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Special project for model railroads SteamSound Simulator Mk.11 Did you build the SteamSound Simulator project described in the April 1991 issue or are you contemplating doing so? Either way, this second version will be of great interest because of its improvements and its ability to be fitted into the loco's tender rather than a following wagon. By DARREN YATES & LEO SIMPSON There's no doubt about it, the SteamSound Simulator created a great deal of interest and already many hundreds (perhaps 500 or more) have been built. But like many new ideas, once it had been done, people could see ways in which it could be improved. And those people were not backward in coming forward so we had quite a lot of feedback on the project. 32 SILICON CHIP One big problem is associated with the idea of building the SteamSound Simulator into a wagon to follow the loco. It means that if you want steam sound with a particular loco it always has to be hitched to the wagon. That does not always lend itself to realism as there are times when the loco needs to be operated on its own (known as "light engine" to rail fans). Alternatively, if you want the steam sound with another locomotive, you either need another SteamSound Simulator or else need to unhitch the wagon and hitch up to the second locomotive. That brings us to the problem of compatibility with the wide range of locomotives available. We found that while the Simulator worked stunningly well with some locomotives it was no good with others; they would be off and running around the track before the circuit emitted so much as a single chuff. The problem relates here to two motor characteristics: back-EMF and motor starting voltage. Some model locomotives have much higher back-EMF for a given speed than others and some of the better brass locomotives with can motors will start with as little as 1V across the track. To be frank , we found that the original circuit just could not cope with the wide range of possible variations, even if some of the components were changed. So the part of the circuit which monitors the loco backEMF has been changed quite markedly. Even so, there are two resistors which need to be selected to suit the particular loco. The second problem with the circuit relates to breakthrough of pulsed track voltage into the SteamSound circuit, particularly at low track voltage settings. With some locos starting with track voltages at 1V or less, the equivalent input voltage to the 12V 3-terminal regulator was not enough to ensure proper regula'tion. To solve this problem, we have re-designed the circuit to work from a 9. 7V rail rather than 12V. The change to a 9.7V supply means that a number of other component values also had to be changed. Still more changes relate to the effort to make the circuit components as small as possible. To this end, we changed the values of the various electrolytics so that they are as small as possible, consistent with good sound effect. And because we changed the capacitors, there has been a "ripple through" effect whereby we had to change a lot of resistor values too. The net effect is that while the circuit configuration of the Mk.II version is very similar to the original, there are a great many value changes. Other changes relate to the design of the PC board. It has now been designed to use much smaller resistors and a TO-92 style 3-terminal regulator instead of the larger TO-220 unit used in the original unit. Finally, to allow the PC board to be crammed into a locomotive tender, it has been designed so that it can be cut The new Mk.11 version of the circuit is built on a PC board which can be cut into two halves & mounted inside the locomotive's tender. Use a piece of foam insulation between the two boards to prevent shorts. in half. The two halves can then be stacked for best fit. New speaker Many of the foregoing changes would not have been contemplated if an alternative to the relatively bulky and expensive speaker origin~lly specified had not become available. This new unit is extremely small, with an overall diameter of only 2 7mm and a front-to-back depth of 9mm. In spite of this, it is surprisingly efficient. In fact , it pumps out a level of steam sound which is truly amazing. There are two sources for this new speaker which has a clear Mylar diaphragm. First, you can buy them from Jaycar Electronics at just $2.50 (Cat AS-3002). A second way to obtain the speaker is to buy an "Executor" sound effects key ring. Touted as a harmless outlet for frustrated motorists, these produce a range of novel sound effects. Inside, they have one of the speakers in question, plus a couple of LR44 mercury cells which can be handy if you have a camera or LCD watch which uses this type. You can buy these "Executor" key rings for around $3 from some supermarkets (we purchased ours from a local Flemings store) or from flea market stands. Using the newly designed PC board, this miniature speaker and a lot of patience, we were able to fit the SteamSound Simulator into the tender of a Mansfield Hobbies brass C38 model (worth over $1000 on current prices). If you have a plastic Lima C38 locomotive, the job is much easier since the tender does not have an internal "slope sheet". Locomotives with larger tenders will be proportionately easier to do. Circuit description TO TRACK BRIDGE RECTIFIER VOLTAGE REGULATOR DIODE MODULATOR +12V WHITE NOISE SOURCE Fig.1: block diagram of the SteamSound Simulator Mk.11. The speed information is derived from the track & this controls the frequency of a sawtooth oscillator. This sawtooth oscillator controls a diode modulator which in turn amplitude modulates a white noise source. The resulting signal is then amplified & fed to a loudspeaker to produce the "chuffing" sound. Since the circuit design has changed so much, we'll start at the beginning rather than hark back to the previous article in the April 1991 issue of SILICON CHIP. The block diagram ofFig.1 has been reproduced but note that many of the components on the circuit which related to the various blocks have been changed. Now refer to the circuit diagram of Fig.2. As before the circuit is priOCT0BER1991 33 33 16V~Y_! 