Hook the free end of your antenna to one end of your coil. At this point, you also need to add a germanium diode, which functions like a silicon-based diode but is better suited to the tiny voltages and currents that we’ll be dealing with. The other end of the diode attaches to one of the wires leading to a high-impedance earphone. A normal modern earphone or headphone will not work in this circuit. The return wire from the headphone is connected to a jumper wire, the other end of which can be clipped to any of the taps in your coil.
One last modification, and you’ll be ready to tune in. You have to ground the jumper wire. By this I mean connect it to something that literally goes into the ground. A cold-water pipe is the most commonly mentioned option, but (duh!) this will work only if the pipe is made of metal. Because a lot of plumbing these days is plastic, check under the sink to see if you have copper pipes before you try using a faucet for your ground.
Another option is to attach the wire to the screw in the cover plate of an electrical outlet, as the electrical system in your house is ultimately grounded. But the sure-fire way to get a good ground connection is to go outside and hammer a 4-foot copper-clad grounding stake into reasonably moist earth. Any wholesale electrical supply house should be able to sell you a stake. They’re commonly used to ground welding equipment.
Figure 5-65. The simple pleasure of picking up a radio signal with ultra-simple components and no additional power.
Figures 5-66 and 5-67 show the completed radio.
Figure 5-66. A signal from the antenna can pass through the coil to ground. If the jumper wire is attached to an appropriate tap on the coil, it resonates with the radio signal, just powerfully enough to energize the earphone which is wired in series with a diode.
Figure 5-67. The real-life version of Figure 5-66.
If you’ve managed to follow these instructions (one way or another), it’s time to tune your radio to the nearest station. Move the alligator clip at the end of your patch cord from one tap to another on your coil. Depending on where you live, you may pick up just one station, or several, some of them playing simultaneously.
It may seem that you’re getting something for nothing here, as the earphone is making noise without any source of power. Really, though, there is a source of power: the transmitter located at the radio station. A large amplifier pumps power into the broadcasting tower, modulating a fixed frequency. When the combination of your coil and antenna resonates with that frequency, you’re sucking in just enough voltage and current to energize a high-impedance headphone.
The reason you had to make a good ground connection is that the radio station broadcasts its signal at a voltage relative to ground. The earth completes the circuit between you and the transmitter. For more information on this and other concepts relating to radio, see the upcoming section “Theory: How radio works.”
Enhancements
The higher your antenna is, the better it should work. In my location, this is a major problem, as I live in a desert area without any trees. Still, just stringing the wire out of my window and tethering it (with rope) to the front bumper of my car enabled me to pick up a faint radio signal.
To improve the selectivity of your radio, you can add a variable capacitor, as shown in the following section. This allows you to “tune” the resonance of your circuit more precisely. Variable capacitors are uncommon today, but you can find one at the same specialty source that I recommended for the earphone and the germanium diode: the Scitoys Catalog (http://www.scitoyscatalog.com).
This source is affiliated with a smart man named Simon Quellan Field, whose site suggests many fun projects that you can pursue at home. One of his clever ideas is to remove the germanium diode from your radio circuit and substitute a low-power LED in series with a 1.5-volt battery. This didn’t work for me, because I live 40 miles from the nearest AM broadcaster. If you’re closer to a transmitter, you may be able to see the LED varying in intensity as the broadcast power runs through it.
Theory
How radio works
When electrical frequencies are very high, the radiation they create has enough energy to travel for miles. This is the principle of radio transmission: A high-frequency voltage is applied to a broadcasting antenna, relative to the ground.
When I say “ground” in this instance, I literally mean the planet beneath us. If you set up a receiving antenna, it can pick up a faint trace of the transmission relative to the ground—as if the earth is one huge conductor. Actually the earth is so large and contains so many electrons, it can function as a common sink, like a gigantic version of the file cabinet that I suggested you should touch to get rid of static electricity in your body before touching a CMOS logic chip.
To make a radio transmitter, I could use a 555 timer chip running at, say, 850 kHz (850,000 cycles per second), and pass this stream of pulses through an amplifier to a transmission tower; if you had some way to block out all the other electromagnetic activity in the air, you could detect my signal and reamplify it.
This was more or less what Marconi (shown in Figure 5-68) was doing in 1901, after he had purchased rights to Edison’s wireless telegraphy patent, although he had to use a primitive spark gap, rather than a 555 timer, to create the oscillations. His transmissions were of limited use, because they had only two states: on or off. You could send Morse code messages, and that was all.
Figure 5-68. Marconi, the great pioneer of radio (photograph from Wikimedia Commons).
Five years later, the first true audio signal was transmitted by imposing lower audio frequencies on the high-frequency carrier wave. In other words, the audio signal was “added” to the carrier frequency, so that the power of the carrier varied with the peaks and valleys of the audio.
At the receiving end, a very simple combination of a capacitor and a coil detected the carrier frequency out of all the other noise in the electromagnetic spectrum. The values of the capacitor and the coil were chosen so that their circuit would “resonate” at the same frequency as the carrier wave. Figures 5-69 and 5-70 illustrate these concepts.
