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Build the SteamSound Simulator Had enough of that silly whine coming out of your HO-scale "38" loco? This steam sound simulator makes a realistic "chuffing" sound that keeps pace with the loco speed. T'S EARLY on a Saturday morning, about 7am, and you're relaxing in the leading carriage of a 5car set sitting on platform 1 at Central Railway Station. Looking around your compartment, you see a couple of black and white photos framed with scenes of the original Zig Zag railway near Lithgow. A "30 class" tank loco draws slowly away from platform 2 with the empty set from the overnight Southern Aurora that arrived half an hour ago. As it moves into the distance, its highpitched whistle signals that it has passed over the points, which now revert to platform 1. It's 7:04am and still no engine. The whole station is alive with the sounds of air compressors, people hurrying to their seats and a stationmaster mumbling something in the background. You shove your head out through I DARREN YATES 22 SILICON CHIP the window and in the distance see big plumes of steam gently drifting into the air. As it moves closer, you can just make out the rear of the tender and the numbers on the back, "3830". All attention is focused on platform 1 as the guard comes up and gets ready for the coupling. The carriage rocks violently back as the loco hits the end buffers. The guard points out to the driver in no uncertain fashion that he came in too fast. The driver just leans out the window and points to his watch. Three minutes to go. The brake lines and coupling hooked up, the familiar beat of the air compressor starts as the fireman stokes up the firebox with rich, black Muswellbrook coal. The guard, now leaning out of the guard's van, blows his whistle and holds out the green flag. The driver acknowledges with a long flowing blast of the whistle and eases the throttle open. The whole train groans as it moves slowly out of the station. The loco loses traction and slips, driving wheels spinning, but the driver recovers it and the Southern Highlands Express makes its way through the yard and onwards. This may be your memory of a steam loco or maybe you're one of the many who ventured to Hawkmount and Fassifern to watch the 60 class Garratts haul everything from coal to concrete. Whatever the case, you have to admit that a steam loco is infinitely more interesting than your average diesel - no bias intended, of course! Model railways Many of us have seen model railway layouts at exhibitions that depict some place about 30 years ago, with all the scenery carefully laid out to look as realistic as possible. Then you'll look down the track and see a steam loco pulling the "pick-up" goods train. Everything looks so realistic, until it passes you and you hear the whine of the electric motor inside the loco ... and the realism is lost. This little project fixes that prob- lem by producing a realistic chuffing sound through a small speaker which fits inside the guard's van or goods wagon directly behind the loco. It's specifically designed to go with our Railpower train controller published in the April and May 1988 issues of SILICON CHIP. However, it it possible to make it work with most other train controllers (we show you how later TO TRACK SAWTOOTH WAVEFORM GENERATOR BRIDGE RECTIFIER VOLTAGE REGULATOR +12V white noise source which is modulated by a sawtooth oscillator to produce the "chuffs". The speed (or frequency) of the sawtooth oscillator is made directly proprortional to the average DC voltage applied to the loco's motor (via the rails) so that the higher the DC voltage, the faster the rate of chuffing. The circuit is connected directly to LOW FREQUENCY AMPLIFIER POWER AMPLIFIER/ BUFFER DIODE MODULATOR WHITE NOISE SOURCE Fig.1: block diagram of the SteamSound Simulator. The speed information is derived from the track & this controls the frequency of a sawtooth oscillator. The sawtooth oscillator in turns controls a diode modulator, which then amplitude modulates a white noise source to produce the "chuffing " sound. in the article). It's also easy to build and uses no hard to get bits. In fact, you'll probably already have most of the parts in your junkbox. Block diagram Refer now to Fig.1 which shows the block diagram of the SteamSound Simulator. The circuit consists of a the track via a bridge rectifier which provides the power requirements fo r the circuit and also provides the throttle setting for the speed of the "chuffs". The bridge rectifi er allows the circuit to work correctly whether the loco is moving forwards or backwards. As mentioned earlier, the