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Design by BRANCO JUSTIC*
Build the
Dog Silencer
. . . and quieten that noisy mutt
Fed up with the barking dog next door? This
Dog Silencer circuit could be the answer to
your prayers. It gives the dog a retaliatory
blast of high-frequency noise that’s beyond
the limits of human hearing.
Barking dogs are one of the worst
sources of noise pollu
tion in Australia. They cause more arguments
between neighbours than any other
problem and are by far responsible
for the majority of noise complaints
to local councils.
18 Silicon Chip
One thing that’s particularly galling
to near neighbours is the selective
deafness of inconsiderate dog owners.
They couldn’t care less how much
distress their dog causes and simply
ignore complaints. In other cases, the
owners are unaware of the problem
because their dog barks only while
they are away.
This device will let you get back at
your neighbour’s barking dog without
anyone else knowing about it. When
the dog barks, you press a button on
the front panel and it gives the dog
a blast of high-intensity frequency-modulated ultrasonic sound. This
lasts for as long as you hold the button
down.
Because this sound will be somewhere in the 20-31kHz range, humans
cannot hear it but most dogs can.
That’s because dogs are able to hear
much higher frequencies than humans
– unless, of course, the dog is old or
deaf, in which case the Dog Silencer
will have no effect.
The barking loop
So why does the dog stop barking?
We’re not too sure but one theory is
that the sudden sound burst interrupts
the “barking loop” (good term, that)
that some dogs get themselves into.
You’ll probably be familiar with this
problem – the dog starts barking and
doesn’t know how to stop. Basically,
the Dog Silencer is an attention-getting device; it distracts the dog and he
forgets to continue barking.
Another theory is that the sound
burst serves as a sharp reprimand.
And of course, if the sound is unpleasant, the dog will quickly learn
to modify its behaviour. Think of this
as being a high-tech equivalent to the
time-honoured “shaddup-barkin-yermangey-so-and-so” bellow out the
bedroom window at 4 o’clock in the
morning.
The real beauty of this device is that
your inconsiderate neighbour doesn’t
know that you’re reprimanding his
equally inconsiderate mutt.
Mind you, such subtlety is not for
one person that we know. Fed up
with the barking dog next door, he
phoned his soundly-sleeping neighbour at three in the morning. And his
response to his neighbour’s complaint
at being phoned at that hour was an
equally subtle “well if your dog’s
keeping me awake, I don’t see why
Warning!
The output from this unit is
extremely loud and could seriously damage your hearing
if you get close to the tweeters
while it is operating. This warning applies even though the unit
operates beyond the range of
human hearing.
For this reason, be sure to
install the tweeters in a location
where they cannot cause hearing damage and observe the
precautions detailed in the text
when testing the unit.
you shouldn’t also be awake”.
Of course, we don’t claim that this
unit will be effective on all dogs. If
the dog is deaf or just plain stupid,
nothing works unless the owner is
prepared to do something. Then again,
it might just depend on the breed of
dog or its temperament.
What we do say is that the Dog
Silencer will deter many dogs from
barking, provided they are not too far
away. It’s a bit hard to set a precise
value for the effective range but it’s
probably somewhere around 30 metres. It certainly wouldn’t stop a dog
that’s barking at the other end of the
street, for example.
Roo scarer
One report we’ve had suggests that
it’s also very effective on kangaroos.
No, it doesn’t stop them from barking;
that’s not what we mean at all. What
we do mean is that it scares them
away. And according to our inform
ant, the roos don’t just casually hop
away. The word he used was “stampede” but we’re not too sure whether
that term is really appropriate for
kangaroos!
Another term he used was “press
the button and whacko! – they’re
gone”. In short, he found that it was
very effective at scaring away kangaroos from the paddock adjacent to his
home in rural Queensland.
This means that there’s another
possible role for the unit – it could be
fitted to a vehicle and used as a “roo
scarer”. This could be useful when
driving on outback roads at night,
for example. We must stress though
that we haven’t tested the unit in this
role and in any case, it might only
“stampede” Queensland kangaroos
(only joking).
