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Showing how a logic gate can be used as an audio
amplifier and a voltage switch.
interference, require a positive
and negative supply, which is
not often available in logic cir-
cuits.
This circuit overcomes the
difficulties encountered in using
opamps and provides a variety
of outputs for virtually any appli-
cation. The component count is
very low
only 17 low cost
components are required (and
that includes the microphone).
The circuit will operate
down to about 3V, drawing a
current of less than 0‹5mA. At
5V, the supply current is around
1mA and even at 9V is only
around 5mA, which compares
quite favorably with an opamp
design. The circuit can be used
at up to 15V, although the cur-
rent drain is then a bit exces-
sive.
meet their own requirements.
Sound activated switches are
useful in many circumstances,
especially when “hands free” op-
eration of a piece of equipment is
required. They are often used, for
example, to automatically switch
on a tape recorder (or a digital
solid state equivalent) to record a
sound or conversation without
“wasting” tape during quiet peri-
ods.
Many inexpensive cassette
recorders have a remote switch
input to enable the recorder to be
switched on from the microphone
and a circuit of this type can eas-
ily be connected to it to start the
recording automatically.
As well as this, sound acti-
vated switches can be useful in
applications such as intercoms,
baby alarms, security alarms or
photographic work, and there are
no doubt many others.
This circuit arose from a re-
quirement for a basic microphone
interface to logic circuits without
having to build one from scratch
each time. Since it was not built
for any specific application, a
number of outputs were provided,
including an amplified version of
the sound waveform. With a few
additional components, however,
the circuit can easily be used as a
sound-operated switch with other
equipment.
It is suited to being supplied
by a 9V PP3 battery. The choice
of case has been left to users, to
CONSIDERATIONS
Most circuits of this type
published over the years use a
microphone signal, which is am-
plified to a suitable level by an
opamp. The signal is then recti-
fied and fed to a comparator,
which switches when the signal
exceeds a certain level. This is
then used to switch a relay or
other device, which in turn con-
trols an appliance.
The problem with using
opamps with digital logic is that
the output of most opamps does
not switch fully between the
supply rails. Thus, with a 5V
supply for instance, the output
will typically switch between
0
‹
5V and 3V. This is not too
much of a problem if a relay
driver transistor is to be con-
nected to the output, but it may
not work satisfactorily with a
logic circuit unless extra inter-
face components are used.
Many of the low-cost
opamps (e.g. 741) also require
a supply voltage greater than
that at which most logic circuits
operate. This means that a sep-
arate supply would need to be
used, together with level shifting
components to bring the output
swing within logic levels.
As well as this, differential
inputs, although improving per-
formance as regard to hum or
LOGICAL AMPLIFIER
Since a logic level output is
required and we are not after hi-
fi standards, a logic gate is used
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Fig.1. Basic CMOS
4069 inverter.
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, August 1999 - www.epemag.com - 803
&RQVWUXFWLRQDO 3URMHFW
287387
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a minute leakage
current.
This will also
be the case if the
input voltage is
increased slowly
until (typically)
the mid-point is
approached,
when the p-
channel device
will begin to turn
off and the n-
channel device
turn on. The sup-
ply current will
also begin to rise because both
transistors are now conducting.
The device will now operate as
a linear amplifier and as the in-
put voltage is further increased,
the output will continue to fall.
At higher voltages, the output
impedance is lower, giving an in-
creased bandwidth. It is also not
the last word in hi-fi from the
point of view of distortion or noise
but, despite this, the sensitivity of
the unit is sufficient to enable it to
respond to a sound at normal
conversation level within one or
two meters.
The electret microphone used
also has a built-in amplifier to re-
duce its output impedance and
noise pick-up, and this no doubt
helps. A problem is that, because
the gain tends to be somewhat
higher at low voltages, the sensi-
tivity of the unit is higher with a
5V supply than at 9V. This can
also be seen from Fig.2b, which
shows the transfer characteristic
for the 4069UB at 5V and 10V.