Figure 5-69. When an audio signal (middle) is combined electronically with a high carrer frequency (top), the result looks something like the compound signal at the bottom. In actuality, the carrier frequency would be much higher compared with the audio frequency, by a radio of perhaps 1,000:1.
Figure 5-70. When the compound signal is passed through a diode, only the upper half remains. An earphone cannot react fast enough to reproduce the high carrier frequency, so it “rides” the peaks and thus reproduces the audio frequency.
The schematic in Figure 5-71 shows the simple circuit that you built by wrapping a coil around an empty vitamin bottle. When a positive pulse was received by the antenna, it resonated with the antenna and the coil, provided that the antenna was long enough and the coil was tapped at the appropriate number of turns.
Theory
How radio works (continued)
Figure 5-71. An antenna at the top of the schematic picks up faint electromagnetic radiation from a distant transmitter. The coil at the left side is tapped at intervals so that its resonance can be adjusted to match the carrier frequency of the radio signal. Other frequencies are grounded (at the bottom of the schematic). The diode passes the “top half” of the signal to the earphone at the right, which is incapable of responding fast enough to reproduce the carrier frequency, and thus filters it out, leaving only the audio frequencies that were superimposed on it.
By adding a capacitor, you can tune the circuit. Now an incoming pulse from the transmitter is initially blocked by the self-inductance of the coil, while it charges the capacitor. If an equally negative pulse is received after an interval that is properly synchronized with the values of the coil and the capacitor, it coincides with the capacitor discharging and the coil conducting. In this way, the right f
requency of carrier wave makes the circuit resonate in sympathy. At the same time, audio-frequency fluctuations in the strength of the signal are translated into fluctuations in voltage in the circuit.
What happens to other frequencies pulled in by the antenna? The lower ones pass through the coil to ground; the higher ones pass through the capacitor to ground. They are just “thrown away.”
The righthand half of the circuit samples the signal by passing it through a germanium diode and an earphone. The power from the transmitter is just sufficient to vibrate the diaphragm in the earphone, after the diode has subtracted the negative half of the signal.
Look back at the diagram of the amplitude-modulated signal. You’ll see that it fluctuates up and down so rapidly, the earphone cannot possibly keep up with the positive-negative variations—hence the need for the diode. It will remain hesitating at the midpoint between the highs and lows, producing no sound at all. The diode solves this problem by subtracting the lower half of the “audio envelope,” leaving just the positive spikes of voltage. Although these are still very small and rapid, they are now all pushing the diaphragm of the earphone in the same direction, so that it averages them out, approximately reconstructing the original sound wave.
Figure 5-72 shows how the circuit can be enhanced with a variable capacitor, to tune it without needing to tap the coil at intervals.
Figure 5-72. By adding a capacitor to the circuit, its resonance can be tuned more precisely. The diagonal arrow indicates that a variable capacitor is used.
The radio can pull in the stations on the AM (amplitude-modulated) waveband that happen to be most powerful in your area. The waveband ranges from 300 kHz to 3 MHz. If you find yourself interested in radio, your next step could be to build a powered radio using a couple of transistors. Alternatively you could build your own (legal) low-power AM transmitter. There’s an ultra-simple kit available from http://www.scitoys.com consisting of just two principal components: a crystal oscillator, and a transformer, shown in Figure 5-73. That’s all it takes.
Figure 5-73. An AM radio transmitter can be made from just two components: a transformer (left) and a crystal oscillator (right), available from http://www.scitoys.com.
Experiment 32: A Little Robot Cart
Robotics is another application of electronics that deserves a book in itself—or several books. So, once again, I’m going to give you an introduction followed by some points that you can follow if you want to go further. As always, I will start with the simplest possible device, which in the world of robotics is a cart that finds its way around your living room.
You will need:
SPST or SPDT microswitches requiring minimal pressure to activate them. A force between 0.02 and 0.1 newtons would be ideal. Quantity: 2. See Figure 5-74.
Figure 5-74. A microswitch has a small button (at the front, righthand side in this picture) that is often actuated by a pivoted metal lever. The switch can respond to a very light pressure, but can handle relatively high currents.
DC gear-motor, rated for any voltage between 5 and 12, drawing a maximum of 100mA in its free-running state, output shaft turning between 30 and 60 RPM. Quantity: 1. A motor is shown in Figure 5-75.
Figure 5-75. For the Little Robot Cart, I found this 5-volt motor, which is supplied with a disc that fits its output shaft. The combination cost less than $10.
Disc or arm that fits securely onto your motor shaft. Quantity: 1.
555 timer. Quantity: 1.
DPDT nonlatching relay rated for the same voltage as your motor. Quantity: 1.
1/4-inch plywood or plastic, one piece about 2 feet square.
#4 sheet-metal screws, 5/8 inch or 3/4 inch long. Quantity: 2 dozen.
