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Diesel sound simulator for model railroads For added realism, why not fit a diesel sound simulator to each of your model locomotives? It's great to see and hear them rolling around the tracks with the sound of the motors rising and falling as they speed up or slow down. By DARREN YATES Well, it was inevitable! After we featured the SteamSound Simulator in the April and September 1991 issues of SILICON CHIP, it didn't take · long for the diesel fraternity to ask "why don't you do the same for diesel locos?" To those of us who prefer steam locomotives, it's almost a case of"why would you bother?" The only problem was that it was the boss who made the final request and after sidestepping the issue for the last 12 months or more, there are only so many medical excuses you can come up with for not doing a project. So I 12 SILICON CHIP came right out and said it: "What a silly project idea"! When I regained consciousness, he reminded me that there are readers who believe that diesels are actually far more interesting than steam locos. So in the interests of remaining fair, impartial and unbiased in any shape, manner or form, I proceeded to do the project. We published a diesel sound generator for model locos some time ago, in the December 1988 issue of SILICON CHIP, but this was intended to be mounted under the layout and could be used with any locomotive scale. This new design can be mounted right inside the loco, provided the scale is HO (1:87) or larger. Mounting the unit inside HO locomotives is made possible by using the 27mm mylar speakers that are now readily available. In addition, we have designed the PC board so that it can be cut in half and folded, if necessary. The design is primarily intended to be used with the pulse width modulated speed controllers described in the April 1988 and April to June 1992 issues of SILICON CHIP, although it can be adapted to other controllers, as described later. Circuit diagram As you can see from Fig.1, the Diesel Sound Simulator contains only two !Cs and a small number of other components. Power is derived from the rails through diode bridge BR1. This is a 1B04 4-pin DIP device capable of providing up to 1 amp. The bridge rectifier feeds diode_Dl and a 100µF capacitor to produce smoothed 4.711 TO TRACK.,. 660k 16 .022 IC1 4046 ZD1 9.1V 400mW 9 vco IN FREQ MAX MIN 11 12 R1 5.6k .,. .,. R2 470k .,. SIG IN COMP IN VCO OUT 16 11 14 - - - - - - - - --1 038 3 12 - - - - - --1028 9 4 ------CKA 04A IC2 4015 DB _____0.1._-+---'l--'-IDA 7 COMP 2 OUT 10 15 15k 5 .,. .,. 8 .,. .,. B 100 EQc VIEWED FROM BELOW .,. DIESEL SOUND SIMULA TOR Fig.1: the circuit of the Diesel Sound Simulator uses a phase lock loop IC connected to an 8~bit shift register to produce the "rumbling" sound. Zener diode ZDl, transistor Qt and diode D2 monitor the back-EMF from the loco's motor to make the pitch of the sound rise and fall with the lo,co's speed. DC which is then regulated to 12V by a 78112 3-terminal regulator. The regulated+ 12V rail powers IC1, IC2 and a power amplifier consisting of four transistors (Q1 Q2, Q3 & Q4). IC1 is a CMOS 4046 phase-locked loop (PLL) which, together with IC2, a CMOS 4015 dual 4-bit shift register, provides most of the circuitry to pro- 811 27mm + 4.7 16VW+ duce the diesel sound. The frequency range of IC1's voltage controlled oscillator (VCO) is set by resistor R1 at pin 11 and resistor R2 at pin 12. By connecting the two clock inputs (pins 1 & 9) of IC2 together and feeding the fourth output of register A into the data in of register B, we've created an 8-bit shift register. The 6th and 7th bits are fed into the SIG IN (pin 14) and COMP IN (pin 3) inputs ofIC1. As a result, the VCO frequency is constantly forced to change back and forth to produce a "rumbling" sound which is taken from pin 2 (COMP OUT) of IC1 via a 15kQ resistor. To provide the effect of turbocharger whine, the VCO output at pin 4 is mixed with the low frequency output via a 27kQ resistor. The mixed high and low frequency signals are then filtered with a 0.22µF capacitor before being fed to the 4-transistor audio amplifier. Speed monitoring The PC board can be separated into two halves which can then be folded together to fit inside the body of the loco. Alternatively, you can mount the board inside a following carriage. As noted above, the frequency of the Diesel Sound Simulator rises and falls in proportion to the motor speed and this gives a very realistic