Make: Electronics Charles Platt (smart books to read txt) 📖

Now that you have a coil, you can hook it up on your breadboard as shown in Figure 5-34, where the green circle is a tactile switch and the two circular red objects are LEDs. Make sure that you use low-current LEDs (otherwise, you may not see anything) and make sure that one of them is negative-side-up, positive-side-down and the other is positive-side-up, negative-side-down. Also, the 220Ω resistor should be rated at 1/4 watt or higher, if possible (see the following caution).

Hot Resistors

You’ll be passing about 50mA through the 220Ω resistor, while the current is flowing. At 12 volts, this works out at 0.6 watts. If you use a 1/8-watt resistor, you will be overloading it, and it will get quite hot and may burn out. If you use a 1/4-watt resistor, it will still get hot, but is unlikely to burn out, as long as you don’t press the button for more than a second or two.

Don’t run the circuit without the coil of wire; you’ll be trying to pass more than 50mA through the LEDs.

Figure 5-34. The breadboarded version of the schematic in Figure 5-33 shows a simple way to set it up for a quick demo. The green button is a tactile switch. The two red LEDs should be placed so that the polarity of one is opposite to the polarity of the other.

When you press the button, one LED should flash briefly. When you release the button, the other LED should flash.

What’s happening here? The coil possesses self-inductance, which means that it reacts against any sudden change in the flow of electricity. First it fights it, and during that brief moment, it blocks most of the current. Consequently, the current looks for an alternative path and flows through D1, the lefthand LED in the schematic. (D2 doesn’t respond, because it can pass current only in the opposite direction.)

Meanwhile, the voltage pressure overcomes the coil’s self-inductance. When the self-inductance disappears, the resistance of the coil is no more than 10 ohms—so now the electricity flows mostly through the coil, and because the LED receives so little, it goes dark.

When you disconnect the power, the coil reacts again. It fights any sudden changes. After the flow of electricity stops, the coil stubbornly sustains it for a moment, because as the magnetic field collapses, it is turned back into electricity. This residual flow of current depletes itself through D2, the LED on the right.

In other words, the coil stores some energy in its magnetic field. This is similar to the way a capacitor stores energy between two metal plates, except that the coil blocks the current initially and then lets it build up, whereas the capacitor sucks up current initially, and then blocks it.

The more turns of wire you have in your coil, the more self-inductance the coil will have, causing your LEDs to flash more brightly.

Here’s one last variation on this experiment to test your understanding of electrical fundamentals. Remove the 220Ω resistor, and substitute a 1K resistor (to protect your LED from sustained current). Remove the coil, and substitute a very large capacitor—ideally, about 4,700 μF. (Be careful to get its polarity the right way around.) What will you see when you press the button? Note that you will have to hold it down for a couple of seconds to get a result. And what will you see when you release the button? Remember: the behavior of capacitance is opposite to the behavior of self-inductance.

Theory

Alternating current concepts

Here’s a simple thought experiment. Suppose you set up a 555 timer to send a stream of pulses through a coil. This is a primitive form of alternating current.

We might imagine that the self-inductance of the coil will interfere with the stream of pulses, depending how long each pulse is, and how much inductance the coil has. If the pulses are too short, the self-inductance of the coil will tend to block them. Maybe if we can time the pulses exactly right, they’ll synchronize with the time constant of the coil. In this way, we can “tune” a coil to allow a “frequency” to pass through it.

What happens if we substitute a capacitor? If the pulses are too long, compared with the time constant of the capacitor, it will tend to block them, because it will have enough time to become fully charged. But if the pulses are shorter, the capacitor can charge and discharge in rhythm with the pulses, and will seem to allow them through.

I don’t have space in this book to get deeply into alternating current. It’s a vast and complicated field where electricity behaves in strange and wonderful ways, and the mathematics that describe it can become quite challenging, involving differential equations and imaginary numbers. However, we can easily demonstrate the audio filtering effects of a loudspeaker and a coil.

Experiment 29: Filtering Frequencies

In this experiment, you’ll see how self-inductance and capacitance can be used to filter audio frequencies. You’re going to build a crossover network: a simple circuit that sends low frequencies to one place and high frequencies to another.

You will need:

Loudspeaker, 8Ω, 5 inches in diameter. Quantity: 1. Figure 5-35 shows a typical example.

Audio amplifier, STMicroelectronics TEA2025B or similar. Quantity: 1. See Figure 5-36.

Figure 5-35. To hear the effects of audio filters using coils and capacitors, you’ll need a loudspeaker capable of reproducing lower frequencies. This 5-inch model is the minimum required.

Figure 5-36. This single chip contains a stereo amplifier capable of delivering a total of 5 watts into an 8Ω speaker when the two channels are combined.

Figure 5-37. A nonpolarized electrolytic capacitor, also known as a bipolar capacitor, looks just like an electrolytic capacitor, except that it will have “NP” or “BP” printed on it.

Nonpolarized electrolytic capacitors (also known as bipolar). 47 μF. Quantity: 2. A sample is shown in Figure 5-37. They should have “NP” or “BP” printed on them to indicate