#6 bolts, 3/4 inch long, with nylon-insert lock nuts. Quantity: 2 dozen.
1/4-inch bolts, 1 inch long, with nuts, to mount the wheels. Quantity: 4.
I’m not specifying one particular motor, because if I did, it might not be available by the time you read this. Motors aren’t like logic chips, which have retained their basic function throughout various improvements over a period of several decades. Motors come and go, and many that you may run across will be surplus parts that will never been seen again. Search online for “gear-motor” or “gearhead motor” and find one as close as possible to the specification that I have provided. The mechanical power output of the motor shouldn’t be important, because we won’t be requiring it to do much work.
The important consideration when you buy your motor is that you should also obtain something that fits onto its output shaft. Typically, this will be a disk or arm that can be screwed into place. To this you can then add a larger wheel of your own, which you can cut with a hole saw or make from the screw-on lid of a jar, or anything else circular that you may find in the house.
A larger wheel will make your cart move faster than a smaller wheel, but will reduce its torque, thus limiting its power to overcome obstacles.
This brings me to my next topic: fabrication. Although this is an electronics book, motors are electromechanical devices, and you have to be able to install them in some kind of a machine to get any interesting results. You can use plywood to complete the two little robotics projects here (ideally, the kind of thin, high-quality plywood sold by hobby stores) but I recommend something that looks better and is easier to work with: ABS plastic. Before you start on the robotic cart, you may want to check the section “Fundamentals: All about ABS.”
Fundamentals
All about ABS
Unless you think the steampunk movement isn’t going back far enough, you probably don’t want your autonomous robot cart to resemble a relic from before the 1800s. Therefore, wood may not be the best construction material. Metal can look nice, but is not easy to work with. For quick results that have a twentieth-century look (maybe even a 21st-century look), plastic is the obvious choice, and I feel that ABS is the best type of plastic to use, because it provides such quick, easy results. ABS stands for “acrylonitrile butadiene styrene.” Lego® blocks are made of ABS. Car-stereo installers and model-railroad buffs use it. You can use it, too. You can saw it, drill it, sand it, whittle it, and drive screws into it, and it won’t warp, split, or splinter. It’s washable, doesn’t need to be painted, and will last almost forever.
Delrin is another type of plastic, but tends to cost more and is a little tougher to drill and cut. It’s a matter of personal preference. ABS machines fairly well, but when you drill it, for example, it can “catch” on the bit and the piece will spin with the bit due to the way that plastic chips off with the bit. Delrin is self-lubricating and has better melting properties under the heat of machining, so it drills and cuts much more cleanly and easily than ABS.
Where to find ABS
Pieces of ABS a couple of feet square are available from online sources such as http://hobbylinc.com or estreetplastics (an eBay store), but you’ll save money if you can truck on down to your nearest plastic supply house and buy it like plywood, in sheets measuring 4 by 8 feet. To discover whether you have a nearby plastic supply house, search for “plastic supply” in your yellow pages or Google Local.
Piedmont Regal Plastics has many supply centers around the nation, but you’ll have to collect it yourself, and they may not be willing to cut small pieces. You can check online at http://www.piedmontplastics.com for their locations.
Stock colors of ABS include black, white, and “natural,” which is beige. Sheets usually are textured on one side, which is the side that should face outward, as it is more scratch-resistant than the smooth side.
Because you won’t be adding paint or other finishes, you’ll have to be careful not to scuff the plastic or scratch it while working. Clean your bench thoroughly before you begin, taking special care to remove any metal particles, which tend to become embedded in the plastic. Use wooden shim
s in the jaws of your vise, and avoid resting the plastic accidentally on any sharp tools or screws. Working with ABS requires a clean environment and a very gentle touch.
Cut with Care
You can saw ABS, but if you use a table saw, the plastic will tend to melt and stick to the blade. These smears will get warm and sticky when you feed the next piece of plastic into the saw, and the result will be extremely unpleasant. The whirling blade will grab the plastic and hurl it at you powerfully enough to break bones. This is known as “kickback” and is a very serious risk when sawing plastic.
If you have extensive experience using a table saw, you are actually more vulnerable, because the reflexes and cautions you have developed while dealing with wood will not be adequate for working with plastic. Please take this warning seriously!
Your first and most obvious precaution is to use a plastic-cutting blade, which has a larger number of thicker teeth to absorb the heat. The blade I have used is a Freud 80T, but there are others. If you use a blade that is not suitable, you will see it starting to accumulate sticky smears. This is the only warning you will get. Clean that blade with a solvent such as acetone, and never use it for ABS again.
Regardless of other precautions, always wear gloves and eye protection when using a table saw, and stand to one side when feeding materials into it. Personally, after one episode of kickback that I thought had broken my arm, I prefer not to use a table saw on plastic at all.
For long, straight cuts, the alternatives include:
Panel saw (big and expensive, but safe and accurate).
Miniature handheld circular saw with a blade around 4 inches in diameter, guided with a straight edge clamped to the sheet.
Make: Electronics Page 36
