 0.0047 μF (which can also be written as 4.7 nF). See Figure 2-15.
Figure 2-15. Ceramic capacitors mostly look like this, although many of them are round or bead-shaped instead of square. The packaging shape is unimportant to us.
Resistors
If you bought only a minimal selection for experiments 1 through 5, now’s the time to buy a larger assortment, so that you won’t be stuck needing the one value that you don’t have. 1/4-watt minimum.
Loudspeaker
Any 8Ω, 1-inch miniature loudspeaker such as part 273-092 from RadioShack. See Figure 2-16.
Figure 2-16. This miniature loudspeaker, just over 1 inch in diameter, is useful for verifying audio output direct from transistor circuits.
Experiment 6: Very Simple Switching
You will need:
AA batteries. Quantity: 2.
Battery carrier for 2 AA batteries. Quantity: 1.
LED. Quantity: 1.
Toggle switches, SPDT. Quantity: 2. See Figure 2-12.
220Ω or similar value resistor, 1/4-watt minimum. Quantity: 1.
Alligator clips. Quantity: 8.
Wire or patch cords. See Figure 2-10, shown previously.
Wire cutters and wire strippers if you don’t use patch cords. See Figure 2-4, shown previously.
In Experiment 3, you illuminated an LED by attaching a battery, and switched it off by removing the battery. For greater convenience our circuits should have proper switches to control power, and while I’m dealing with the general topic of switches, I’m going to explore all the varieties, using a circuit to suggest some possibilities.
Assemble the parts as shown in Figures 2-17 and 2-18. The long lead on the LED must connect with the resistor, because that is the more positive side of the circuit.
You’ll notice that you have to include a couple lengths of wire. I suggest green wire to remind you that these sections are not connected directly to positive or to negative power. But you can use any color you like. You can also substitute patch cords, if you have them. However, learning to strip insulation from pieces of wire is a necessary skill, so let’s deal with that now.
Figure 2-17. If the LED is on, flipping either of the switches will turn it off. If the LED is off, either of the switches will turn it on. Use alligator clips to attach the wires to each other, and to the switches if your switches don't have screw terminals. Be careful that the clips don't touch each other.
Figure 2-18. Full-size toggle switches with screw terminals make it easy to hook up this simple circuit.
Tools
If automatic wire strippers (Figure 2-19) don’t grip skinny 22-gauge wire very effectively, try the Ideal brand of wire strippers shown back in Figure 2-4, or use plain and simple wire cutters as shown in Figure 2-20. When using wire cutters, you hold the wire in one hand and apply the tool in your other hand, squeezing the handles with moderate pressure—just enough to bite into the insulation, but not so much that you chop the wire. Pull the wire down while you pull the cutters up, and with a little practice you can rip the insulation off to expose the end of the wire.
Figure 2-19. Using automatic wire strippers, when you squeeze the handles the jaw on the left clamps the wire, the sharp grooves on the right bite into the insulation. Squeeze harder and the jaws pull away from each other, stripping the insulation from the wire.
Macho hardware nerds may use their teeth to strip insulation from wires. When I was younger, I used to do this. I have two slightly chipped teeth to prove it. Really, it’s better to use the right tool for the job.
Figure 2-20. To remove insulation from the end of a thin piece of wire, you can also use wire cutters. This takes a little practice.
Figure 2-21. Those who tend to misplace tools, and feel too impatient to search for them, may feel tempted to use their teeth to strip insulation from wire. This may not be such a good idea.
Connection Problems
Depending on the size of toggle switches that you are using, you may have trouble fitting in all the alligator clips to hold the wires together. Miniature toggle switches, which are more common than the full-sized ones these days, can be especially troublesome (see Figure 2-22). Be patient: fairly soon we’ll be using a breadboard, which will eliminate alligator clips almost completely.
Figure 2-22. Miniature toggle switches can be used—ideally, with miniature alligator clips—but watch out for short circuits.
Testing
Make sure that you connect the LED with its long wire toward the positive source of power (the resistor, in this case). Now flip either of the toggle switches. If the LED was on, it will go off, and if it was off, it will go on. Flip the other toggle switch, and it will have the same effect. If the LED does not go on at all, you’ve probably connected it the wrong way around. Another possibility is that two of your alligator clips may have shorted out the battery.
Assuming your two switches do work as I described them, what’s going on here? It’s time to nail down some basic facts.