circuit was designed primarily for use with a pulse typ e train controller so before we go any further, let's go over the basic principles of PWM train control so that we understand what is meant by the terms "positive pulses" and "varying pulse width". All model locos use a simple method of transforming electricity to movement: you apply a voltage to the little motor and the loco moves. The higher the voltage, the faster it goes. OK, that should be obvious. However, at low voltages and due to dirt on the rails or the wheels, the motor will tend to not operate smoothly and may often stall on gradients and curves. By applying a pulsed DC voltage to the motor, we get much better speed regulation an d hence smooth running at low speeds. The loco will also start smoothly, without any of the jerkiness associated with conventional controllers. Fig. 2 shows how a PWM controller works. If narrow pulses are applied to the rails (as at the top of the diagram), then the motor averages these pulses out, so that in effect we have a small voltage across the motor. As the pulse width increases, the average voltage increases, which in turn, APRIL 1991 23 SLOW SPEEO PRODUCES NARROW PULSES MEOIUM SPEEO PRODUCES HALF-WIDTH PULSES j FAST SPEEO PRODUCES VERY WIDE POSITIVE PULSES Fig.2: how a PWM controller works. At low speed settings, only narrow pulses are applied to the rails to produce a low average voltage. At higher speed settings, the pulse width is increased to produce a higher average voltage across the motor. makes the loco speed up. Finally, if we have very wide positive pulses, the average voltage is very high and so the loco speeds around the track. · Now if we go back to the block diagram of Fig.1, the output of the bridge rectifier is fed to a voltage regulator which provides +12 volts DC to power the circuit. It is also sent to a voltage inverter stage and this controls the sawtooth oscillator. The reason for the inversion is to provide the correct control voltage for the oscillator so that we get the desired output; ie, to produce a low frequency output, we need a high voltage on the input and to produce a high freqeuncy output, we need a low voltage on the input. In effect, the sawtooth oscillator works the wrong way around, so we need to invert the incoming voltage to compensate. From the oscillator, we get a sawtooth output with a frequency proportional to the pulse width of the track voltage. To put it simply, the faster the train is going, the higher the frequency from the oscillator. This output is then fed to a diode modulator. To explain briefly, the conductivity of a diode changes depending on the voltage across it. This means that a diode with 0.6 volts across it will conduct more current than a diode with only 0.2 volts across it. In effect, we are using the diode as a voltage-controlled resistor but more about this later. Meanwhile, the white noise generator produces about 80mV of signal which makes up the steam and chuffing sound. By modulating or varying this signal, we can produce the effect of a train chuffing up a fairly steep hill or blasting along the flat. The white noise signal is modulated by feeding it to a low frequency amplifier and by using the diode modulator to vary the gain of this Our prototype SteamSound Simulator was built into a HO baggage van from Powerline Models Pty Ltd. Power for the circuit can be picked up by running leads through to the loco motor or by using a pick-up system from the rails. Using the SteamSound Simulator with the Simple Train Controller OK, can you use the SteamSound Simulator with the Simple Train Controller described in our November 1990 issue? The answer is yes but you do have to make a few minor modifications. You can use the same modifications to make the SteamSound Simulator work with just about any train controller. First, because of the way in which the Simple Train Controller works (ie, without a pulsed DC output), you will not be able to power the SteamSound Simulator directly from the rails. Instead, it will have to be powered from a separate DC sup- 24 SILICON CHIP ply. That in turn means that the project can no longer be mounted inside a carriage but you can mount it in a fixed position under the layout. If you're willing to accept that limitation, here are the modifications: (1 ). Disconnect the anode of diode D5 from the bridge rectifier and connect it instead to the external DC supply. This could be a 12V DC plugpack supply (which will give an output of about 16-1 ?V when lightly loaded}. Alternatively, you could use the supply rail to the train controller itself provided it is in the range 1518V DC; or you can use some other external DC supply up to about 25V. (2). Delete the 1.8kQ resistor connected to the bridge rectifier. (3). Change the 330kQ resistor on O2's base to 120kQ, the 27kQ resistor to 150kQ, and the 150kQ resistor to a 10kQ trimpot (tie the wiper to one of the outside pins). Note that you still must connect the SteamSound Simulator to the track via the bridge rectifier to derive the speed information. The trimpot is simply adjusted for best effect (ie, steam only when the throttle is closed, with the "chuffs" starting as the throttle is opened). 