Make Electronics

Experiment 29: Filtering Frequencies

256 Chapter 5

theory

Waveforms (continued)

You can create your own waveform as an input for your au-

dio amplifier with the basic astable 555 timer circuit shown

in Figure 5-51. You have to be careful, though, not to over-

load the amplifier input. Note the 680K series resistor on the

output pin of the timer. Also note the 500Ω potentiometer.

1

2

3

4 5

6

7

8

555

timer

1K

680K 100K

0.1uF

0.01uF

9V

DC

To input of

audio amplifier

500

ohms

Figure 5-51. A 555 timer is wired in astable mode using the

component values shown here to generate a wide range of

audible frequencies when the 100K potentiometer is adjusted.

After the output is reduced in power, it can feed into the ampli-

fier chip that was used previously.

Disconnect your music player and connect the output from

the 555 circuit to the input point (the 33K resistor) in the

amplifier circuit shown earlier in Figure 5-41. You don’t have

to worry about a separate connection on the negative side

as long as the 555 timer shares the same breadboard and

the negative side of its power supply.

Make sure that the 500Ω potentiometer is turned all the

way to short the output from the timer to the negative side

of the power supply. This functions as your volume control.

Also make sure the 100K potentiometer is in the middle of

its range. Switch on the power and slowly turn up the 500Ω

potentiometer until you hear a tone.

Now adjust the 100K potentiometer to create a low-pitched

note. You’ll find that it doesn’t have a “pure” sound. There

are some buzzing overtones. This is because the 555 timer

is generating square waves such as those shown in Figure

5-52, not sine waves, and a square wave is actually a sum

of many different sine waves, some of which have a high

frequency. Your ear hears these harmonics, even though

they are not obvious when you look at a square-shaped

waveform.

Route one of the connections to your loudspeaker through

your spool of hookup wire, and now you should hear a

much purer tone, as the buzzing high frequencies are

blocked by the self-inductance of the coil. Remove the coil

and substitute the 10 μF capacitor, and now you hear more

buzzing and less bass.

You’ve just taken a small step toward sound synthesis. If this

subject interests you, you can go online and search for oscil-

lator circuits. For a thorough understanding of the relation-

ship between waveforms and the sounds you hear, you’ll

really need an oscilloscope, which will show you the shape

of each waveform that you generate and modify.

Time

Voltage

Figure 5-52. The output from a 555 timer is either “on” or “off,”

with a very fast transition between those two states. The result

is an almost perfect square wave. Theoretically, this can be

disassembled into a complex set of sine waves that have many

different frequencies. The human ear hears the high frequen-

cies as harsh overtones.