effect. To achieve this effect, we need to monitor the speed of the model locomotive's motor. In practice, we can't monitor the speed directly but we can monitor the motor's back-EMF. The is the voltage produced by the motor which opposes the current flow through it and this voltage is directly proportional to the speed. But how do we separate the motor back-EMF from the voltage applied to it to make it go? You might remember that we said that this sound simulator we designed especially to go with the DECEMBER 1992 13 the amplifier is around 350Hz, although the bass really sounds quite impressive when fed to a large speaker. Construction Take care with the orientation of the two ICs when installing them on the PC board & don't get them mixed up. You can cut the board in half, if necessary, after all the parts have been mounted in position. PWM controllers designed by SILICON CHIP. These controllers produce a pulse output at about 200Hz and have a DC amplitude of about 20V peak. The power to the locomotive is varied over · the full range by varying the width of the pulses. Now since the pulse output from the controller is usually off for 50% or more of the time, that gives us a chance to monitor the back-EMF of the motor because the motor naturally keeps spinning between the pulses and therefore its back-EMF can be measured at these times. The waveform of Fig.2, taken from a motor driven by a PWM controller, demonstrates this effect. You can see the large pulse output of the controller and in between each pulse is a spiky waveform which represents the motor's backEMF. The spikes come from the motor's commutator and brushes by the way. We can monitor the back-EMF by using a circuit which ignores the full pulse output of the speed controller but measures the voltage in between ~~ ~ pulses. This is done using zener diode ZD1, transistor Ql and diode DZ. ZD1 conducts whenever the voltage across the rails is more than about 11 V; ie, whenever high voltage pulses are present. When ZD1 conducts, so does Ql and therefore the monitoring circuit ignores these pulses. At other times, Ql is off and so diode DZ conducts, via the 15kQ resistor, to charge a 4. 7µF capacitor. This capacitor is proportional to the motor speed of the loco and is fed into the VCO IN (pin 9) ofIC1 to control its frequency. Hence, increasing the motor speed increases the VCO frequency and thus the diesel sound rises in pitch. Audio amplifier The audio amplifier is based around transistors Q2-Q5. This is a simple complementary output stage which has a gain of about 5. 7, as set by the 470Q and 2.2kQ resistors connected to the emitter of Q2. The low frequency rolloff is set by the 4.7µF capacitor in series with the 470Q resistor. The actual cutoff frequency for -FEB-1 QQ ? 1? '.'l5 CH1= 500m\/ DC P*1 · win dow w 2m s /d rr " ~ f'l "' 14 L r, .L & fl'' ,, f~ SILICON C H I P lot, . ( ~T fl T., ,~ l - ~~~ ~ Fig.2: this oscilloscope waveform shows the voltage from a PWMspeed controller at a low speed setting. The pulses have an amplitude of about 17V. The spiky waveform between the pulses is the motor backEMF. The hash on the waveform is caused by the brushes & the commutator in the motor. All of the components, except the speaker, are installed on a small PC board measuring 102 x 25mm and coded 09111921. Regardless of how you are going to finally install the unit, we suggest that you build the board ih its supplied form and get it going. Then, if it is necessary, you can cut the board in half to fit it into the loco. Before beginning any constructional work, check the PC board carefully for any shorts or breaks in the tracks. If you find any, use a small artwork knife or a dash of solder to fix the problem as appropriate. When inserting components, start by fitting the two wire links which go under IC2. Next, solder in the resistors. You can use either the standard 0.25W 1 % metal film resistors or Philips MRS16T 0.4W types which have a body that's only 3mm long. If you are using the standard 7mm body (0.25W) resistors, some of them will need to be installed "end on" while the 3mm body types can be installed flush with the board. Our photo shows a mixture of both types ofresistors on the board. If you can't make out the colour bands on the resistors, use your multimeter to make sure of the value. Next, solder in the diodes and the bridge rectifier. The "+" pin on the bridge rectifier package should be.adjacent to the end of the PC board, as shown on the wiring diagram