05 1N4004 100 . + l5VW+ WHITE NOISE SOURCE 14 .,. 0.1 .,. 1.2M FROM TRACK .001 1k. LOW FREQUENCY AMPLIFIER 150k 06 1N914 .0471 HIGH GAIN AMPLIFIER .,. 07 01+ 1N914 POWER AMPLIFIER/BUFFER DIODE MODULATOR +12V 27k +12V 0.11 15k B .,. ~ 1.8k E'Oc VIEWED FROM BELOW .,. +12V SAWTOOTH GENERATOR .,. STEAM SOUND SIMULATOR Fig.3: the final circuit is based mainly on an LM324 quad op amp IC. D1-D4 rectify the track voltage & this controls the frequency of the sawtooth generator based on Q2 & ICla. Ql is the white noise source. Its output is fed to IClb where it is amplitude modulated by diode modulator D7 to produce the "chuffing" sound. The output ofIClb is then amplified & fed to the loudspeaker. stage. This low-pass active filter stage amplifies the white noise and removes the high frequencies so that our "chuffs" have a bit more grunt to them. The output from the low frequency amplifier is then fed to a high gain amplifier. This stage amplifies the signal to a level suitable for driving the power amplifier and loudspeaker stages. Main circuit Take a look now at Fig.3 . It's based mainly on a single LM324 quad op amp to keep the parts count fairly low. Diodes Dl-D4 full wave rectify the PWM track voltage to produce the positive DC voltage pulses. This is then coupled via isolating diode D5 to a .7812 3-terminal regulator which produces a +12V DC rail. This rail directly provides power for the audio output transistors (Q3 & Q4) and is also decoupled using a 22Q resistor and 470µF capacitor to provide power for the small-signal circuitry (Ql, QZ and ICl). The positive-going pulses from the bridge rectifier are also fed to transistor QZ which forms the voltage inverter. The output signal appears at QZ's collector and is fed to a filter network consisting of a 6.8kQ resistor and l0µF capacitor. This filter network has two functions: first, it filters and averages the pulses to provide a steady DC voltage; and second, it forms part of the timing network for the sawtooth oscillator based on ICla. ICla is 1/4 of an LM324 quad op amp, connected as a standard Schmitt trigger squarewave oscillator but with a couple of changes. To start with, the oscillator uses the voltage derived from QZ to determine its frequency, so that it really acts as a voltage-controlled oscillator or VCO. Diode D6, connected ·in the negative feedback loop of the op amp, causes the output signal to be a series of short pulses. Let's now take a closer look at how The top trace of this CRO photograph shows the waveform across the speaker, while the bottom trace shows the waveform at the output of the sawtooth generator (pin 9 of ICla). CRO settings: upper trace 0.lV/ cm & 20ms/div; lower trace 0.5V/ cm & 20ms/div. APRIL 1991 25 15k 01-04;:: 0B ~ i'roh'<at>stl:k]~- +·•:. . i ,.1•1~©lT<at>; • !® I · • • 0 •••• 2.e., 27 A©-- 100uf A©---wi! _ Jf ~ + 22Q 01 330k 05 1a12 A, 'y ©- ©-----4 k©---- 22k -C::- l'G· o 0.1 ~ 330k 2!i ~ • 68k ..-<at> ,~ §Pi (!) :5 68k ~lOOk • 12 M . 10 [!]12;~r<at>O uF :g ~ ~ 1 . 03 + C)1oouF 04 • 25VW + ro: TRACK Fig.4: check each resistor with a multimeter before installing it on the PC board to make sure you have the correct value. The resistors are all mounted end-on to save space. The type numbers & pinouts for the transistors (Q1-Q4) can be gleaned from the main circuit diagram (Fig. 3). this oscillator works. Initially, the lOµF capacitor has no voltage across it and so the output of ICla (pin 8) is high. The capacitor now quickly charges via the 1.ZkQ resistor and D6 until it reaches the upper threshold level of the op amp (ie, the voltage on pin 10), as set by the two 68kQ and the 120kQ resistors. When it reaches this level, pin 8 switches low but this plays no part in discharging the lOµF capacitor because D6 is now reversed biased. Instead, the lOµF capacitor discharges via the 6.8kQ resistor to whatever voltage is at QZ 's collector. This voltage determines the time it takes for the capacitor to discharge to the lower threshold level, at which point pin 8 switches high again and the cycle repeats. The lower the voltage at QZ's collector, the