Circuit details
Refer now to Fig.1 for the full circuit
details of the Dog Silencer. It’s built
around IC1 which is a TL494 pulse
width modulation (PWM) controller.
The TL494 is normally used in
switchmode power supplies but is
suitable for virtually any PWM application. In this circuit though, we don’t
pulse width modulate the output.
Instead, the outputs either operate at
full duty-cycle or are off.
Fig.1: the circuit is based on a TL494 PWM controller IC. This IC provides complementary square signals at its
pin 9 & 10 outputs and these drive transistors Q2 and Q3, the centre-tapped transformer T1 and the tweeters.
JULY 1999 19
Fig.2: this block diagram shows the internal circuitry of the TL494 PWM controller. It includes a sawtooth oscillator,
a PWM comparator, a dead-time control comparator, two error amplifiers and a 5V reference. Emitter followers Q1
& Q2 provide the complementary square-wave output signals at pins 9 & 10.
Fig.2 shows a block diagram of
the TL494. It contains the following
circuitry:
• An internal oscillator which has its
frequency set by capacitor CT at pin 5
and resistor RT at pin 6.
• A stable +5V reference at pin 14.
• A “dead-time” comparator with
one input driven from the oscillator.
• Two error amplifiers with their outputs ORed together via diodes (pin 3).
• A PWM comparator with one input
derived from the oscillator and the
other from the ORed output of the two
error amplifiers.
• A flipflop which is driven (via a
NOR gate) by the dead-time and PWM
comparators.
• Two 200mA output transistors
with uncommitted emitters (pins 9
& 10) and collectors (pins 8 & 11).
The bases of these two transistors are
driven in anti-phase by the outputs of
the flipflop.
As used in the Dog Silencer, the
internal oscillator of the TL494
Fig.3: the top waveform in this scope shot shows the 2V
p-p sawtooth waveform at the anode of PUT1. This
waveform is used to frequency modulate the output.
20 Silicon Chip
operates at somewhere between
40kHz and 60kHz and this produces
complementary pulse trains (at half
this frequency) at the emitters of the
internal output transistors (E1 & E2).
Notice that, in this circuit, the collectors of these two transistors are tied to
the positive supply rail, so that they
function as emitter followers.
The E1 and E2 outputs from the
TL494 drive NPN transistors Q2 and
Q3 (TIP41C) in push-pull fashion and
these in turn drive centre-tapped trans-
Fig.4: the bottom waveform in this shot (collector of Q2)
shows the drive to transformer T1, while the top
waveform shows the signal drive to one of the tweeters.
Parts List
1 PC board (available from
Oatley Electronics)
1 plastic case with label
1 prewound centre-tapped
transformer (T1)
2 10µH inductors (L3,L4) – see
text
1 pushbutton switch (S1)
2 piezoelectric tweeters
1 5kΩ horizontal-mount linear
trimpot (VR1)
The inductors and the centre-tapped transformer (T1) are supplied prewound,
to make the assembly as easy as possible. Make sure that all polarised parts are
oriented correctly.
former T1. The secondary winding
of the trans
former then drives two
piezoelectric tweeters which, together with inductors L1-L4, form two
series resonant circuits connected in
parallel.
OK, so that’s how the circuit works
in a nutshell. In practice, it’s a little
more complicated than that, as we
shall see.
Rather than provide a fixed frequency output, this circuit uses an external oscillator to provide frequency
modulation. This circuit is based on
programmable unijunction transistor
PUT1, which is set up as a relaxation
oscillator. R1 & R2 bias the gate of the
PUT to about 3V, while R3 & C1 set
the frequency of oscillation.
In operation, the PUT conducts
each time its anode voltage rises 0.6V
above the gate voltage and stops conducting when C1 discharges (ie, when
the holding current drops below the
threshold value). The result is a 2.7Hz
2V peak-to-peak sawtooth waveform
at the anode. This signal is buffered by
emitter-follower stage Q1 and applied
to pin 6 of IC1.
