Make: More Electronics (2014)
Chapter 1. Experiment 1: Sticky Resistance
I want to start with some simple entertainment, because I think electronics should always contain an element of fun.
For this experiment, I’m going to use glue and cardboard. I realize that these materials are not commonly used in electronics books, but they’re going to serve two purposes. First, they will remind us that electricity isn’t necessarily confined to wires and boards. Second, the experiment will lead to a deepening understanding of that most fundamental and vital component, the bipolar transistor. And third, this experiment will lead into a general conversation about ions, resistance, and resistivity.
I realize that if you read Make: Electronics you already learned the basics about transistors, but after I do a small amount of recapitulation, I’m going to move beyond the basics.
§ Remember that you will find components for each experiment listed at the back of the book. See Appendix B.
A Glue-Based Amplifier
Figure 1-1 shows the plan. The cardboard will be your foundation for the circuit; you won’t be using a breadboard for this project. Begin by pushing the legs of the transistor into the cardboard. The 2N2222 is sold in two versions, one featuring a little metal cap, the other using a small lump of black plastic. If you happen to be using the metal type, the tab that sticks out should be on the left, viewed from the point of view in the figure. If you have the black plastic type, the 2N2222 or PN2222 will have its flat side on the right—but if you happen to buy the P2N2222 variant (which often pops up as “equivalent” when you search for the other part numbers), the flat side should be on the left. Check the part number with a magnifying glass, and see Symbology if you are unclear about this.
Figure 1-1. Your first experiment: All you need is a transistor, a 220Ω resistor, a 9V battery, patch cords and pieces of wire, and some white glue and cardboard.
Connect the components as shown. The long lead of the LED is on the right, and the short lead is on the left. The resistor attached to the long lead of the LED has a value of 470Ω. Don’t allow any of the alligator clips to touch each other where they grip the leads of the transistor. Now take your container of Elmer’s glue and squeeze out a zigzag path that is about 12” long and less than 1/8” thick. If you can taper it from top to bottom, as shown in Figure 1-2, this is good. Make sure there are no breaks in the trail.
Why am I suggesting Elmer’s glue? Because most people have a bottle lying around the house somewhere, and it just happens to have the electrical characteristics that I want. It’s not an insulator, but it’s not a very good conductor, either.
Figure 1-2. The actual experiment, just in case you were wondering if it really works.
You have to work fairly fast, before the glue dries. Take the green wire (which connects with the center lead on the transistor) and touch it half-way down the glue trail. The LED should glow quite brightly. Now touch it near the bottom of the glue trail, and the LED should glow less brightly.
If you have read my previous book, you’ll know why—but I’m going to tell you anyway.
The path of glue that you squeezed out should have a resistance of about 1 megohm from top to bottom, or 10K per inch. If you want to check this with your meter, use pieces of wire to extend the probes, so that you don’t get glue on them.
The transistor acts as an amplifier. It amplifies the current flowing into its base (the center lead). The amplified output emerges from its emitter (the lefthand lead, in Figure 1-1). In the experiment, you restricted the current flowing into the base of the transistor by passing the current through some of the glue, which has a high resistance. The LED is responsive to current, and shows you what’s happening by varying its brightness.
To get a visual impression of what the transistor is doing, remove it from the circuit, as in Figure 1-3. The green alligator clip now connects with the series resistor, which connects with the LED, and the LED should remain dark. The resistance of the glue is so high, not enough current gets through to light the LED. If you move the green alligator all the way up to less than a quarter inch from where the positive power supply is connected with the glue, the LED should glow dimly.
Figure 1-3. When the transistor is no longer amplifying current to the LED, the resistance of the glue is too high to allow enough current to make the LED light up.
Just in case you have trouble remembering the schematic symbol for an NPN transistor, and the pinouts of actual components, I’ve included Figure 1-4 to remind you. The tab that sticks out of the metal-can type of transistor may be in either of the orientations shown, or somewhere in between, but it will always be closer to the emitter than to the other leads. As for the schematic symbol, you know that this represents an NPN transistor because the arrow “Never Points iN.”
Figure 1-4. The schematic symbol for an NPN transistor, and simplified views of components seen from above. See the important warning in the text regarding reversed leads on the P2N2222.
