6. The wire that you’re joining is much heavier than the 22-gauge wire that you worked with previously, so it will suck up more heat, and you must touch the soldering iron to it for a longer time. Make sure that the solder flows all the way into the joint, and check the underside after the joint is cool. Most likely you’ll find some bare copper strands there. The joint should become a nice solid, rounded, shiny blob. Keep the heat-shrink tubing as far away from the joint as possible while you’re using the soldering iron, so that heat from the iron doesn’t shrink the tubing prematurely, preventing you from sliding it over the joint later.
Figure 3-59.
Figure 3-60.
Figure 3-61.
Figure 3-62.
Figure 3-63.
Figure 3-64.
7. When the joint has cooled, slide the heat-shrink tubing over it, and apply the heat gun. Now repeat the process with the other conductor. Finally, slide the larger piece of tubing over the joint. You did remember to put the large tubing onto the wire at the beginning, didn’t you?
Figure 3-65. Completion of the shortened power cord for a laptop power supply.
Figures 3-59 through 3-65 show the steps all the way through to the end.
If you have completed the soldering exercises so far, you now have sufficient basic skills to solder your first electronic circuit. But first, I want you to verify the vulnerability of components to heat.
Experiment 13: Broil an LED
In Chapter 1, you saw how an LED can be damaged if too much current flows through it. The electricity caused heat, which melted the LED. Unsurprisingly, you can just as easily melt it by applying too much heat to one of its leads with a soldering iron. The question is: how much heat is too much? Let’s find out.
You will need:
30-watt or 40-watt soldering iron
15-watt pencil-type soldering iron
A couple of LEDs (that are expendable)
680Ω resistor
Wire cutters and sharp-nosed pliers
“Helping hand” gadget to hold your work
I don’t want you to use alligator clips to join the LED to a power supply, because the alligator clip will divert and absorb some of the heat from your soldering iron. Instead, please use some sharp-nosed pliers to bend each of the leads from an LED into little hooks, and do the same thing with the wires on a 680Ω load resistor. Finally bend the new wires on your AC adapter so that they, too, are tiny hooks. Now you can put the hooks together like links in a chain, as shown in Figure 3-66.
Figure 3-66. By literally hooking together the leads from a resistor and a white-light LED, we minimize pathways for heat to escape during the subsequent test.
Grip the plastic body of the LED in your helping hand. Plastic is not a good thermal conductor, so the helping hand shouldn’t siphon too much heat away from our target. The resistor can dangle from one of the leads on the LED, and the wire from the AC adapter can hang from that, a little farther down. Gravity should be sufficient to make this work. Set your AC adapter to deliver 12 volts as before, plug it in, and your LED should be shining brightly. I used a white LED in this experiment, because it’s easier to photograph.
Make sure your two soldering irons are really hot. They should have been plugged in for at least five minutes. Now take the pencil-style iron and hold its tip firmly against one of the leads on your glowing LED, while you check the time with a watch. Figure 3-67 shows the setup.
Figure 3-67. Applying heat with a 15-watt soldering iron. A typical LED should withstand this treatment for two or three minutes, but if you substitute a 30-watt soldering iron, the LED is likely to burn out in under 15 seconds.
I’m betting that you can sustain this contact for a full three minutes without burning out the LED. This is why you use a 15-watt soldering iron for delicate electronics work—it doesn’t endanger the components.
Allow your LED wire to cool, and then apply your more powerful soldering iron to the same piece of wire as before. Again, make sure it is completely hot, and I think you’ll find that the LED will go dark after as little as 10 seconds (note, some LEDs can survive higher temperatures than others). This is why you don’t use a 30-watt soldering iron for delicate electronics work.
The large iron doesn’t necessarily reach a higher temperature than the small one. It just has a larger heat capacity. In other words, a greater quantity of heat can flow out of it, at a faster rate.
Throw away your burned-out LED. Substitute a new one, connected as before, but add a full-size copper alligator clip to one of the leads up near the body of the LED, as shown in Figure 3-68. Press the tip of your 30-watt or 40-watt soldering iron against the lead just below the alligator clip. This time, you should be able to hold the powerful soldering iron in place for a full two minutes without burning out the LED.
Figure 3-68. When a copper alligator clip is used as a heat sink, you should be able to apply a 30-watt soldering iron (below the clip) without damaging the LED.
Imagine the heat flowing out through the tip of your soldering iron, into the wire that leads to the LED—except that the heat meets the alligator clip along the way, as shown in Figure 3-69. The clip is like an empty vessel waiting to be filled. It offers much less resistance to heat than the remainder of the wire leading to the LED, so the heat prefers to flow into the copper clip, leaving the LED unharmed. At the end of your experiment, if you touch the clip, you’ll find that it’s hot, while the LED remains relatively cooler.
Figure 3-69. The heat sink intercepts the heat, sucks it up, and protects the LED from damage.
