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NORTH POLE OF PERMANENT MAGNET
The positive and negative terminals of the battery connect so that each end of the electromagnet has the same polarity as the permanent magnet next to it. Like poles of magnets repel each other. This repelling action moves the electromagnet and causes the axle to spin. As the axle spins, the positive and negative connections to the electromagnet swap places, so the magnets continue to push the axle around. A simple mechanism consisting of a commutator (a segmented wheel with each segment connected to a different end of the electromagnet) and brushes that touch the commutator cause the connections to change. The commutator turns with the axle and the brushes are stationary, with one brush connected to the positive battery terminal and the other brush to the negative battery terminal. As the axle, and therefore the commutator, rotates, the segment in contact with each brush changes. This in turn changes which end of the electromagnet is connected to negative or positive voltage.
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116 Part II: Aisle 5, Component Shack: Stocking Up_
If you want to get a feel for the mechanism inside a DC motor, buy a cheap one for a few dollars and tear it apart.
The axle in a DC motor rotates a few thousand times per minute — a bit fast for most applications. Suppliers sell DC motors with something called a gear head pre-mounted; this device reduces the speed of the output shaft to under a hundred revolutions per minute (rpm). This is similar to the way that changing gears in your car changes the speed of the car.
Suppliers’ catalogs typically list several specifications for the motors they carry. Two key things that you need to consider are
Speed: Listed as rpm (revolutions per minute). The speed that you need depends on your project. For example, when turning the wheels of a model car, you may aim for 60 rpm, with the motor rotating the wheels once per second.
Operating voltage: The operating voltage is given as a range. Electronics projects typically use a motor that works in the 4.5- to 12-volt range.
Also notice the manufacturer’s nominal voltage and stated rpm for the motor. The motor runs at this rpm when you supply the nominal voltage. If you supply less than the nominal voltage, the motor runs slower than the stated rpm.
DC motors have two wires (or terminals that you solder wires to), one each for the positive and negative supply voltage. You run the motor by simply supplying a DC voltage that generates the speed that you want and switching off the voltage when you want the motor to stop.
You can use a more efficient method of controlling the speed of the motor called pulse width modulation. This method turns voltage on and off in quick pulses. The longer the on intervals, the faster the motor goes. If you’re building a kit for something motor-controlled, such as a robot, the electronics for the kit supplies this speed control.
If you’re attaching things such as wheels, fan blades, and so on to the motor shaft, be sure that you have attached the component securely before you apply power to the motor. If not, the item may spin off and hit you, or someone near and dear to you, in the face.
So You Want to Make Some Noise ?
So, you’ve probably asked yourself at one time or another, just what is sound? Sound is simply a series of vibrations traveling through the air. When you talk, for example, your vocal cords vibrate to create sound waves that travel to a listener’s ear.
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Chapter 5: Filling Out Your Parts Bin
In electronics, you can use speakers and buzzers to create sound. In fact, the world of electronics tends to be a noisy one: You can activate music, buzzers, alarms, and other sounds with your electronic gadgets. In the following sections, we explore these devices that you can use to get your project heard.
Speaking of speakers
Most speakers simply consist of one permanent magnet, an electromagnet, and a cone. Figure 5-9 shows how the components of a speaker are arranged.
The electromagnet is attached to the cone. When electric current moves through the electromagnet, it either gets pulled toward the permanent magnet or (if the electric current goes in the other direction) pushed away from the permanent magnet. The motion of the electromagnet causes the cone to vibrate, which creates sound waves.
You state the frequency range over which a speaker generates sound in Hz (hertz) or kHz (kilohertz). The human ear can hear sound over a frequency range of about 20Hz to 20kHz. Speakers generate sound over various ranges, depending on their size and design (for example, in a stereo set up, one speaker may generate in the bass range and another in a higher range). If you’re just looking for a general-purpose speaker, don’t worry too much about the frequency range. If you’re building a super-duper high-end audio system, you probably want to spend a lot of time investigating and picking out speakers that meet your needs.