Elenco Basic Electronic Experiments - Manual (Page 44)

EXPERIMENT #29: ELECTRONIC SOUND
Now it’s time to make some noise. To do this we need a speaker. A speaker converts electrical energy into sound. It does
this by using the energy of an AC electrical signal to create mechanical vibrations. These vibrations create variations in air
pressure, called sound waves, which travel across the room. You “hear” sound when your ears feel these air pressure
variations. You need high current and low voltage to operate a speaker, so we will always use the transformer with the
speaker. (Remember that a transformer converts high-voltage/low-current to low-voltage/high-current). We create an AC
signal for the speaker using the oscillator circuit introduced in the last experiment, with minor changes. A speaker has a
schematic symbol like this:
Connect the circuit, notice that two transformer taps are not connected although their wires cross in the schematic. Also
notice there are 4 resistors and 4 capacitors connected to the 3.3k
Ω resistor (we are using the (+) row of holes at the
bottom to make the connections easier) and 2 loose wires connected to the transformer. We are also using the disc
capacitors for the first time, refer back to page 19 to review them if you need to. Connect the transformer to one resistor
and one capacitor at a time, then press the switch and listen. All the combinations are listed below, you don’t need to try
all of them but try some and see if there is a pattern in the frequency or pitch (a term used in music) of the sound. Record
a few comments about the sound you hear.
10k
Ω
33k
Ω
100k
Ω
1M
Ω
0.005 μF
0.047 μF
10
μF
100 μF
You may start to see the same thing we told you about the blinking LED frequency - that the frequency increases when you
lower the resistance or capacitance. It also increases if you lower the inductance, but you don’t have any other inductors
you can substitute.
Oscillators are among the most important circuits in electronics and most of your remaining experiments will use an
oscillator of some form. Although the oscillator circuits used here are simple ones, some oscillators can be the most difficult
circuits to design.
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SPEAKER Symbol for SPEAKER