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Fig.2. Comparison of switching slopes for
buffered (a) and unbuffered (b) CMOS
inverters.
as the input amplifier. The cir-
cuit uses two of the six inverters
inside a 4069 CMOS chip to
amplify the signal, which is
picked up by a small electret
microphone. The 4069 is one of
the cheapest devices in the
CMOS range, and with the addi-
tion of a single feedback resis-
tor (which effectively biases the
output to mid-supply voltage) it
makes a very useful amplifier.
The internal circuit of one
CMOS 4069 inverter (excluding
the input protection compo-
nents) is shown in Fig.1 and
consists of two transistors: a p-
channel and an n-channel
MOSFET.
It must be noted, however,
that most devices in the 4000
CMOS series contain a buffer
following the logic function, uti-
lizing two cascaded inverters of
the type shown to achieve
sharper input-output voltage
characteristics and reduced
switching times. They are la-
beled “B’’ for buffered (e.g.
4049B), whereas the unbuffered
types are labeled “UB’’ (e.g.
4069UB).
In normal operation, if the
input is held at 0V, the output of
a logic inverter will be at the
positive supply rail with the p-
channel transistor conducting
and the n-channel device cut
off, with the device drawing only
CIRCUIT DIAGRAM
The circuit diagram of the
Sound Activated Switch is shown
in Fig.3. The microphone is bi-
ased by resistor R1 and the AC
signal coupled to the input of the
first inverter, IC1a, which is bi-
ased into its linear mode by resis-
tor R2.
The output of this stage con-
sists of an amplified version of
the sound signal and this is fed to
a similar stage built around an-
other inverter (IC1b), which am-
plifies it further. The output signal
from IC1b can be tapped at test
point TP1, from where it may be
used to feed a suitable power am-
plifier, depending on the applica-
tion.
As mentioned earlier, this sig-
nal is not by any means in the hi-
fi category. It will be about 1V
peak-to-peak for normal conver-
sation levels. Louder sounds will
obviously result in a larger output
(with increasing distortion) limited
by the supply voltage.
Interestingly, while distortion
obviously increases as the output
SLOPING OFF
Because a buffered device has
a fairly high gain (especially if two
such devices are cascaded) a small
increase in the input voltage will
cause such a large output swing
that the output will switch com-
pletely, with the n-channel device
hard on and the p-channel transis-
tor hard off (see Fig.2a) and the
circuit drawing a microamp or less.
With the UB device, this ac-
tion is more gentle (as shown in
Fig.2b) and a situation can easily
be arranged where both transis-
tors are conducting and the de-
vice functions as a stable linear
amplifier, with the output at about
the mid-supply voltage. Since
both transistors will be conducting
in this state, the current flow will
be in the low milliamps range and
will depend on the supply voltage.
The major limitation of this
circuit as an amplifier is that its
gain also depends to a large ex-
tent on the supply voltage, as
does its relatively high output
impedance, which, together with
any load capacitance, determines
the bandwidth.
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, August 1999 - www.epemag.com - 804
&RQVWUXFWLRQDO 3URMHFW
age is exceeded, the output of
this stage will switch to a low
level. This is illustrated in Fig.4,
which shows the waveforms at
different points in the circuit.
Inverter IC1c, therefore, pro-
vides faster switching, with its
output pulses coinciding with the
peaks of the sound picked up by
the microphone.
This may be useful in some
applications where, for example,
the frequency of the sound needs
to be measured, but could not be
used to switch on a tape recorder,
for which a steady “on” signal is
required. This condition is
achieved by using the negative-
going output of IC1c to charge up
capacitor C4 via diode D1.
The diode prevents the capaci-
tor from discharging when the out-
put goes high again. When a sound
is picked up by the microphone, the
voltage on this capacitor will there-
fore fall to the negative supply,
causing the output of inverter IC1d
to go high.
When the sound ceases, ca-
pacitor C4 discharges via resistor
R6, until the voltage at the input
of IC1d eventually rises above
the logic threshold, causing the
output of IC1d to switch low
again. By varying the value of R6
and/or C4, the length of time for
which the output of IC1d stays
high after the sound has ceased
can be varied to suit the applica-
tion.