Fundamentals
All about switches
When you flip the type of toggle switch that you used in Experiment 6, it connects the center terminal with one of the outer terminals. Flip the switch back, and it connects the center terminal with the other outer terminal, as shown in Figure 2-23.
Figure 2-23. The center terminal is the pole of the switch. When you flip the toggle, the pole changes its connection.
The center terminal is called the pole of the switch. Because you can flip, or throw, this switch to make two possible connections, it is called a double-throw switch. As mentioned earlier, a single-pole, double-throw switch is abbreviated SPDT.
Some switches are on/off, meaning that if you throw them in one direction they make a contact, but in the other direction, they make no contact at all. Most of the light switches in your house are like this. They are known as single-throw switches. A single-pole, single-throw switch is abbreviated SPST.
Some switches have two entirely separate poles, so you can make two separate connections simultaneously when you flip the switch. These are called double-pole switches. Check the photographs in Figures 2-24 through 2-26 of old-fashioned “knife” switches (which are still used to teach electronics to kids in school) and you’ll see the simplest representation of single and double poles, and single and double throws. Various toggle switches that have contacts sealed inside them are shown in Figure 2-27.
Figure 2-24. This primitive-looking single-pole, double-throw switch does exactly the same thing as the toggle switches in Figures 2-23 and 2-27.
Figure 2-25. A single-pole, single-throw switch makes onl
y one connection with one pole. Its two states are simply open and closed, on and off.
Figure 2-26. A double-pole, single-throw switch makes two separate on/off
connections.
Fundamentals
All about switches (continued)
Figure 2-27. These are all toggle switches. Generally, the larger the switch, the more current it can handle.
To make things more interesting, you can also buy switches that have three or four poles. (Some rotary switches have even more, but we won't be using them.) Also, some double-throw switches have an additional “center off” position.
Putting all this together, I made a table of possible types of switches (Figure 2-28). When you’re reading a parts catalog, you can check this table to remind yourself what the abbreviations mean.
Figure 2-28. This table summarizes all the various options for toggle switches and pushbuttons.
Now, what about pushbuttons? When you press a door bell, you’re making an electrical contact, so this is a type of switch—and indeed the correct term for it is a momentary switch, because it makes only a momentary contact. Any spring-loaded switch or button that wants to jump back to its original position is known as a momentary switch. We indicate this by putting its momentary state in parentheses. Here are some examples:
OFF-(ON): Because the ON state is in parentheses, it’s the momentary state. Therefore, this is a single-pole switch that makes contact only when you push it, and flips back to make no contact when you let it go. It is also known as a “normally open” momentary switch, abbreviated “NO.”
ON-(OFF): The opposite kind of momentary single-pole switch. It’s normally ON, but when you push it, you break the connection. So, the OFF state is momentary. It is known as a “normally closed” momentary switch, abbreviated “NC.”
(ON)-OFF-(ON): This switch has a center-off position. When you push it either way, it makes a momentary contact, and returns to the center when you let it go.
Other variations are possible, such as ON-OFF-(ON) or ON-(ON). As long as you remember that parentheses indicate the momentary state, you should be able to figure out what these switches are.
Figure 2-29. This evil mad scientist is ready to apply power to his experiment. For this purpose, he is using a single-pole, double-throw knife switch, conveniently mounted on the wall of his basement laboratory.
Fundamentals
All about switches (continued)
Sparking
When you make and break an electrical connection, it tends to create a spark. Sparking is bad for switch contacts. It eats them until the switch doesn’t make a reliable connection anymore. For this reason, you must use a switch that is appropriate for the voltage and amperage that you are dealing with. Electronic circuits generally are low-current, and low-voltage, so you can use almost any switch, but if you are switching a motor, it will tend to suck an initial surge of current that is at least double the rating of the motor when it is running constantly. You should probably use a 4-amp switch to turn a 2-amp motor on and off.
Checking a switch
You can use your meter to check a switch. Doing this helps you find out which contacts are connected when you turn a switch one way or the other. It’s also useful if you have a pushbutton and you can’t remember whether it’s the type that is normally open (you press it to make a connection) or normally closed (you press it to break the connection). Set your meter to measure ohms, and touch the probes to the switch terminals while you work the switch.
