Sensors take energy in forms such as sound or light and transform that energy into a signal. By using a sensor, you can detect heat, light, and sound, for example. When a signal is sensed, to the sensor produces an electrical signal that is used by your circuit to control some activity. For example, an infrared detector can work in conjunction with an infrared remote control device to stop or start a little go-kart.

IR detector: This converts infrared (IR) light into an electric signal. The version that we use in Chapters 11 and 9 contains a photodiode that detects infrared light and an integrated circuit that produces either +V or 0 volts on its output pin. In order to reduce noise from ambient IR light, this detector is designed to only respond to IR light that is pulsed at 38 kHz.

Tilt/vibration sensor: This type of sensor (which we use in "Couch Pet-Ato project) detects motion or vibrations when the switch is mounted with the body of the sensor horizontal. When the sensor detects motion, it closes a switch, just like a toggle switch works.

Capacitors are kind of like a voltage sandwich in that they have two plates, with a slab of voltage between them. A so-called condenser mike (also called a capacitor microphone) contains one plate made of a very light material that acts as a diaphragm. This plate vibrates when sound waves hit it. This moves the two plates apart, which changes capacitance (the ability to store electrons). Moving the plates farther apart decreases capacitance (discharging current), and moving them together increases capacitance (charging current). Condenser microphones aren’t cheap, but they give high-quality sound, so they are often the best choice for an audio-intensive project.

Today, the most popular type of condenser microphone is the electret microphone (which gets its name from the combination of electrostatic and magnet), invented in 1962. The electret material used in this type of microphone is made by embedding a permanent charge in a material called a dielectric. A charge is embedded in a dielectric by aligning the charges in the material — sort of like how you make a magnet by aligning the atoms in a piece of iron. There is a preamplifier in an electret microphone, to which you provide a supply voltage. That’s why the projects in this book that use electret microphones have a connection through a resistor running between the plus (+) lead of the microphone and the +V bus to power the preamplifier. (The resistor reduces the voltage at the + lead of the microphone to the desired supply voltage.)

When you order electret microphones, pay attention to the diameter and thickness because some can be hard to handle and solder. For most of our projects in this book, we use microphones with a diameter of about 3⁄8" and a thickness of about 2⁄10". A microphone cartridge with a diameter of about 1⁄4" and a thickness of about 1⁄10" turns out to be much harder to handle and solder to than a microphone cartridge of about 3⁄8" and a thickness of about 2⁄10". (Check out Chapter 6, where we bit the bullet and used a small microphone cartridge because that project needed some of the capabilities we couldn’t find in a larger microphone cartridge.) Many microphone cartridge sizes are specified in millimeters. To help you translate this, typical diameters of microphone casings are 6 mm (about 1⁄4") and 9.7 mm (about 3⁄8").

Sensitivity is another issue that you should pay attention to with microphone cartridges. Sensitivity is measured in decibels (dB) — and just to confuse you, this measurement is given as a negative number. A microphone cartridge with a sensitivity of –40 dB, for example, is more sensitive (provides higher voltage at a given level of sound) than a microphone cartridge with a sensitivity of –60. For example, for the project in Chapter 6 (which has to pick up very faint sounds as part of a parabolic microphone), you need a highly sensitive microphone cartridge. We use one with a sensitivity of –35 dB. In Chapter 14, in which you talk directly into the microphone to record a message, we use a less-sensitive microphone cartridge, rated at –64 dB.

To connect electret microphone cartridges to your project, you can get electret microphone cartridges with solder pads or with leads that you can insert into a breadboard. We use both in our projects.

A diode sends out light when you pass an electric current through it. LEDs, which we use quite a bit in the projects in this book, are similar to the tiny, twinkly lights you use to decorate a Christmas tree, and they come in a variety of colors, such as red, orange, yellow, green, blue, and white. Blue and white LEDs are a lot more expensive, so you don’t see them used that often in this part of web site. (We’re thrifty!)

LED color isn’t controlled by the plastic that surrounds the light. Rather, the semiconductor material used in the LED determines the color. The plastic surrounding the semiconductor material can be clear or treated so that it diffuses the light. In addition, you can get LEDs in several sizes and shapes. The standard LED, which is a cylinder with a diameter of 5mm, is referred to as T-1 3⁄4.

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 amplifier 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.

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 buzzer essentially 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 piezoelectric crystal), 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 bzzz sound. There are electromagnet buzzers, but the piezo buzzer works just fine for electronic projects, so we stick to using them in this part of web site.