Figure 2-112. Top: The basic functions of the noisemaking oscillator circuit shown as a block diagram. Bottom: The same functions with a slow oscillator added to control the fast oscillator.
Well, the first section of the circuit that you assembled created a pulsing signal about twice per second. You used it to flash an LED. Maybe we can get rid of the LED and feed the output from the first section to the second section. The lower block diagram in Figure 2-112 explains this concept.
Can it really be that simple? Well, yes and no. The trick is to make the output from the first section compatible with the input to the second section. If you simply connect a wire from the cathode of the first PUT to the anode of the second PUT, that’s not going to work, because the second PUT is already oscillating nicely between low and high voltage, about 1,000 times each second. Add more voltage, and you will disrupt the balance that enables oscillation.
However, remember that the voltage on the gate of a PUT affects its threshold for conducting electricity. Maybe if we connect the output from Q1 to the gate of Q2, we’ll be able to adjust that threshold automatically. The voltage still has to be in a range that the PUT finds acceptable, though. We can try various resistors to see which one works well.
This sounds like trial and error—and that’s exactly what it is. Doing the math to predict the behavior of a circuit like this is far too complicated—for me, anyway. I just looked at the manufacturer’s data sheet, saw the range of resistor values that the PUT would tolerate, and chose one that seemed as if it should work.
If you remove the LED and substitute R10 as shown in the breadboard diagram in Figure 2-113, you’ll find that the fluctuating output from Q1 makes Q2 emit a two-tone signal. This is more interesting, but still not what I want. I’m thinking that if I make the pulses out of Q1 less abrupt, the result could be better, and the way to smooth a pulsing output is to hook up another capacitor that will charge at the beginning of each pulse and then release its charge at the end of each pulse. This is the function of C3 in Figure 2-114, and it completes the circuit so that it makes a whooping sound almost like a “real” alarm.
If you don’t get any audio output, check your wiring very carefully. It’s easy to make a wrong connection on the breadboard, especially between the three legs of each transistor. Use your meter, set to DC volts, to check that each section of the circuit has a positive voltage relative to the negative side of the power supply.
Figure 2-115 shows how your circuit should actually look on the breadboard.
Figure 2-113. R10 connects the slow-running oscillator at the top of the breadboard to the gate of Q2, the PUT in the middle of the breadboard. This modulates the audio oscillator, with addition of a smoothing capacitor.
Figure 2-114. This schematic shows the same circuit as in Figure 2-113:
R10: 10K
C3: 2.2 μF
Figure 2-115. This photograph shows the complete alarm-audio circuit on a breadboard.
Tweaking it
There’s still a lot of room for creativity here:
Adjust the frequency of the sound: Use a smaller or larger capacitor instead of C2 (half or twice the current value). Use a smaller or larger value for R5.
Adjust the pulsing feature: Use a smaller or larger capacitor instead of C1 (half or twice the current value). Use a smaller or larger value for R2.
General performance adjustments: try a larger value for R1. Try smaller or larger values for C3.
Try running the circuit at 7.5 volts, 10 volts, and 12 volts.
The circuits in this book are suggested as only a starting point. You should always try to tweak them to make them your own. As long as you follow the general rule of protecting transistors and LEDs with resistors, and respecting their requirements for positive and negative voltage, you’re unlikely to burn them out. Of course, accidents will happen—I myself tend to be careless, and fried a couple of LEDs while working on this circuit, just because I connected them the wrong way around.
Step 5: Enhancements
A noisemaking circuit is just the output of an alarm. You would need several enhancements to make it useful:
1. Some kind of an intrusion sensor. Maybe magnetic switches for windows and doors?
2. A way to start the sound if any one of the sensors is triggered. The way this is usually done is to run a very small but constant current through all of the switches in series. If any one switch opens, or if the wire itself is broken, this interrupts the current, which starts the alarm. You could make this happen with a double-throw relay, keeping the relay energized all the time until the circuit is broken, at which point, the relay relaxes, opening one pair of contacts and closing the other pair, which can send power to the noisemaker.
The trouble is that a relay draws significant power while it’s energized, and it also tends to get hot. I want my alarm system to draw very little current while it’s in “ready” mode, so that it can be powered by a battery. Alarm systems should never depend entirely on AC house current.
If we don’t use a relay, can we use a transistor to switch on the rest of the circuit when the power is interrupted? Absolutely; in fact, one transistor will do it.
3. But how do we arm the alarm in the first place? Really, we need a three-step procedure. First, check a little light that comes on when all the doors and windows are closed. Second, press a button that starts a 30-second countdown, giving you time to leave, if that’s what you want to do. And third, after 30 seconds, the alarm arms itself.
4. If the alarm is triggered, what then? If someone forces open a window, should the alarm stop sounding as soon as the window is closed again? No, the alarm should lock itself on, until you turn it off.