This is a hassle, though, because you have to wait while the meter makes an accurate measurement. When you just want to know whether there is a connection, your meter has a “continuity tester” setting. It beeps if it finds a connection, and stays silent if it doesn’t. See Figures 2-30 through 2-32 for examples of meters set to test continuity. Figure 2-33 offers an example of a toggle switch being tested for continuity.
Figure 2-30.
Figure 2-31.
Figure 2-32. To check a circuit for continuity, turn the dial of your meter to the symbol shown. Only use this feature when there is no power in the component or the circuit that you are testing.
Figure 2-33. When the switch connects two of its terminals, the meter shows zero resistance between them and will beep if you have set it to verify continuity.
Use the continuity-testing feature on your meter only on circuits or components that have no power in them at the time.
Background
Early switching systems
Switches seem to be such a fundamental feature of our world, and their concept is so simple that it’s easy to forget that they went through a gradual process of development and refinement. Primitive knife switches were quite adequate for pioneers of electricity who simply wanted to connect and disconnect electricity to some apparatus in a laboratory, but a more sophisticated approach was needed when telephone systems began to proliferate. Typically, an operator at a “switchboard” needed a way to connect any pair of 10,000 lines on the board. How could it be done?
In 1878, Charles E. Scribner (Figure 2-34) developed the “jack-knife switch,” so called because the part of it that the operator held looked like the handle of a jackknife. Protruding from it was a plug, and when the plug was pushed into a socket, it made contact inside the socket. The socket, in fact, was the switch.
Figure 2-34. Charles E. Scribner invented the “jack-knife switch” to satisfy the switching needs of telephone systems in the late 1800s. Today’s audio jacks still work on the same basis.[1]
Audio connectors on guitars and amplifiers still work on the same principle, and when we speak of them as being “jacks,” the term dates back to Scribner’s invention. Switch contacts still exist inside a jack socket.
Today, of course, telephone switchboards have become as rare as telephone operators. First they were replaced with relays—electrically operated switches, which I’ll talk about later in this chapter. And then the relays were superceded by transistors, which made everything happen without any moving parts. Before the end of this chapter, you’ll be switching current using transistors.
[1] The photo on which this drawing is based first appeared in The History of the Telephone by Herbert Newton Casson in 1910 (Chicago: A. C. McClurg & Co.).
Introducing Schematics
In Figure 2-35, I’ve redrawn the circuit from Experiment 6 in a simplified style known as a “schematic.” From this point onward, I will be illustrating circuits with schematics, because they make circuits easier to understand. You just need to know a few symbols to interpret them.
Compare the schematic here with the drawing of the circuit in Figure 2-17. They both show exactly the same thing: Components, and connections between them. The gray rectangles are the switches, the zigzag thing is the resistor, and the symbol with two diagonal arrows is the LED.
The schematic LED symbol includes two arrows indicating that it emits light, because there are some kinds of diodes, which we’ll get to later, that don’t. The triangle inside the diode symbol always points from positive to negative.
Trace the path that electricity can take through the circuit and imagine the switches turning one way or the other. You should see clearly now why either switch will reverse the state of the LED from on to off or off to on.
This same circuit is used in houses where you have a switch at the bottom of a flight of stairs, and another one at the top, both controlling the same lightbulb. The wires in a house are much longer, and they snake around behind the walls, but because their connections are still the same, they could be represented with the same basic schematic. See Figure 2-36.
A schematic doesn’t tell you exactly where to put the components. It just tells you how to join them together. One problem: Diffe
rent people use slightly different schematic symbols to mean the same thing. Check the upcoming section, “Fundamentals: Basic schematic symbols,” for the details.
Figure 2-35. This schematic shows the same circuit as in Figure 2-17 and makes it easier to see how the switches function.
Figure 2-36. The two-switch circuit shown in Figures 2-17 and 2-35 is often found in house wiring, especially where switches are located at the top and bottom of a flight of stairs. This sketch shows what you might find inside the walls. Wires are joined with “wire nuts” inside boxes that are hidden from everyday view.
Fundamentals
Basic schematic symbols
Schematic symbols are like words in a language: they have mutated over the years into a confusing range of variations. A simple on/off (single-pole, single-throw, or SPST) switch, for instance, can be represented by any of the symbols shown in Figure 2-37. They all mean exactly the same thing.
Figure 2-37. Variations on a theme: Just some of the different styles used to depict a single-pole, single-throw switch in schematic diagrams. The bottom version is the style used in this book.
Make: Electronics Page 7
