When current moves through the electromagnet, which is attached to the
cone, it gets pushed toward or pulled away from the permanent magnet. This
depends on which way the electric current is moving. This movement of the
electromagnet is what makes the cone vibrate, and that produces sound waves.
Speakers come with a rated impedance(the degree to which a component
resists electrical current): for example, 4 ohm, 8 ohm, 16 ohm, or 32 ohm. A
speaker is often referred to by its impedance: for example, “I’m going out to
buy an 8 ohm speaker.” When you use a speaker in a circuit, it should have an
impedance rating that matches the minimum impedance rating that the ampli-
fier hooked up to the speaker can drive. If you use a speaker with higher
impedance than the amplifier can drive, you won’t get the maximum amount
of sound; conversely, if you use a speaker with lower impedance than the
amplifier can drive, you might overheat the amplifier. You can find this rating
in the datasheet on your supplier’s Web site.
For example, in Chapter 8, we use an 8 ohm speaker because the LM386
amplifier can drive a speaker with impedance as low as 8 ohms. And in
Chapter 14, we use a 16 ohm speaker because the ISD1110 voice record/
playback chip can drive a speaker with impedance as low as 16 ohms.
Speakers also come with a power rating, such as 0.2 watt, 1 watt, or 2 watt.
Choose a speaker with at least as high of a power rating as the maximum
output of the amplifier. Again, you can find this maximum output in the ampli-
fier datasheet.
When you buy a speaker for electronics projects, buy one that comes with
convenient holes in the corners of metal or plastic flanges that you can slip
screws through. These help you to easily attach the speaker to the box
you’re putting the circuit in. See Chapter 4 for more about building and
assembling projects.
Buzzers ..................................................................................................
If you have an annoying friend who plays practical jokes, you’ve probably
been on the receiving end of the buzzer and handshake joke. A buzzeressen-
tially generates a sound, which you can use in projects in a variety of ways.
For example, a buzzer could be the horn on a remote controlled car or an
alarm that goes off when a sensor detects motion.
In a buzzer, you apply voltage to a crystal (a piezoelectriccrystal), which then
expands or contracts. By attaching a diaphragm to the crystal, you cause the
diaphragm to vibrate when you change the voltage. This vibration causes that
bzzzsound. There are electromagnet buzzers, but the piezo buzzer works just
fine for electronic projects, so we stick to using them in this book.
Chapter 3: Assembling Your Electronics Arsenal 51