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Fig.4. Waveforms at the circuit test points.
approaches the upper and lower
supply limits, this amplifier pro-
gressively “rounds off” the sig-
nal peaks in a manner more
reminiscent of a valve amplifier,
rather than clipping them, as
would occur in an opamp.
age) to another inverter would
result in the output of this stage
oscillating between the two supply
rails, due to stray noise (both au-
dio and electrical) reaching the
input.
By placing a potential divider
(R4/R5) at the output of amplifier
IC1b, the input of the next stage,
IC1c, will be held slightly below
its threshold voltage and the out-
put of this stage will remain at the
positive supply (logic 1 level)
when there is no sound reaching
the microphone.
When a sound is picked up
by the microphone, the output of
the amplifier stage will swing be-
tween the logic levels in sympa-
thy with the signal and, since the
threshold of the next inverter,
IC1c, is at around half the supply
rail voltage, each time this volt-
OUTPUTS
CMOS inverters, even the
UB types, have a relatively
steep input logic transfer char-
acteristic. With a 5V supply, an
input change of less than 1V
produces an output swing of
nearly the full supply. With a
10V supply, an input change of
around 3V is required to do this.
Since the transition occurs
at around the mid-supply volt-
age, connecting the output of
the amplifier stage (which is bi-
ased to the mid-supply rail volt-
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Fig.3. Complete circuit diagram for the Sound Activated Switch.
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, August 1999 - www.epemag.com - 805
&RQVWUXFWLRQDO 3URMHFW
case, the positive terminal
should be connected to the pos-
itive supply line.
The relatively gentle nature
of the input/output characteris-
tics of the unbuffered 4069 in-
verter, coupled with the slow
discharge of C4 via R6, means
that the output of IC1d tends to
switch uncleanly. With the in-
verter now spending more time
in the linear region while C4 dis-
charges, intermittent oscillation
can occur and its output is not
suitable for connecting to other
logic circuits. The two final in-
verters, IC1e and IC1f are
therefore combined with resis-
tors R7 and R8 to form a
Schmitt Trigger circuit, which
uses positive feedback to
sharpen the response.
APPLICATIONS
Depending on the applica-
tion, the outputs of the unit can
be connected to a variety of de-
vices.
Output TP1 provides the
amplified, but otherwise unmod-
ified, microphone signal from
IC1b.
Output TP2 provides a
roughly rectangular waveform
having the frequency of the
original signal. Since it switches
between 0V and the positive
supply rail, it may be used as an
output to other logic circuits for
further processing, such as fre-
quency measurement.
Outputs TP3 and TP4 pro-
vide logic low and logic high
levels for as long as the sound
persists. Output TP4 can be
used directly for interfacing with
other logic circuits or, for exam-
ple, to the relay driver shown in
Fig.6.
The relay can be used to
switch on any appliance, e.g.
tape recorder, lamp etc., de-
pending on its ratings.
To prevent the relay from
switching on and off during
short periods of silence, the
value of resistor R6 or capacitor
C4 may need to be increased.
This should be done by trial and
error: too small a value will
cause the relay to keep switch-
ing on and off very often, while
too high a value will result in the
relay staying on for a long pe-
riod after the sound has ceased.
For applications where a
longer delay is required, and
where the device may be trig-
gered by a loud initial sound
which then dies away, the value
of C4 may be increased to allow
lower values of R6 to be used. It
is possible to use electrolytic
capacitors here and, in this
PHONE TRIGGERED
A useful application for this
circuit would be to switch on a
lamp to signal that the telephone
is ringing. This would be helpful in
a noisy office or workshop, or per-
haps at home for someone who is
hard of hearing.
For this sort of application
to be successful, it is important
to ensure that there are no
“false alarms”. Frequent trips to
the telephone, only to find that
the lamp has been triggered by
a passing car or some other
noise and not the phone, will not
endear you to your granny!
Place the microphone close
to the telephone and reduce the
circuit’s sensitivity so that only
the telephone can make a loud
enough sound. This can be
done by decreasing the value of
R5, using trial and error or by
fitting a preset. The maximum
value should not exceed 470k
(with R4 as specified) as values
greater than this may prevent
the circuit from operating prop-
erly.
