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Switching Basics and More 75

Experiment 10: Transistor Switching

Your finger is conducting positive voltage to the base of the transistor. Even
though your skin has a high resistance, the transistor still responds. It isn’t just
switching the LED on and off; it is amplifying the current applied to its base.
This is an essential concept: a transistor amplifies any changes in current that
you apply to its base.

Check Figure 2-88 to see more clearly what’s happening.

If you studied the section “Background: Positive and negative” in Chapter 1,
you learned that there is really no such thing as positive voltage. All we re-
ally have is negative voltage (created by the pressure of free electrons) and
an absence of negative voltage (where there are fewer free electrons). But be-
cause the idea of a flow of electricity from positive to negative was so widely
believed before the electron was discovered, and because the inner workings
of a transistor involve “holes” which are an absence of electrons and can be
thought of as positive, we can still pretend that electricity flows from positive
to negative. See the following section, “Essentials: All about NPN and PNP tran-
sistors,” for more details.

R1

Q1

12v

DC

Finger

tip R3

D1

Figure 2-87

R1

41

R3

)LQJHU

WLS

Figure 2-88. These two diagrams show

the same components as before, with a

fingertip substituted for R2. Although only

a trickle of voltage now reaches the base

of the transistor, it’s enough to make the

transistor respond.