5. How do you turn it off? Some kind of secret-code keypad would be good.
6. But to avoid driving everyone crazy if the alarm is triggered when you’re not there, it should eventually stop itself, perhaps after about 10 minutes. At that point it should remain quiet but should light an LED to tell you what happened. You can then press a reset button to switch off the LED.
Implementing the Wish List
I’ve compiled a wish list that seems likely to make the project at least five times as complicated as it is already. Well, that’s what tends to happen when you go beyond little demo circuits and try to design something that will be useful in everyday life. Suddenly you find yourself having to accommodate all kinds of circumstances and situations.
Actually, I can and will show you how to take care of all the enhancements on the wish list, but I’m thinking that they will require us to get a little more serious about electronics projects in general first. If you’re going to build something ambitious, you’ll want to make it more permanent, and probably more compact, than a breadboard with components pushed into it.
You will need to know how to connect everything permanently with solder, on a piece of perforated board that you can install in a neat little project box with switches and lights on the outside.
I’m going to deal with soldering and packaging in the next chapter. After that, we’ll get back to the alarm project.
Larger versions of all schematics and breadboard photos are available online at this book's website: http://oreilly.com/catalog/9780596153748.
Larger versions of all schematics and breadboard photos are available online at this book's website: http://oreilly.com/catalog/9780596153748.
Larger versions of all schematics and breadboard photos are available online at this book's website: http://oreilly.com/catalog/9780596153748.
3. Getting Somewhat More Serious
I don’t know how far you’ll want to delve into electronics, but I do know that I’ve shown you about as much as I can with just a handful of components, some wires, a breadboard, and a few tools. To continue, you’ll need:
Some more tools and
supplies
Basic soldering skills
Additional knowledge about:
Integrated circuits
Digital electronics
Microcontrollers
Motors
The tools are not particularly exotic or expensive, and the soldering skills are easily acquired. Learning to join wires with solder is far easier than mastering high-level crafts such as jewelry making or welding.
As for additional areas of knowledge about electronics, they are no more challenging than those that I have covered already.
By the end of this section, you should be able to transplant components from a breadboard onto perforated board, where you will solder everything together, and then mount the board in a little box with switches and warning lights on the front, for everyday use.
Shopping List: Experiments 12 Through 15
Tools
Each of the following tools is rated Essential, Recommended, or Optional. The Essential tools will take you through this chapter of the book. If you supplement them with the Recommended tools, they should be sufficient to get you to the end of the book. The Optional tools will make your work easier, but whether they’re worth the money is for you to decide. Remember that URLs for manufacturers and sources of supply are all listed in the appendix.
I am assuming that you already have some commonly used workshop basics, such as an electric drill.
Essential: Pencil-type 15-watt soldering iron
Examples are RadioShack part 64-2051, McMaster-Carr catalog item 7016A34, or Xytronic model 252. See Figure 3-1. Soldering irons rated at 15 watts are less common than those that deliver 25 watts or more. Still, the 15-watt size is desirable for the small-scale work you’ll be doing, and greatly reduces the risk of damaging components by inflicting excess heat.
Figure 3-1. The low wattage of this pencil-style soldering iron enables you to use it safely on sensitive components, and the sharp tip helps to apply heat selectively.
When comparing prices, remember that a plated tip, which costs a little more, will last longer, will be easier to keep clean, and will conduct heat more reliably than a plain copper tip. If the manufacturer’s specification doesn’t mention a plated tip, the soldering iron probably doesn’t have one.
Essential: General-duty soldering iron, 30 to 40 watts
Although most of the projects in this book entail small, heat-sensitive components and thin wire, at some point you’re likely to want to make a solder joint with larger components and/or thicker wire. A 15-watt soldering iron will be unable to deliver enough heat. You should consider having a larger soldering iron in reserve, especially because they are relatively inexpensive.
Personally, I like the Weller Therma-Boost, shown in Figure 3-2, because it has an extra button that delivers more heat on demand. This is useful when you want the iron to get hot quickly, or if you are trying to solder very thick wire, which absorbs a lot of heat.
Figure 3-2. This higher-wattage soldering iron delivers the additional heat necessary for thicker wire or larger components. The discoloration quickly occurs as a result of everyday use and has no effect on the capability of the iron, as long as the tip of it is clean.
If you can’t find or don’t like the Weller, almost any 30-watt or 40-watt soldering iron will do. Check eBay or your local hardware store.
Essential: Helping hand
The so-called “helping hand” (or “third hand”) has two alligator clips that hold components or pieces of wire precisely in position while you join them with solder. Some versions of the “helping hand” also feature a magnifying lens, a wire spiral in which you can rest your soldering iron, and a little sponge that you use to clean the tip of your iron when it becomes dirty. These additional features are desirable. Helping hands are available from all electronics hobby sources. Examples are the catalog item HH55 from Elenco or model 64-2991 from RadioShack. See Figure 3-3.
Figure 3-3. The helping hand is fitted with two alligator clips to hold your work. The metal spiral is a safe place to holster a hot soldering iron, and you use the sponge to wipe its tip.