faster the capacitor discharges and therefore the higher the CAPACITOR CODES 0 0 0 0 0 Value IEC Code EIA Code 0.1µF .068µF .047µF .001µF 100n 68n 47n 1n 104 683 473 102 output frequency. Since the voltage at QZ's collector is inversely proportional to the track pulse width, it follows that the oscillator speeds up as the train speed increases. The waveform across the lOµF capacitor is sawtooth shaped and this matches the waveform of a real "chuff" amazingly well. This signal is then fed to the anode of diode D7 via 150kQ and 330kQ resistors. The O. lµF capacitor at the junction of these two resistors is used to filter the PARTS LIST 1 PC board, code SC09104911, 108 x 28mm 1 32mm 8Q loudspeaker (IRH KSS-3108) Semiconductors 1 LM324 quad op amp (IC1) 1 7812 12V regulator 2 BC548 NPN transistors (01 ,02) 1 BC337 NPN transistor (03) 1 BC327 PNP transistor (04) 5 1N4004 rectifier diodes (D1-D5) 2 1N914 signal diodes (D6,D7) Capacitors 1 470µF 25VW electrolytic 2 100µF 25VW electrolytic 26 SILICON CHIP 2 10µF 16VW electrolytic 6 0.1 µF monolithic 1 .068µF monolithic 1 .047µF monolithic 1 .001 µF monolithic Resistors (0.25W, 5%) 1 1.2MQ 1 15kQ 1 560kQ 1 6.8kQ 2 330kQ 1 1.8kQ 2 150kQ 1 1.2kQ 1 120kQ 1 1kQ 3 100kQ 1 820Q 2 68kQ 1 22Q 2 27kQ 1 10Q 1 22kQ Miscellaneous Solder, hookup wire, etc. waveform and to prevent "clicks" in the output. Diode modulator D7 is the diode modulator stage depicted in Fig.1. Its cathode is connected to a voltage divider (27kQ & 15kQ) which sets the bias to about 4.3 volts. This provides us with a preset level and balance so that we don 't get too much steam and not enough chuff. The 0. lµF capacitor on D7 's cathode provides a low-impedance AC path to ground, so that we get maximum effect from the modulation. Whenever the DC level of the sawtooth waveform rises above 4.3 volts , the diode begins to turn on and this decreases its AC impedance. The .068µF capacitor thus sees a progressively lower impedance to ground as the voltage across D7 increases. Because IClb is connected as a noninverting amplifier, these impedance variations directly control its gain. If . the impedance goes down, the gain goes up. Conversely, if the impedance goes up, the gain goes down. Thus, D7 modulates the gain of IC7b to provide the "chuffing" effect. White noise source Transistor Ql is used as the white noise source. This transistor is connected as a reverse biased diode (ie, the base-emitter junction is reversed biased) and the resulting noise is coupled via a O. lµF capacitor to pin 12 of IClb. IClb functions as a noninverting amplifier with modulated gain, as detailed above. The .OOlµF capacitor in the feedback loop rolls off the upper frequency response of this stage. The modulated output from IClb appears at pin 14 and is direct coupled to non-inverting amplifier stage IClc. From here, the signal is passed to pin 3 of ICld which, together with transistors Q3 and Q4 , forms the output stage. Q3 and Q4 buffer the output of the op amp to provide current gain and are connected inside the feedback loop to minimise distortion. The lOQ resistor and the 0. lµF capacitor at the output form a Zobel network, which stops the circuit from oscillating. The output signal is coupled to the loudspeaker via a lOµF capacitor. A value of lOµF might seem a bit puny for a normal amplifier but since it is only handling modulated white noise there is very little low frequency information and so a small capacitor can be used. For the same reason (ie , no low frequencies), a small speaker can be used and still provide quite a surprising level of steam sound output. Construction All components except for the loudspeaker are mounted on a small PC board. This is coded SC09104911 and measures 108 x 28mm. The board, along with the recommended loudspeaker, can be installed in any carriage that's long enough to accommodate it; eg, a guard 's van or goods wagon. Before you start assembly of the board, carefully check the tracks for shorts or breaks. Any faults should be corrected at this stage. Also make sure that the board will fit into the selected van or wagon - you don't want to be doing surgery on it when it's full y assembled. Once you are satisfied with the PC board itself, take a look at the wmng diagram (Fig.4) , which shows how the components should be installed. Make sure you follow it precisely otherwise you may have problems fitting all the components onto the board because of the cramped conditions. Begin by installing the four wire links, making sure that they are flush with the board and as straight as possible. This done , you can install the resistors. These are all installed end on to