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the easy way into electronics
This month we feature two Protoboard circuits based on the 555
timer. This chip must be one of the most popular devices ever
developed but when it came out in around 1973, people wondered
what to do with it! The two circuits are quite similar to look at
but they produce different functions and you can change from
one circuit to the other in just a few moments.
Most electronics enthusiasts know
that the 555 can be used in timer and
oscillator circuits and these are what
we are presenting here. However, in
each case we use two 555s and one is
used to control the other.
The first circuit (Fig.1) is a siren
with one 555 being used to frequency modulate the second. The second
circuit could be used as part of an
alarm. It consists of a 555 oscillator
which runs for a time determined by
the second 555.
Fig.3 shows the wiring layout for the
siren circuit on a protoboard. As we
did last month, the two ICs straddle
the central channel of the Protoboard
and the eight pins of each IC connect
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to individual conducting strips. The
other components and jumper leads
are then inserted to make up the
circuit.
Note that the three electrolytic capacitors must be connected the right
way around (ie, positive voltage to
their posi
tive terminal) otherwise
they will be damaged although initially, the circuit would probably work.
You will need two potentiometers, both 100kΩ, but other values
from 50kΩ-250kΩ will work just as
well although producing a different
range of frequencies and times, as
we shall see.
The two pots are mounted on the
vertical panel of our Protoboard’s
metal chassis and then wired to the circuit. You will also need a momentary
contact pushbutton switch and this is
also mounted on the panel.
The diode (D1) is wired in series
with the DC socket for the 9V plugpack
but it should also be included if you
are using a 6V or 9V battery because
without D1, a reversed DC supply
will mean instant death for the two
ICs. Note that supply from a 9V DC
plugpack is likely to be closer to 12V
because this circuit does not have a
heavy current drain.
Hook up all the components for the
circuit of Fig.1, as shown in the diagram of Fig.3. Don’t hurry the job as
you might make a tragic mistake. Even
Fig.1: IC1 runs at a slow rate (about 0.5Hz to 2.8Hz) and it is used to frequency
modulate IC2 which drives a loudspeaker.
so, it will only take a few minutes to
wire it up. On the other hand, if you
are not confident with the task, just
wire up the circuit involving IC2 first.
This will produce an oscillator with
a frequency which can range from
around 200Hz to 700Hz, depending
on how you set pot VR2.
Scope waveforms
The scope waveforms of Fig.4 and
Fig.5 show the frequency extremes
which can be expected at pin 3 of IC2.
Fig.4 shows a waveform with negative
pulses which are 700µs wide at a
repetition rate of 200Hz while Fig.5
shows the same 700µs wide negative
pulses but at a repetition rate of 675Hz.
The negative pulse width of 700µs
(or 0.7 milliseconds) is set by the 0.1µF
capacitor at pins 2 & 6, combined with
the 1kΩ resistor to pin 7.
The variable part of the waveform,
the positive section, is determined
by the 10kΩ resistor and 100kΩ pot
connected to pin 7, the 1kΩ resistor
to pins 2 & 6 and the aforementioned
0.1µF capacitor. So by varying the pot
setting, we vary the overall frequency.
By the way, both those waveforms
were taken with the loud
speaker
disconnected (the noise gets to you
after a while!) and this means that
the amplitude of over 11V peak-to-
peak is larger than it would be. With
the speaker connected the amplitude
drops to between 6V and 9V peakto-peak, depending on the frequency
setting.
OK, now let’s have look at IC1. This
is almost identical to the circuit for IC2
except that the capacitor at pins 2 & 6
is 47µF instead of 0.1µF. This means
that the frequency range will be about
500 times lower.
Fig.6 shows the waveforms at pin
2 and pin 3 of IC1. The upper trace
shows the sawtooth or triangle waveform at pin 2. The frequency can be
made to vary between about 0.5Hz
and 2.8Hz, depending on the setting
of pot VR1.
Actually, the waveform at pin 2
looks more “sawtoothy” at the low
Fig.2: in this circuit, IC1 is used as monostable and it allows IC2 to produce a burst of oscillation lasting for about one
second. If you want it longer, increase the 47µF capacitor at pin 6.
October 2000 61
Fig.3: this shows the Protoboard component layout for the siren circuit of Fig.1. Note that you can build the circuit
in stages, starting with the parts for IC2 (see text).
frequency setting of VR1, with a long
upslope and then a relatively sudden
down-slope.
We use the sawtooth waveform from
pin 2 of IC1 to modulate the frequency
of IC2. Hence, pin 2 of IC1 is connected
via a 10kΩ resistor to the CV (control
voltage) input, pin 5, of IC2. This
causes the internal thresholds of the
555 window comparator to be moved
up and down. The result is that IC2’s
frequency jumps up with each downslope of the sawtooth and then falls
away, just like a wailing siren.
Fig.7 is an attempt to show this
modulation effect but the digital
scope can’t really give a true picture
because its timebase speed of 100ms/
The completed Siren circuit, viewed from the front. The two potentiometers
(VR1 & VR2) are fitted to holes in the front panel at top left.
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div (to display the sawtooth) is far
too slow to show the siren frequency
(lower trace).
OK, we have just about covered all
the waveforms that you can check on
this modulated siren circuit but there
are still a few wrinkles to uncover.