Warning: Nonstandard Leads
For as long as anyone could remember, when you looked at a plastic-packaged 2N2222 transistor from above, and held it with the flat side on the right, the leads were always identified as collector, base, and emitter when reading from top to bottom. Some manufacturers called the transistor the PN2222, but the pinouts were still the same.
For reasons that remain unclear, some time around 2010, a variant with part number P2N2222 was produced by On Semiconductor, Motorola, and possibly some other manufacturers. Its performance was identical to that of the 2N2222 and the PN2222, but the sequence of its leads was reversed.
Suppose you search an online supplier for 2N2222, which is a reasonable thing to do, as 2N2222 is the most generic version of this transistor’s part number. Chances are, you will be offered a P2N2222, because your search term is contained within P2N2222. If you go ahead and buy that component because its specification seems to be identical, you are likely to insert it the wrong way around in your circuit.
Compounding the problem, transistors will work to some extent when reversed, although some degradation may occur. Therefore you can use the P2N2222 the wrong way around and get some results from a circuit, although not quite what you expect. If you then discover your error and reverse the P2N2222, quite probably you still won’t get the results you expect, because the transistor has been damaged by reversed polarity.
Anyone buying components online should be careful to read the part number and take note of the configurations in Figure 1-4. And, as always, check datasheets carefully!
Background: Conductors and Insulators
You can learn more from your experiment if you wait for the glue to dry. The drier it gets, the weaker the LED response becomes. Why is this? Because some of the water in the glue is evaporating, while the rest of it is absorbed into the cardboard.
As you may recall from Make: Electronics, electric current is a flow of electrons. Atoms or molecules which have surplus electrons, or a deficit of electrons, are called ions. I don’t know what Elmer’s glue is made of, but apparently it contains a chemical that allows ion transfer. The water in the glue helps to enable this, as the ions move through the water.
Water on its own is not a good conductor. To demonstrate this, you need some pure water—not the water that comes out of your faucet, which usually contains mineral impurities. Pure water used to be called distilled water, which was created by boiling water to make steam (leaving the impurities behind), and then condensing the steam. These days people still sometimes talk about distilled water, but it is becoming uncommon because the process of making it is too energy intensive. Instead, you are likely to find “deionized” water, which is usually created by a process such as reverse osmosis. “Deonized” tells you there are no ions in it, right? So, it should be no surprise that the water doesn’t conduct electricity very well.
Insert the probes of your meter into a cup of distilled or deonized water, a couple of inches apart. You should find that the resistance is more than 1 megohm. Now dissolve some salt in the water, and the resistance should drop radically, because the salt is a source of ions.
You may wonder where the dividing line is between a conductor and an insulator. To answer that question, you need to know how “resistivity” is measured. It’s very simple: if R is the resistance of an object in ohms, A is its area in square meters, and L is its length in meters:
Resistivity = (R * A) / L
Resistivity is measured in ohm-meters. A very good conductor, such as aluminum, has resistivity of about 0.00000003 ohm-meters. That’s 3 divided by 100 million. At the other extreme, a very good insulator, such as glass, has resistivity of about 1,000,000,000,000 (one trillion) ohm-meters.
Somewhere in the middle are semiconductors. Silicon, for instance, has a resistivity of about 640 ohm-meters, although this can be reduced by “doping” the silicon with impurities and biasing it with an electrical potential to encourage electrons to flow through it.
What’s the resistivity of Elmer’s glue? I’ll leave you to figure that out, with the aid of your multimeter. And what about cardboard? Its resistivity is so high, how could you ever measure it? See if you can think of a way.
Make Even More
If you repeat Experiment 1, what happens if you use a trace of glue that is three or four times as wide? What happens if you put two LEDs in parallel—or in series?
Maybe you think you know what the results will be. But it’s always good to validate an assumption by testing it.
I mentioned earlier that if you insert a transistor the wrong way around, it will still work to some extent. It can tolerate a small reversed voltage between the base and the emitter (usually less than 6V), but using a 9V battery, you’re more likely to cause some damage. Does that actually happen when you try it? And if so, why? If you search for more information about this, you’re likely to find yourself learning how the layers in a transistor are structured, and how charges move from one to another. This is good to know.
After you have reversed a transistor in a circuit, it may have sustained some damage and should not be used in other circuits. You can, however, test it as described in the next experiment, and compare its performance with that of a fresh transistor that has not been abused.