Essential: Magnifying lens
No matter how good your eyes are, a small, handheld, powerful magnifying lens is essential when you are checking solder joints on perforated board. The three-lens set in Figure 3-4 is designed to be held close to your eye, and is more powerful than the large lens on a “helping hand.” The folding lens in Figure 3-5 stands on your workbench for hands-free operation. Both are available from RadioShack and similar items are stocked by art supply stores and hobby shops. Plastic lenses are quite acceptable if you treat them carefully.
Figure 3-4. As long as you treat it carefully, a cheap set of plastic magnifying lenses is perfectly acceptable. Handheld magnification is essential for inspecting the solder joints that you make on perforated board.
Figure 3-5. This kind of folding magnifier can stand on your desktop and is useful for checking part numbers on tiny components.
Essential: Clip-on meter test leads
The probes that came with your multimeter require you to hold them in contact while you make a reading. This requires both hands, preventing you from doing anything else at the same time.
When you use a pair of “minigrabber” probes with little spring-loaded clips at the end, you can attach the Common (negative) lead from your meter to the negative side of your circuit and leave it there, while you touch or attach the positive probe elsewhere.
The Pomona model 6244-48-0 (shown in Figure 3-6) from Meter Superstore and some other suppliers is what you need. If you have trouble finding it or you object to the cost, you may consider making your own by buying a couple of “banana plugs” (such as RadioShack part 274-721) that will fit the sockets on your meter, and then use 16-gauge or thicker stranded wire to connect the plugs with IC test clips, such as Kobiconn 13IC331 or RadioShack “mini hook clips,” part number 270-372C. See Figures 3-7 and 3-8.
Figure 3-6. These “minigrabber” add-ons for meter leads make it much easier to measure voltage or current. Push the spring-loaded button, and a little copper hook slides out. Attach it to a wire, release the button, and you have your hands free for other tasks. It’s a mystery that meters are not supplied with these grabbers as standard equipment.
Figure 3-7. To make your own minigrabber meter leads, first attach a banana plug to a wire by sliding the wire through the cap, into the plug, and out through a hole in
the side.
Figure 3-8. Then screw a collar over the protruding piece of wire, and screw on the cap. The other end of the wire is soldered to a probe.
Essential: Heat gun
After you join two wires with solder, you often need to insulate them. Electrical tape, sometimes called insulating tape, is messy and tends to come unstuck. You’ll be using heat-shrink tube, which forms a safe, permanent sheath around a bare-metal joint. To make the tube shrink, use a heat gun, which is like a very powerful hair dryer. They’re available from any hardware supply source, and I suggest you buy the cheapest one you can find. See Figure 3-9.
Figure 3-9. Like an overpowered hair dryer, the heat gun is used with heat-shrink tubing to create a snug, insulated sheath around bare wire.
Essential: Solder pump
This little gadget sucks up hot, melted solder when you are trying to remove a solder joint that you made in the wrong place. Available from All Electronics (catalog item SSR-1) or RadioShack 64-2086. See Figure 3-10.
Essential: Desoldering wick
Also known as desoldering braid. See Figure 3-11. You use this to soak up solder, in conjunction with the Solder Pump. Available from All Electronics (catalog item SWK) or RadioShack (part 64-2090).
Essential: Miniature screwdriver set
Dinky little electronic parts often have dinky little screws in them, and if you t
ry to use the wrong size of screwdriver, you’ll tend to mash the heads of the screws. I like the Stanley precision set, part number 66-052, shown in Figure 3-12. But any set will do as long as it has both small Phillips and straight-blade screwdrivers.
Recommended: Soldering stand
Like a holster for a gun, you rest your soldering iron in this stand when the iron is hot but not on use. Examples are catalog item 50B-205 from All Electronics, RadioShack model 64-2078, or check eBay. See Figure 3-13. This item may be built into the helping hand, but you need an extra one for your second soldering iron.
Figure 3-10. To remove a solder joint, you can heat the solder until it’s liquid, then suck it up into this squeezable rubber bulb.
Figure 3-11. An additional option for removing liquid solder is to soak it up in this copper braid.
Figure 3-12. A set of small screwdrivers is essential.
Figure 3-13. A safe and simple additional stand for a hot soldering iron.
Recommended: Miniature hand saw
I assume that you will want to mount a finished electronics project in a decent-looking enclosure. Consequently, you are likely to need tools to cut, shape, and trim thin plastic. For example, you may want to cut a square hole so that you can mount a square power switch in it.
Power tools are not suitable for this kind of delicate work. A miniature handsaw (a.k.a. a “hobby saw”) is ideal for trimming things to fit. X-Acto makes a range of tiny saw blades. I suggest the #15 blade, plus the handle that it fits in, shown in Figure 3-14. Available online from Tower Hobbies, Hobbylinc, ArtCity, and many other arts/crafts sources. Also look for the larger X-Acto saw blade, #234 or #239, which you can use for cutting perforated board.
Make: Electronics Page 13