The scope shot of Fig.3 shows this
2.7Hz sawtooth waveform. It varies
the voltage applied to pin 6 of IC1 and
the result is a frequency modulated
waveform which constantly sweeps
over a range of about 3kHz.
To explain this point further, depending on the setting of trimpot
VR1, the output frequency can vary
from 21 to 24kHz and back again, 2.7
times a second. While this is beyond
the limit of our hearing, it would
sound like a shrieking siren to a dog.
The frequency modulated waveform
is shown as the lower trace in Fig.3
but the scope shows it as a jumbled
waveform because the frequency is far
above the sampling rate at its sweep
setting of 100ms/div.
Trimpot VR1 sets the basic oscillator frequency. At one extreme, it varies the frequency modulated output
from about 18-21kHz, while at the
other extreme the output varies from
28-31kHz.
Power for the sawtooth oscillator
circuit is derived from the VREF output
(pin 14) of IC1. This output provides
a regulated +5V rail.
Trigger circuit
Switch S1 and its associated parts
provide the trigger circuit. This connects via R8 to the dead-time (DT)
input of IC1 at pin 6. Normally, the DT
control input is pulled high via R8 &
R9, which means that the dead-time
is at maximum. This also means that
the two internal transistors are held
off, so there is no drive to Q2 & Q3.
When S1 is pressed, pin 4 is pulled
low via R8 and so the dead time decreases to its minimum value. As a
result, IC1’s E1 and E2 outputs provide
maximum drive to Q2 and Q3. D1 &
D2 protect Q2 & Q3 from damage due
to inductive switching spikes.
Note that the non-inverting inputs
(IN+) of the two error amplifiers are
Semiconductors
1 TL494 PWM controller (IC1)
1 BC548 NPN transistor (Q1)
2 TIP41C NPN transistors
(Q2,Q3)
1 programmable unijunction
transistor (PUT1)
2 BA159 diodes (D1,D2)
Capacitors
1 470µF 25VW (C6)
1 100µF 16VW electrolytic (C5)
1 1µF 16VW electrolytic (C2)
2 0.47µF MKT polyester (C1,C4)
1 .0022µF greencap (C3)
Resistors (0.25W, 1%)
2 470kΩ (R3,R8)
1 100kΩ (R2)
1 68kΩ (R1)
1 47kΩ (R6)
1 22kΩ (R9)
1 12kΩ (R7)
1 10kΩ (R5)
1 1kΩ (R4)
2 120Ω (R10,R12)
2 47Ω (R11,R13)
Miscellaneous
Machine screws & nuts,
insulated hookup wire.
connected to the VREF, while the two
inverting inputs are connected together. This effectively disables the error
amplifiers and ensures the maximum
duty-cycle at the outputs.
The lower waveform in Fig.4 was
taken from the collector of Q2 and
shows the drive to the transformer
(T1). Note that this is a square-wave
signal. Q3 drives T1 in exactly the
same manner, except that its output
is 180° out of phase with Q2’s.
However, because each tweeter is
connected in a series resonant circuit
across T1’s secondary, the resultant
tweeter signal voltage is not only
JULY 1999 21
Fig.5: install the parts on the PC board as shown
in this wiring diagram. Note that Link 1 and Link2
should be replaced with 200µH inductors if square
tweeters are supplied.
sinusoidal but is also much greater
in amplitude. This is shown as the
top waveform in Fig.4, which has an
amplitude of 59.2V peak-to-peak or
about 21V RMS. As a result, the total
output power from both tweeters is
equivalent to about 100W (assuming
8Ω tweeters).
Power for the circuit can be derived
from any 10-16V DC source capable of
supplying at least 1A. A 12V battery
or 1A 12V DC plugpack supply would
be ideal for this job.
Building it
All the parts for this design are
available from Oatley Electronics,
so you don’t have to scrounge about
for individual bits and pieces. The
accom
panying panel shows all the
details.
The job of assembly mainly consists
of installing the parts on the PC board.
