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