ofFig.3. Make sure that the diodes and electrolytic capacitors are installed with correct polarity. Take care when soldering in the transistors to make sure that they are correctly oriented. Q3 and Q4 have their faces butted together, while. Q2 and Q5 face away from each other. The two CMOS ICs go in last - make sure they are correctly oriented. Testing To test the Diesel Sound Simulator, you'll need to hook it up to your layout with a loco ready to run on the track. This is because the Diesel Sound Simulator relies on the back-EMF produced by the motor to get the correct speed information. We also suggest that you don't use the specified 27mm SIAS (Voc) ·i. ."' I ! > : JU? • •I r"! ! '' JI i l !·!J'"i!_' GEN ■ ii ~ M E N U S :__ = J Power analyser Fully autoranging for voltage, current and frequency. 700VRMS 20ARMS DC-20KHz 0 Accurate measurements to 0.1 % O Accurate with distorted waveforms O Accurate with all power factors O Measures any short term events from inrush current to burst power O PC Software and interfaces provided with most instruments. I • : , ii • he "smart" digital technology in these Voltech analysers and testers, results in measurement made easy. 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'C!:~~![~9h2~se INDUSTRIAL PRODUCTS ro miniature speaker at tnis stage - it is rather fragile and should be handled as little as possible. Use a conventional larger speaker during testing. When you first apply power, and with the speed controller set for minimum output, the Diesel Sound Simulator should be in "idle" mode; ie, simulating the effect of the motor at idle. As the speed setting is increased to make the loco move, the pitch of the diesel sound should begin to rise. Wind up the speed controller to maximum and observe that the diesel sound is still realistic. It should sound like a diesel running at a good pace, not like a motorbike doing 8000RPM. If the maximum diesel pitch is too high, you can reduce it by increasing the value ofRl. Similarly, if the maximum pitch is too low (unlikely), you can increase it by reducing the value ofRl. Similarly, if the idle speed is too CAPACITOR CODES 0 0 0 0 Value IEC Code EIA Code 224 104 223 SPEAKER Fig.3: here's how to install the parts on the PC board. Make sure that you install the wire links before fitting IC2. high, you can reduce it by increasing the value of R2. If the PC board does not work, the most likely reason will be shorts between tracks caused by tiny solder splashes. Careful inspection of the board under a good light with a magnifying glass will locate these problems. Mounting it in a loco As noted above, the Diesel Sound Simulator is intended for locomotives in HO, 00 or larger scales. However, some HO diesel locos are just too small or are so crammed inside that it will not be possible to install the sound components in them. In these cases, the only practical approach will be to install the sound components inside a dummy loco (ie, without a motor) or inside a carriage or wagon. Either way, you must arrange to pick up power from the rails. This can be done from the loco's wheels and then via a miniature 2-way cable and connector. Alternatively, you will need to pick up power from the rails via collectors on the wheels of the dummy loco or wagon. On the hand, if you can possibly manage to fit the sound components inside the loco itself, the effect will be even more realistic. You can then have PARTS LIST 1 PC board, code 09111921, 102 x25mm 1 27mm 8Q Mylar loudspeaker Semiconductors 1 4046 phase-locked loop (IC1) 1 4015 dual 4-bit shift register (IC2) 1 78L 12 + 12V regulator 2 BC548 NPN transistors (01 ,02) 1 BC558 PNP transistor (03) 1 BC337 NPN transistor (04) 1 BC327 PNP transistor (05) 1 9.1V 400mW zener diode (201) 1 1804 bridge rectifier (BR1) 1 1N4004 rectifier diode (01 ) 1 1N914 signal diode (02) Capacitors 1 100µF 35VW electrolytic 1 100µF 16VW electrolytic 1 10µF 16VW electrolytic 2 4. 