This board comes with a screened
parts overlay and the transformer
and inductors are all supplied prewound, to make the assembly as easy
as possible.
Fig.5 shows the parts layout on
the PC board. Begin by installing the
resistors, diodes and the wire links,
then install the capacitors. Take care
to ensure that the three electrolytic
capacitors are all correctly oriented.
Note that Link 1 and Link 2 are installed only if you are using rectangular tweeters. If you are supplied with
square tweeters, these links should
be replaced with inductors L1 and L2
(both 200µH); ie, the square tweeters
each require two series inductors
while the rectangular tweeters only
require one. The extra inductors will
automatically be supplied in the kit if
you are supplied with square tweeters.
The transistors can go in next, along
with the IC and the trimpot. Make
sure that the semiconductors are all
correctly oriented. Q2 & Q3 are both
mounted with their metal tabs towards
transformer T1, while IC1 has its pin 1
adjacent to the 470µF capacitor.
The transformer can now be soldered into position, after which you
can install the external wiring for the
power supply, Trigger switch (S1) and
the tweeters. Use medium-duty hook
up wire for the tweeter and switch
leads and heavy-duty hookup wire
for the supply leads.
Work can now begin on the plastic
case. The PC board is mounted on the
lid of the case using machine screws
and nuts, as shown in the photo. You
can use the PC board as a template
for drilling the four mounting holes
in the lid.
The decorative label is affixed to
the bottom of the case and this becomes the front panel. There’s only
one hole to drill and that’s for the
Trigger switch.
You will also have to file three
notches in the top rim of the case,
to provide clearance for the external
leads. Two of these notches provide
clearance for the tweeter leads, while
the third provides clearance for the
power supply leads.
If you are using a plugpack supply,
Resistor Colour Codes
No.
2
1
1
1
1
1
1
1
2
2
22 Silicon Chip
Value
470kΩ
100kΩ
68kΩ
47kΩ
22kΩ
12kΩ
10kΩ
1kΩ
120Ω
47Ω
4-Band Code (1%)
yellow violet yellow brown
brown black yellow brown
blue grey orange brown
yellow violet orange brown
red red orange brown
brown red orange brown
brown black orange brown
brown black red brown
brown red brown brown
yellow violet black brown
5-Band Code (1%)
yellow violet black orange brown
brown black black orange brown
blue grey black red brown
yellow violet black red brown
red red black red brown
brown red black red brown
brown black black red brown
brown black black brown brown
brown red black black brown
yellow violet black gold brown
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you could solder its leads directly to
the PC board. Make absolutely certain
that you get these leads the right way
around. This design doesn’t have a
reverse-polarity protection diode, so
some of the parts will be damaged if
you get it wrong.
Testing
Before testing the unit, check your
work carefully for wiring errors. This
done, solder a .0033µF capacitor in
parallel with C3 (it can be tacked to
the copper side of the board). This will
reduce the output frequency to around
10kHz, so that it will be audible and
you can tell whether or not the unit
is working.
Be warned, however, that the output will be extremely loud, although
you might not think so because it’s
operating at a high frequency. This
means that it could damage your
hearing if you are not careful. For this
reason, always position the tweeters
face down on the bench and cover
them with a blanket for testing.
By the way, this warning is equally valid when the unit is operating
beyond the limits of human hearing.
Even though you cannot hear the
noise, it could still seriously damage your hearing if you are careless
enough to get close to the tweeters.
Do not, under any circumstances,
get in front of the teeters while they
are operating.
Another way to reduce the output
for testing is to solder a 1kΩ resistor
in series with each tweeter. Once you
have everything set up, apply power
and press the Trigger switch. If the
unit is working properly, you will hear
a modulated high-frequency sound.
If the unit fails to work, switch off
immediately and check for wiring
errors. If all appears to be OK, reapply
power and check for +12V on pins 8,
11 & 12 of IC1 and at the collectors of
Q1 & Q2. Q1’s collector should be at
+5V, while the gate of PUT1 should
be at about 3V.