The alligator clip is known as a heat sink, and it should be made of copper, because copper is one of the best conductors of heat.
Because the 15-watt soldering iron failed to harm the LED, you may conclude that the 15-watt iron is completely safe, eliminating all need for a heat sink. Well, this may be true. The problem is, you don’t really know whether some semiconductors may be more heat-sensitive than LEDs. Because the consequences of burning out a component are so exasperating, I suggest you should play it safe and use a heat sink in these circumstances:
If you apply 15-watt iron extremely close to a semiconductor for 20 seconds or more.
If you apply a 30-watt iron near resistors or capacitors for 10 seconds or more. (Never use it near semiconductors.)
If you apply a 30-watt iron near anything meltable for 20 seconds or more. Meltable items include insulation on wires, plastic connectors, and plastic components inside switches.
Rules for Heat Sinking
1. Full-size copper alligator clips do work better.
2. Clamp the alligator clip as close as possible to the component and as far as possible from the joint. (You don’t want to suck too much heat away from the joint.)
3. Make sure there is a metal-to-metal connection between the alligator clip and the wire to promote good heat transfer.
Fundamentals
All about perforated board
For the remainder of this book, you’ll be using perforated board whenever you want to create permanent, soldered circuits. There are three ways to do this:
1. Point-to-point wiring. You use perforated board that has no connections behind the holes. Either the board has no copper traces on it at all, as in Figure 3-70, or you will find a little circular copper circle around each hole, as in Figure 3-71. These circles are not connected with each other and are used only to stabilize the components that you assemble.
Point-to-point wiring allows you to place the components in a convenient, compact layout that can be very similar to a schematic. Under the board you bend the wires to link the components, and solder them together, adding extra lengths of wire if necessary. The advantage of this system is that it can be extremely compact. The disadvantage is that the la
yout can be confusing, leading to errors.
Figure 3-70.
Figure 3-71. Either this type of perforated board or the type in Figure 3-70 can be used for point-to-point wiring in Experiment 14.
2. Breadboard-style wiring. Use perforated board that is printed with copper traces in exactly the same pattern as the conductors inside a breadboard. Once your circuit works on the breadboard, you move the components over to the perfboard one by one, maintaining their exact same positions relative to each other. You solder the “legs” of the components to the copper traces, which complete the circuit. Then you trim off the surplus wire. The advantage of this procedure is that it’s quick, requires very little planning, and minimizes the possibility for errors. The disadvantage is that it tends to waste space. A cheap example is shown in Figure 3-72.
3. You can etch your own circuit board with customized copper traces that link your components in a point-to-point layout. This is the most professional way to complete a project, but it requires more time, trouble, and equipment than is practical in this book.
Point-to-point wiring is like working with alligator clips, on a much smaller scale. The first soldered project will use this procedure.
Figure 3-72. Perforated board etched with copper in variants of a breadboard layout. This example is appropriate for Experiment 15.
Experiment 14: A Pulsing Glow
You will need:
Breadboard
15-watt pencil-type soldering iron
Thin solder (0.022 inches or similar)
Wire strippers and cutters
Plain perforated board (no copper etching necessary)
Small vise or clamp to hold your perforated board
Resistors, various
Capacitors, electrolytic, 100 µF and 220 µF, one of each
Red LED, 5 mm, rated for 2 volts approximately
2N6027 programmable unijunction transistor
Your first circuit using a PUT was a slow-speed oscillator that made an LED flash about twice each second. The flashes looked very “electronic,” by which I mean that the LED blinked on and off without a gradual transition between each state. I’m wondering if we can modify this circuit to make the LED pulse in a more gentle, interesting way, like the warning light on an Apple MacBook when it’s in “sleep” mode. I’m thinking that something of this sort might be wearable as an ornament, if it’s small enough and elegant enough.
I’m also thinking that this first soldering project will serve three purposes. It will test and refine your skill at joining wires together, will teach you point-to-point wiring with perfboard, and will give you some additional insight into the way that capacitors can be used to adjust timing.
Look back at the original circuit in Experiment 11, on page 82. Refresh your memory about the way it worked. The capacitor charges through a resistor until it has enough voltage to overcome the internal resistance in the PUT. Then the capacitor discharges through the PUT and flashes the LED.
If you drew a graph of the light coming out of the LED, it would be a thin, square-shaped pulse, as shown in Figure 3-73. How can we fill it out to make it more like the curve in Figure 3-74, so that the LED fades gently on and off, like a heartbeat?
Figure 3-73.
Figure 3-74. The original PUT oscillator circuit in Experiment 11 made the LED emit sharp, short flashes. The upper graph shows what we might find if we measured light output over time. The second graph shows a gentler onset to each flash, followed by a slow fade-out. Capacitors can be used to create this effect.
