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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 SMART FASTCHARGERS® 2 NEW MODELS WITH OPTIONS TO SUIT YOUR NEEDS & BUDGET Now with 240V AC + 12V DC operation PLUS fully automatic voltage detection Use these REFLEX® chargers for all your Nicads and NIMH batteries: Power tools  Torches  Radio equip.  Mobile phones  Video cameras  Field test instruments  RC models incl. indoor flight  Laptops  Photographic equip.  Toys  Others  Rugged, compact and very portable. Designed for maximum battery capacity and longest battery life. AVOIDS THE WELL KNOWN MEMORY EFFECT. SAVES MONEY & TIME: Restore most Nicads with memory effect to capacity. Recover batteries with very low remaining voltage. CHARGES VERY FAST plus ELIMINATES THE NEED TO DISCHARGE: charge standard batteries in minimum 3 min., max. 1 to 4 hrs, depending on mA/h rating. Partially empty batteries are just topped up. Batteries always remain cool; this increases the total battery life and also the battery’s reliability. DESIGNED AND MADE IN AUSTRALIA For a FREE, detailed technical description please Ph (03) 6492 1368; Fax (03) 6492 1329; or email smartfastchargers<at>bigpond.com 2567 Wilmot Rd., Devonport, TAS 7310 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. ELECTRONIC COMPONENTS & ACCESSORIES • RESELLER FOR MAJOR KIT RETAILERS • • PROTOTYPING EQUIPMENT • FULL ON-SITE SERVICE AND REPAIR FACILITIES • LARGE RANGE OF ELECTRONIC DISPOSALS (COME IN AND BROWSE) CB RADIO SALES AND ACCESSORIES Ph (03) 9723 3860 Fax (03) 9725 9443 Come In & See Our New Store M W OR A EL D IL C ER O M E 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.