save space, as shown in the wiring diagram and in the photograph. Uffi a pair of needle nose pliers to make neat right angle bends in the resistor leads. This will give your board a much neater appearance and reduce the possibility of shorts between components. Now you can install the monolithic capacitors. These should all have a fixed spacing of 5mm between their leads, regardless of their value. Don't try using greencaps here - they will be too bulky. After you've installed these capacitors, wire in the signal and power diodes. Make sure that the correct type is used at each position and that they are install·ed the right way around, otherwise the circuit may end up acting like a short circuit! Next, install the four transistors. The reason for doing these now is that they are lower in profile than the RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 No. Value 4-Band Code (5%) 5-Band Code (1%) 1 1 2 2 1 3 2 2 1.2MQ 560kQ 330kQ 150kQ 120kQ 100kQ 68kQ 27kQ 22kQ 15kQ 6.8kQ 1.8kQ 1.2kQ 1kQ 820Q 22Q 10Q brown red green gold green blue yellow gold orange orange yellow gold brown green yellow gold brown red yellow gold brown black yellow gold blue grey orange gold red violet orange gold red red orange gold brown green orange gold blue grey red gold brown grey red gold brown red red gold brown black red gold grey red brown gold red red black gold brown black black gold brown red black yellow brown green blue black orange brown orange orange black orange brown brown green black orange brown brown red black orange brown brown bla9k black orange brown blue grey black red brown red violet black red brown red red black red brown brown green black red brown blue grey 6Iack brown brown brown grey black brown brown brown red black brown brown brown black black brown brown grey red black black brown red red black gold brown brown black black gold brown 1 1 APRIL 1991 27 Three holes were drilled in the bottom of the carriage to let the sound out. The miniature loudspeaker sits in an adjacent rectangular cutout and protrudes slightly from the underside of the carriage. electrolytic capacitors. Once again, make sure that they are installed correctly. The pinout diagrams are on the circuit schematic (Fig.3). Now you can install the electrolytic capacitors. Although they may look a tight fit, these capacitors fit in snugly if you use the recommended voltage rating. Check that the polarity of each capacitor is correct, too. Finally, solder in the 7812 regulator and the LM324 op amp IC. You can easily identify pin 1 of the IG by the adjacent notch (or dot) in the plastic body. When you've finished, check _the board carefully for solder splashes and dry joints. If everything _is OK, you can connect up the loudspeaker and the train controller. To test the board, connect the track leads directly to the controller and apply power with the throttle fully closed. If the overload alarm sounds , switch off immediately and check your wiring for a short or an incorrectly installed component. If all is well, the circuit will make a continuous sound that simulates the noise of escaping steam. If you now open the throttle (that's railway talk for increasing the speed), the steam sound should slowly decrease in volume until the circuit begins to chuff. As you continue to open the throttle , the speed of the chuffing should also increase. Installation When you install the board inside the carriage, you can use Blu-tac® to Fig.5: this is the full-size artwork for the PC board._ 28 SILICON CHIP hold it down. The way in which the power supply is connected is up to you. You may wish to connect the power directly from the motor of the loco or you may wish to use a collector system from the wheels or rails. The choice is yours. We mounted our prototype into a New South Wales HO baggage car made by Powerline Models Pty Ltd (047 39 6204). We drilled three 10mm holes in the base of the carriage to let the sound escape and also made a rectangular cutout to accept the loudspeaker which protrudes slightly from the underside of the carriage. If you intend mounting the SteamSound Simulator underneath your layout baseboard, you can use a much larger speaker and thereby get a lot more sound. Another good idea which we tried was to have one SteamSound Simulator behind the loco and one underneath the layout baseboard. Because the two simulators are not synchronised, they give an interesting echo effect as the train moves around the layout. Which ever way you do it, you can now stop using your imagination and actually have the sounds of steam around your layout! (Oh, what joy! No more diesels ... oops! Only joking, of course!) SC