First, disconnect the 10kΩ resistor
from pin 5 of IC2; just plug the unused
end into one of the unused Protoboard
boles. Now IC2 is unmodulated and
produces a steady tone. Want to make
it louder? There are two ways. First,
you can reduce the value of the 68Ω
resistor in series with the loudspeaker.
This will allow more current through
the speaker but is not the most efficient way.
Second, try mounting an empty
toilet roll over the speaker and then
“tune” VR2 for maximum loudness.
You can have quite a lot of fun with
this effect.
Finally, you might wonder why we
bothered putting a 100µF coupling
capacitor in series with the speaker.
Why have it there when the circuit
will work without it? If you don’t believe us, try shorting the capacitor out.
Still works, doesn’t it? But notice that
Fig.4: this is the waveform at pin 3 of IC2 in the circuit
of Fig.1. It has negative pulses which are 700µs wide at
a repetition rate of 200Hz.
Fig.5: this shows the same circuit conditions as at Fig.4
but VR2 has been set to increase the pulse repetition rate
to 675Hz.
Fig.6: these are the waveforms at pin 2 and pin 3 of
IC1 in the circuit of Fig.1. The upper trace shows the
sawtooth or triangle waveform at pin 2. The frequency
can be made to vary between about 0.5Hz and 2.8Hz,
depending on the setting of pot VR1. The lower trace is
the negative pulse train; the pulse width is about 38ms.
Fig.7: these waveforms attempt to show the modulation
effect of the circuit of Fig.1. The digital scope can’t
really give a true picture because its timebase speed of
100ms/div (to display the sawtooth) is far too slow to
show the siren frequency (lower trace).
Fig.8: this scope shot shows the current waveform
through the 68Ω resistor and loudspeaker when the
100µF capacitor is shorted out. The amplitude is 3.76V
peak-to-peak.
Fig.9: this shot shows the current waveform through the
loudspeaker and 68Ω resistor when the 100µF capacitor
is in circuit. Note that the peak-to-peak and RMS
current values are substantially higher than in Fig.8.
October 2000 63
of Fig.2, the timed alarm. Pin 4 of
IC2 now goes to pin 3 of IC1 instead
of to the +9V line. The 10kΩ resistor
between pin 2 of IC1 and pin 5 of IC2
is removed.
Also, pin 2 of IC1 now goes to a
pushbutton instead of to pins 6. This
converts IC1 from a low frequency
oscillator (ie, an astable) to a pulse
generator (ie, a monostable).
Bursts of sound
This rear view shows how the completed is connected to the DC socket, the pots
(VR1 & VR2) and the loudspeaker using flying leads.
it is not as loud as with the capacitor
doing its job?
Now why is that? We’ll give you a
hint. With the capacitor out of circuit,
the 555 only “sources” current into the
loudspeaker when its output at pin 3
is high. So we get a train of positive
pulses through the speaker.
But when the 100µF capacitor is in
series with the speaker, current can
flow in two directions: first, when
pin 3 is high, it charges up the 100µF
capacitor via the 68Ω resistor and
speaker and second, when pin 3 is low,
it then discharges the 100µF capacitor
via the resistor and speaker. So we get
more drive to the speaker.
To prove it, we used the scope again.
Fig.8 shows the cur
rent waveform
through the 68Ω resistor when the
100µF capacitor is shorted out. The
amplitude is 3.76V peak-to-peak.
Then, when the capacitor is back
in circuit, the frequency remains the
same, as you would expect, since
the output loading has no effect on
the frequency of a 555 oscillator. But
note that the amplitude is now almost
doubled, in Fig.9. It sounds quite a lot
louder too.
Timed alarm
Now that you’ve had fun with the
siren, let’s change the circuit to that
Fig.10: this shows the
burst effect that can be
obtained with the circuit
of Fig.2 (see text).
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Actually, as part of the change,
you can leave the pushbutton out
of circuit for the moment and just
connect pin 4 of IC2 to pin 3 of IC1
(leave pin 2 connected to pin 6).
Now if you apply power, IC1 will
cause IC2 to produce bursts of sound
from the speaker at a rate which can
be varied by VR1. Fig.10 shows the
effect.
If you want the timed burst to be
longer, you can increase the value of
the 47µF capacitor at pin 6 of IC1 and
VR1 can be increased in value.
How the circuits work
After all that, you probably have
nutted out how each of the circuits
work but we will go through it briefly
for the sake of completeness.
Apart from the timed alarm circuit
of Fig.2 where IC1 works as a mono
stable, all the circuits work as astable
multivibrators. The resistors and
potentiometer at pins 2 & 6 charge
the capaci
tor (0.1µF or 47µF) up
to two-thirds of the supply voltage
whereupon the internal transistor at
pin 7 discharges the capacitor down
to one-third of the supply voltage.
The one-third and two-thirds voltage
thresholds are set internally for the
555’s window comparator.
While the capacitor at pin 6 is
charging, the output at pin 3 is high
and while it is being discharged, the
output is low. You can check this effect on the various scope waveforms
in this article.
Pin 4 (RESet) is normally tied high
and if it is low it inhibits oscillation.
We use this fact in the timed alarm of
Fig.2. Pin 2 is normally tied to pin 6
in the astable oscillator circuit but is
used in the timed alarm as the trigger
input for the monostable circuit.
Pulling pin 2 suddenly low starts
the monostable; pin 3 goes high while
the capacitor at pin 6 charges and then
goes low when it reaches two-thirds
SC
of the supply voltage.
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