Make Electronics

Switching Basics and More 87

Experiment 11: A Modular Project

I want you to keep the slow-flashing circuit separately, untouched, because I
have an idea to make use of it a little later. You can leave the LED blinking.

The loudspeaker should be wired in series with a 100Ω resistor to limit the cur-
rent that flows out of the PUT. The loudspeaker doesn’t have any polarity, even
though it is fitted with a red wire and a black wire. You can connect it either
way around.

Initially, you may be disappointed, because the circuit will not seem to be do-
ing anything. However, if you place your ear very, very close to the loudspeak-
er, and if you wired the circuit correctly, you should hear a faint buzz, like a
mosquito. Obviously, this isn’t loud enough to serve any practical purpose. We
need to make it louder. In other words, we need to amplify it.

Maybe you remember that the 2N2222, which you played with previously, can
function as an amplifier. So let’s try using that.

Step 3: Amplification

Disconnect the loudspeaker and its 100Ω series resistor. Then add the 2N2222,
which is linked with the output from the PUT via a 1K resistor to protect it from
excessive current. See Figure 2-107.

The emitter of the 2N2222 is connected to ground, and the collector is sup-
plied through the loudspeaker and its 100Ω series resistor. This way, small fluc-
tuations in the output from the PUT are sensed by the base of the 2N2222
which converts them into bigger fluctuations between the collector and the
emitter, which draw current through the loudspeaker. Check the schematic in
Figure 2-108.

Now the sound should be louder than an insect buzz, but still not really loud
enough to be useful. What to do?

Well—how about if we add another 2N2222? Bipolar transistors can be placed
in series, so that the output from the first one goes to the base of the second
one. The 24:1 amplification of the first one is multiplied by another 24:1, giving
a total amplification of more than 500:1.

There are limits to this technique. The 2N2222 can only conduct so much cur-
rent before getting overloaded, and excess amplification can cause distortion.
But when I built this circuit, I used a meter to verify that we’re still within the
design limits of a 2N2222, and for this project, I don’t care whether the sound
is slightly distorted.

BAckground

Mounting    a   

loudspeaker

The diaphragm or cone of a

loudspeaker is designed to radi-

ate sound, but as it oscillates to

and fro, it emits sound from its

back side as well as its front side.

Because the sounds are opposite

in phase, they tend to cancel each

other out.

The perceived output from a

loudspeaker can increase dramati-

cally if you add a horn around it in

the form of a tube to separate the

output from the front and back

of the speaker. For a miniature

1-inch loudspeaker, you can bend

and tape a file card around it. See

Figure 2-106.

Better still, mount it in a box so

that the box absorbs the sound

from the rear of the loudspeaker.

For purposes of these simple

experiments, I won’t bother to go

into the details of vented enclo-

sures and bass-reflex designs.

Figure 2-106. A loudspeaker emits

sound from its bottom surface as

well as its top surface. To increase

the perceived audio volume, use

a cardboard tube to separate the

two sound sources, or mount the

speaker in a small box.