7µF 25VW electrolytic 1 0.22µF 63VW MKT polyester 1 0.1 µF 63VW MKT polyester 1 .022µF 63VW MKT polyester Resistors (0.25W, 1%) 1 680kQ 1 10kQ 1 470kQ 1 5.6kQ 2 100kQ 1 2.2kQ 2 47kQ 1 1kQ 1 27kQ 1 470Q 2 15kQ 1 4.7Q 0.22µF 0.1µF .022µF 220n 100n 22n 0 0 0 0 0 0 0 0 0 0 0 0 0 No. Value 4-Band Code (1%) 5-Band Code (1%) 1 680kQ 470kQ 100kQ 47kQ 27kQ 15kQ 10kQ 5.6kQ 2.2kQ 1kQ 470Q 4.7Q blue grey yellow brown yellow violet yellow brown brown black yellow brown yellow violet orange brown red violet orange brown brown green orange brown brown black orange brown green blue red brown red red red brown brown black red brown yellow violet brown brown yellow violet gold brown blue grey black orange brown yellow violet black orange brown brown black black orange brown yellow violet black red brown red violet black red brown brown green black red brown brown black black red brown green blue black brown brown red red black brown brown brown black black brown brown yellow violet black black brown yellow violet black silver brown diesel sound all the time, even when the loco is running "light engine" (that's railway speak for when it's running by itself). RESISTOR COLOUR CODES rn 2 2 1 2 SILICON CHIP The prototype fitted neatly inside a HO scale Victorian Railways V/Line G Class Mk2 loco but only after we had relocated the internal wiring for the lighting. To obtain adequate sound output, the loudspeaker should be fitted inside a 30mm length of tubing (not shown here). As a practical exercise, we mounted the Diesel Sound Simulator inside a HO scale Victorian Railways V/Line G Class Mk2 loco. This model is quite roomy inside although it still requires some internal surgery to make everything fit. First, it was necessary to replace the steel weights with lead. Health regulations prevent toy and model suppliers from selling products containing lead but there is nothing to stop model enthusiasts from putting in lead after purchase. It was also necessary to relocate the diodes and the wiring associated with the directional lighting. These diodes were originally terminated to solder lugs under screws which retained the steel weights. We relocated the diodes to a small piece of Veroboard which was then mounted inside the roof above one of the cabins. As noted above, the PC board has been designed to be cut in half if necessary. This is best done with a miniature high speed grinding wheel fitted to a Dremel Moto-tool or an Arlec Mini-tool. This enables you to fold the board in half, with both halves connected together with very short lengths of miniature hook-up wire. The bisected board is then squeezed into the cleared space above the lead weights. We put insulating tape over the lead weight to prevent shorts to the board. Finally, we fixed the speaker inside the roof of the loco adjacent to louvres which conveniently let the sound through. All that you can see in the relevant photos. How did it sound? Not too brilliant - at least not initially. While it was satisfactory at idle, once the loco got going the sound of the motor and following rolling stock tended to drown the sound from the speaker. It really needed a baffle. Hence, after the photos were taken, we mounted the speaker inside a short length of tubing (anything over 30mm long does the job) which is mounted above the unpowered bogie. This can only be done if the top half of the bogie housing (essentially it is a dummy motor) is removed to make space for the tube. That done , the sound output is much louder and is quite satisfactory. Under the layout mounting While the circuit was specifically designed to run with our PWM controller designs, it is possible to use it with other controllers which simply vary smoothed cir unfiltered DC. An example is the simple controller featured in the November 1990 issue of SILICON CHIP. In these cases though, it is not possible to install the system inside the locomotive. Instead it will have to be installed underneath the layout. You can then use a much larger loudspeaker and obtain even more sound output. The circuit will require a few modifications, however. These are as follows: (1). Since the DC from the rails can vary from Oto 12V or thereabouts, the circuit will have to be run from an external power supply which can be a 12V DC plugpack. (2). The anode of diode Dl will need to be disconnected from the bridge rectifier BR1 and connected instead to the positive lead from the external DC power supply. (3). You will probably also have to change the value ofRl to obtain satisfactory operation. We found that 4 7kQ was suitable. Apart from these modifications, the Diesel Sound Simulator is connected as in the loco except that its inputs are wired directly across the track (ie, across the output of your speed controller). Acknowledgement Fig.4: this is the full-size etching pattern for the PC board. Our thanks to Powerline Models Pty Ltd who kindly supplied the V/ Line G Class Mk2 locomotive used in this article. They can be contacted at PO Box 264, Penrith NSW 2750. Phone (047) 396 204. SC D ECE M BE R 1992 17