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Switching Basics and More 73

Experiment 10: Transistor Switching

Experiment 10: Transistor Switching

You will need:

• AC adapter, breadboard, wire, and meter.


• LED. Quantity: 1.


• Resistors, various.


• Pushbutton, SPST. Quantity: 1.


• Transistor, 2N2222 or similar. Quantity: 1.

A transistor can switch a flow of electricity, just like a relay. But it’s much more
sensitive and versatile, as this first ultra-simple experiment will show.

We’ll start with the 2N2222 transistor, which is the most widely used semi-
conductor of all time (it was introduced by Motorola in 1962 and has been in
production ever since).

First, you should get acquainted with the transistor. Because Motorola’s pat-
ents on the 2N2222 ran out long ago, any company can manufacture their
own version of it. Some versions are packaged in a little piece of black plastic;
others are enclosed in a little metal “can.” (See Figure 2-83.) Either way, it con-
tains a piece of silicon divided into three sections known as the collector, the
base, and the emitter. I’ll describe their function in more detail in a moment,
but initially you just need to know that in this type of transistor, the collector
receives current, the base controls it, and the emitter sends it out.

Use your breadboard to set up the circuit shown in Figure 2-85. Be careful to
get the transistor the right way around! (See Figure 2-84.) For the three brands
I have mentioned in the shopping list, the flat side should face right, if the
transistor is packaged in black plastic, or the little tab should face toward the
lower left, if the transistor is packaged in metal.

Figure 2-85. The transistor blocks voltage that reaches it through R1.

But when pushbutton S1 is pressed, this tells the transistor to allow

current to pass through it. Note that transistors are always identified

with letter Q in wiring diagrams and schematics.

S1: Pushbutton, momentary, OFF (ON)

R1: 180Ω

R2: 10K

R3: 680Ω

Q1: 2N2222 or similar

D1: LED

S1

R2

R1

Q1

12v

DC

D1

R3

Figure 2-83. A typical transistor is pack-

aged either in a little metal can or a molded

piece of black plastic. The manufacturer’s

data sheet tells you the identities of the

three wire leads, relative to the flat side of

a black plastic transistor or the tab that

sticks out of a metal-can transistor.

Collector

Base

Emitter

Collector

Base

Emitter

Figure 2-84. The 2N2222 transistor may be

packaged in either of these formats. Left:

RadioShack or Fairchild. Right: STMicro-

electronics (note the little tab sticking out

at the lower-left side). If you use a different

brand, you’ll have to check the manufac-

turer’s data sheet. Insert the transistor in

your breadboard with the flat side facing

right, as seen from above, or the tab point-

ing down and to the left, seen from above.