Running down discrete components:
Resistors, capacitors, and transistors
Whereas discreetcomponents should be good at keeping secrets, discrete
components are so-named because they are one single, solitary thing, rather
than a collection of components like those contained in an integrated chip
(IC). (Read more about ICs later in this chapter.) Discrete components are
single electrical items, typically resistors, capacitors, and transistors.
Diodes are also discrete components, but the only kind we use in this book
are light emitting diodes (LEDs). See the later section, “Let there be light:
Light emitting diodes,” for more about these.
Resistors
The job of a resistor is to restrict the flow of current, which is essentially the
flow of electrons. The more electrons, the higher the current. For example,
you might want to stop LEDs (which love to eat current) from burning them-
selves out, and so you would add a resistor to your circuit. You’ll find these
little guys used all the time in electronics circuits. We measure resistance in
ohms, which are so tiny that the measurement of them is usually given in
thousands (kohms) or millions (megohms) of ohms. The value of a resistor is
indicated by colored bands, with each band representing a number. However,
rather than counting colored bands, just read the packaging your resistor
comes in or test it with a multimeter.
One variation on a resistor is a variable resistor, also called a potentiometer.A
variable resistor allows you to constantly adjust from 0 (zero) ohms to a max-
imum value. These can be mounted on the face of a gadget, where you adjust
them with a knob; or, you can mount them on a circuit board, where you have
to adjust them by using a screwdriver. A typical use of a potentiometer is to
control the volume of an amplifier in a sound circuit.
Capacitors
A capacitorstores an electric charge. Quite often, you’ll see capacitors used
hand in hand with a resistor — for example, in a circuit whose job is to set
timing. Because it takes time for a capacitor to fill with a charge, you can set
your watch by them (so to speak) if you use a resistor to control how fast the
charge (that is, current) flows in. Also, they make good filters for DC signals
because although AC passes through a capacitor with ease, DC signals are
stopped in their tracks.
Capacitanceis a measurement of a capacitor’s ability to store a charge. The
larger the capacitance, the more charge is stored. You measure capacitance
Chapter 3: Assembling Your Electronics Arsenal 39