One thing is obvious: the LED is going to be emitting a greater total amount of light in each cycle. Therefore, it’s going to need more power. This means that C1, in Figure 3-75, must be a larger capacitor.
When we have a larger capacitor, it takes longer to charge. To keep the flashes reasonably frequent, we’ll need a lower-value resistor for R1 to charge the capacitor quickly enough. In addition, reducing the values of R2 and R3 will program the PUT to allow a longer pulse.
Most important, I want to discharge the capacitor through a resistor to make the onset of the pulse gradual instead of sudden. Remember, when you have a resistor in series with a capacitor, the capacitor not only charges more slowly, but discharges more slowly.
Figure 3-75 shows these features. Compare it with Figure 2-103 on page 85. R1 is now 33K instead of 470K. R2 and R3 are reduced to 1K. R4 also is 1K, so that the capacitor takes longer to discharge. And C1 is now 100 µF instead of 2.2 µF.
Figure 3-75. The first step toward creating a gentler flashing effect is to use a larger capacitor for C1 and discharge it through a resistor, R4. Lower-value resistors are necessary to charge the capacitor rapidly enough.
R1: 33K
R2: 1K
R3: 1K
R4: 1K
C1: 100 µF electrolytic
Q1: 2N6027
Assemble this circuit on a breadboard, and compare the results when you include R4 or bypass it with a plain piece of jumper wire. It softens the pulse a bit, but we can work on it some more. On the output side of the PUT, we can add another capacitor. This will charge itself when the pulse comes out of the PUT, and then discharge itself gradually through another resistor, so that the light from the LED dies away more slowly.
Figure 3-76 shows the setup. C2 is large—220 µF—so it sucks up the pulse that comes out of the PUT, and then gradually releases it through 330Ω resistor R5 and the LED. You’ll see that the LED behaves differently now, fading out inside of blinking off. But the resistances that I’ve added have dimmed the LED, and to brighten it, you should increase the power supply from 6 volts to 9 volts.
Remember that a capacitor imposes a smoothing effect only if one side of it is grounded to the negative side of the power supply. The presence of the negative charge on that side of the capacitor attracts the positive pulse to the other side.
I like the look of this heartbeat effect. I can imagine a piece of wearable electronic jewelry that pulses in this sensual way, very different from the hard-edged, sharp-on-and-off of a simple oscillator circuit. The only question is whether we can squeeze the components into a package that is small enough to wear.
Figure 3-76. The second step toward a gentler flashing effect is to add another capacitor, C2, which charges quickly with each pulse and then discharges slowly through R5 and the LED below it.
Same components as before, plus:
R5: 330Ω
C2: 220 µF electrolytic
Power supply increased to 9 volts
Figure 3-77. On a dark night in a rural area, the heartbeat flasher may be attractive in unexpected ways.
Resizing the Circuit
The first step is to look at the physical components and imagine how to fit them into a small space. Figure 3-78 shows a 3D view of a compact arrangement. Check this carefully, tracing all the paths through the circuit, and you’ll see that it’s the same as the schematic. The trouble is that if we solder the components together like this, they won’t have much strength. All the little wires can bend easily, and there’s no easy way to mount the circuit in something or on something.
Figure 3-78. This layout of components replicates their connections in the schematic diagram while squeezing them into a minimal amount of space.
The answer is to put it on a substrate, which is one of those terms that people in the electronics field like to use, perhaps because it sounds more technical than “perfboard.” But perforated board is what we need, and Figure 3-79 shows the components transferred onto a piece of board measuring just 1 inch by 0.8 inch.
Figure 3-79. Perforated board can be used to support the layout of components. Their leads are soldered together under the board to create the circuit. The middle diagram shows the wires under the board as dashed lines. The bottom diagram shows the b
oard from underneath, flipped left to right. Orange circles indicate where solder joints will be necessary.
The center version of this diagram uses dotted lines to show how the components will be connected with each other underneath the board. Mostly the leads that stick out from underneath the components will be long enough to make these connections.
Finally, the bottom version of the perfboard diagram shows the perfboard flipped left-to-right (notice the L and the R have been transposed to remind you, and I’ve used a darker color to indicate the underside of the board). Orange circles indicate where solder joints will be needed.
The LED should be unpluggable, because we may want to run it at some distance from the circuit. Likewise the power source should be unpluggable. Fortunately we can buy miniature connectors that fit right into the perforated board. You may have to go to large online retail suppliers such as Mouser.com for these. Some manufacturers call them “single inline sockets and headers,” while others call them “boardmount sockets and pinstrip headers.” Refer back to Figure 3-29 and check the shopping list for more details.
This is a very compact design that will require careful work with your pencil-style soldering iron. Because a piece of perforated board as small as this will tend to skitter around, I suggest that you apply your miniature vise to one end to anchor it with some weight while still allowing you to turn it easily.
Make: Electronics Page 16