Performance could be further
improved by choosing a value for
R6 to keep output TP4 high only
for the duration of each ring
(around 2·2M: with C4 at
100nF). This will cause the lamp
to flash in the characteristic UK
telephone ringing pattern, making
it not only more noticeable, but
easier to recognize in the event of
triggering due to other noises,
which would cause the lamp to
flash in a different way.
CONSTRUCTION
Component and track layout
details of the printed circuit
board are shown in Fig.5. This
board is available from the EPE
Online Store (code 7000
240) at
www.epemag.com
Assemble the components
in order of size, and use a
socket for IC1. Remember that
the IC is a CMOS device and
should be handled accordingly,
discharging static electricity
from yourself before handling it.
Two short pieces of tinned
copper wire, such as discarded
resistor leads, need to be sol-
dered carefully to the two pads
on the back of the microphone
capsule. Although it is not ap-
parent from the circuit symbol,
electret microphones have a
built in amplifier and must
therefore be connected to the
circuit with the correct polarity.
The 0V terminal is the one con-
nected to the metal body of the
capsule.
Use terminal pins for off-
board wiring connections to
other circuitry, as required by
the application.
BABY MINDER
Another application would
be as a baby alarm, as in Fig.7.
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, August 1999 - www.epemag.com - 806
&RQVWUXFWLRQDO 3URMHFW
Here, output TP4 would be used
to control the output of an audio
amplifier switching it (or the
loudspeaker) on when a sound
is detected.
The actual sound (from out-
put TP1) would be fed to the
amplifier input allowing the baby
sitter to hear if the baby was
crying or simply stirring and
thus determine whether or not it
needed attention.
COMPONENTS
Resistors
R1 10k
R2, R4, R7 47k (3 off)
R3 1M
R5 330k (see text)
R6 4M7 (see text)
R8 470k
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All 0.25W 5% carbon film
Capacitors
C1, C3, C4 100n ceramic (3 off)
C2, C5 100p ceramic (2 off)
C6 10u radial electrolytic, 16V
Semiconductors
D1 1N4148 signal diode
IC1 4069UB hex inverter
Miscellaneous
MIC1 electret microphone,
2-terminal
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PA CONTROL
A similar application could
be devised where, for example,
public announcements need to
be made during which the nor-
mal background music has to
be interrupted. It should be ap-
preciated, however, that al-
though the circuit is fast, it still
takes time to operate, especially
if a relay is used for the switch-
ing. It could be found that the
beginning of any announcement
is not transmitted.
In such applications, the
use of a transmission gate such
as the 4066 quad analog multi-
plexer is recommended to
switch the signal. A possible
scheme is shown in Fig.8. Here
the four gates within the 4066
are used in pairs and controlled
by outputs TP3 and TP4.
Since TP3 is normally high,
the signal at the music input will
be transmitted to the volume
control, while its lower end will
be held at 0V by the other trans-
mission gate connected to out-
put TP3. Whenever an an-
nouncement is made, these two
gates will switch off and output
TP4 will go high switching on
the other two gates.
The audio signal from the
microphone will then be applied
to one end of the volume con-
trol potentiometer, while its
other end will be grounded. The
relative volume of the music
Printed circuit board available
from
the
EPE Online Store
, code
7000240 (
www.epemag.com
);
terminal pins; 14-pin DIL socket;
connecting wire, solder, etc.
Fig.5. PCB component lay-
out and (approximately) full-
size copper foil master
track pattern.
See also the
SHOP TALK Page!
Approx. Cost
Guidance Only
$15
would, of course, have to be de-
coupled to ensure that there
were no sudden thumps when
the speaker or the inputs were
switched. The values of R6 and/
or C4 may also need to be ad-
justed for best results.
In this application, the per-
son making the announcement
would, no doubt, be very close
to the microphone and so the
and announcement signals can
therefore be set as required.
Note that neither of these
circuits take into account any
DC offset voltages which may
exist in the signal path. These
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Fig.6. Interfacing to a relay.
Fig.7. Baby alarm
application.
Copyright © 1999 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, August 1999 - www.epemag.com - 807
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