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The prototype was built into a low-cost plastic case, with the PC board mounted
on the lid. Note the notches filed into the case for the tweeter and supply leads.
Truscott’s
ELECTRONIC WORLD Pty Ltd
ACN 069 935 397
27 The Mall, South Croydon, Vic 3136
email: truscott<at>acepia.net.au
www.electronicworld.aus.as
JULY 1999 23
The PC board is attached to the lid of the case using machine screws and nuts.
Use medium-duty hookup wire for the tweeter and switch leads and heavy-duty
hookup wire for the supply leads.
Fig.6: transistors
Q2 and Q3 must be
heatsinked if you
intend building a
“roo scarer”. Be
sure to isolate their
metal tabs from
the heatsink metal
using a TO-220
mounting kit, as
shown here.
Assuming that everything works
properly, remove the 1kΩ series resistors (if fitted) from the tweeters and
the .0033µF capacitor from the back
of the board. Now, with the tweeters
face down on the benchtop, briefly
press the button again. This time, you
shouldn’t be able to hear anything
because the unit will be operating in
the ultrasonic range. If you do hear a
faint high-pitched noise, adjust VR1
until all is quiet.
Don’t keep the Trigger switch press
ed for too long when testing the unit
at this stage, otherwise Q2 & Q3 could
overheat. The unit is designed for
intermittent use only and provided
it is used in the manner, there’s no
need to fit heatsinks to the two driver
transistors.
Installation
The best location to mount the
tweeters is under the eaves of the
house, so that they are protected from
the weather. Try to position them so
Where To Buy The Parts
All parts for the Dog Silencer are available from Oatley Electronics. The
pricing details are as follows:
Complete kit (includes box, label, wiring kit and two tweeters
but does not include plugpack supply) ............................................ $43.00
PC board plus all on-board parts and one tweeter ......................... $30.00
Extra tweeter ..................................................................................... $5.00
Box, label, switch and wiring kit ......................................................... $8.00
13.8V 1A plugpack power supply ......................................................... $10
Please add $6.00 for postage and packing. To order, contact Oatley Electronics at PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563; fax (02)
9584 3561; email oatley<at>world.net
24 Silicon Chip
that are as close to the offending dog
as possible, while keeping them hidden from view. Mounting them up out
of the way also means that humans
cannot get too close.
You should also cover the tweeters
with a thin plastic membrane or house
them in a suitable cover, to prevent
them from getting wet.
After that, it’s simply matter of
pressing the button for a few seconds
each time the mutt next door barks.
Over time, you may find that the dog
realises that it’s going to cop this every
time it barks and so eventually ceases
to be a problem.
Building a “roo scarer”
Finally, if you intend fitting this circuit to a vehicle as a “roo scarer”, use
a rocker or toggle switch for S1 so that
the unit can be operated continuously.
A toggle switch with an illuminated
rocker is preferable here, so that you
know when the unit is on.
Heatsinking will also be required
for the two TIP41C output transistors.
One tweeter should be quite sufficient
in the roo scarer role, so a couple of
flag heatsinks should do the job. These
will have to be securely anchored,
to prevent the transistor leads from
lifting the pads on the PC board due
to vibration.
Note that the heatsinks must not
short against anything else or touch
each other, since they will be at collector potential.
A better idea would be to build the
circuit into a rugged metal diecast
case. Q2 & Q3 could then be bolted
to the case for heatsinking and connected to the PC board using flying
leads. Both transistors will have be
electrically isolated from the case
using standard TO-220 mounting kits
(mica washer plus mounting bush), as
shown in Fig.6.
After mounting the transistors, it’s
a good idea to check that their metal
tabs are indeed isolated from the
case using the low-ohms range of a
multimeter.
Power should be taken from the
fused side of the ignition switch, so
that the unit can only be operated
when the ignition is on. Note that all
external wiring connections should
be run using automotive cable. The
tweeter can be mounted behind the
grille and must be waterproofed by
covering it with a thin plastic memSC
brane.
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