0.1 I spffKER 1on! -:- -:- Ra : 1M (SELECT ON TEST) Rb : 50k (SELECT D°N TEST) ~ B 1N<at>ouT EOc VIEWED FROM BELOW -:- STEAMSOUND SIMULATOR Mk.11 Fig.2: the circuit of the SteamSound Simulator is based mainly on an LM324 quad op amp IC. Diodes D1 -D4 rectify the track voltage & this controls the frequency of the sawtooth oscillator which is based on Q2 & ICtb. This sawtooth oscillator in turn controls diode modulator D8. Qt is the white noise source. Its output is fed to ICla where it is amplitude modulated by D8 to produce the "chuffing" sound. The output ofICta is then amplified & fed to the loudspeaker. marily intended for use with the pulse width modulated "Railpower" controller published in the April and May 1988 issues of SILICON CHIP. However, it can be adapted to most train controllers. Diodes D1-D4 full wave rectify the pulsed track voltage to produce positive DC voltage puls es. These are then coupled via diode D5 to a 7805 3terminal regulator which has a 4. 7V zener diode connected to its GND terminal. This effectively increases the regulator's output voltage to +9. 7V (nominal). This rail directly provides power for the audio output transistors (Q3 & Q4) and is also decoupled using a 2200 resistor and 47µF capacitor to provide power for the smallsignal circuitry (Qi, QZ & IC1). The positive-going pulses from the bridge rectifier are also fed to a network consisting of five resistors, a 0. lµF capacitor and diode D6 . The purpose of this network is to extract and filter the loco motor's back-EMF from the track voltage. Diode D6 is crucial to this functi on because it discharges the 0. lµF capacitor in between 34 SILICON CHIP track pulses, down to the level of the back-EMF. Transistor QZ inverts the track voltage signal and feeds it to a filter network consisting of resistor Rb and a 2.ZµF capacitor. As well as its filtering function, these components form part of the timing network for the sawtooth oscillator based on IClb. Squarewave oscillator IClb is 1/4 of an LM324 quad op amp, connected as a standard Schmitt trigger squarewave oscillator but with the voltage derived from QZ setting its frequency. It thus acts as a voltage-controlled oscillator or VCO. Diode D7, connected in the negative feedback loop of the op amp, causes the output signal to be a series of short pulses. The lower the voltage at QZ's collector, the higher is the output frequency. Since the voltage at QZ's collector is inversely proportional to the loco motor's back EMF, the oscillator speeds up as the train speed increases. The waveform across the 2.ZµF 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 D8 via 2 70kQ and 330kQ resistors. The 0.lµF capacitor at the junction of these two resistors is used to filter the waveform and to prevent "clicks" in the output. D8 is the diode modulator stage depicted in Fig. l. Its cathode is connected to a voltage divider (150kQ & 270kQ) which sets the bias to about 3.3V. This provides us with a preset level so that we don't get too much steam and not enough "chuff". The 0.lµF capacitor on DB'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 3.3 volts, the diode begins to turn on and this decreases its AC impedance . The 0.lµF capacitor at DB's anode thus sees a progressively lower impedance to g:r.ound as the voltage across D8 increases. Because op amp ICla is connected as a non-inverting 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, D8 modulates the gain of ICla to provide the "chuffing" effect. PARTS LIST 1 PC board, code SC09110911, 114 x 29mm 1 miniature speaker (Jaycar Cat AS-3002; see text) Semiconductors 1 LM324N quad op amp (IC1) 1 78L05 +5V 100mA regulator 2 BC548 NPN transistors (01,02) 1 BC337 NPN transistor (03) 1 BC327 PNP transistor (04) 1 4. 7V zener diode (ZD1) 6 1N4004 power diod~s (D1 -D6) 2 1N914 signal diodes (D7-D8) This view shows the completed SteamSound Simulator board, before it was cut into two halves for mounting in the tender. The loco, by the way, is a Mansfield Hobbies brass C38 model and is worth over $1000. ~ ~-o~ 0~ [®- 2C)F .:·;."';~.: C!llil. ·1~1 i a1.•10n &si -I~~ . I -~3e-J lffEI~(!) I 0~ D)&--A-~~ost ~,(oTh T O 02A<at>D4A<at>-\..!:1/©zo, I ~i· .,(. cl)~~ L _ ·-Gs 1a os . (I) 110k (!l A(i)o · ~Q2 68 ~ io"' . ~tlji;,i1~ <at> . ~ . ~ _-. · sii ,~Y.;:....:... .Q, - ~~ . l •0 .1•!.,...-!-,-SPEAKER · r'tl~[-o.11t•IA,ri,, ~ ~ o~Q~t. ~--· ~ _ ~ ~ ~·-GJ~ N _ _, -_ ~ 04C)~3uF -03 ______:L_J N FROM TRACK Fig.3: to save space, the prototype used Philips MRS16T miniature resistors but you can also use conventional resistors mounted end-on. Check the resistor values with a multimeter before installing them on the board. Transistor Q1 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 developed across the 47kQ resistor is coupled via a 0.1µF capacitor to pin 3 of IC1a. ICla functions as a non-inverting amplifier with modulated gain, as detailed above. The modulated output from IC1a appears at pin 1 and is direct coupled to non-inverting amplifier stage IC1c. From here, the signal is passed to pin 10 of ICld which, together with transistors Q3 & Q4, forms the output stage. Q3 & Q4 buffer the output of the op amp to provide current gain and are connected inside the feedback loop to minimise distortion. The 10Q resistor and the 0.1µF capacitor at the output form a Zobel network, which stops the circuit from oscillating. The output signal is coupled to the loudspeaker via a 33µF capacitor. The value of 33µF 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. Resistor selection Two resistor values on the circuit, Ra and Rb, are not specified. They need to be selected to match the operating characteristics of the particular model loco. Ra is set so that the circuit starts "chuffing" at the exact point where the locomotive starts to move. With a careful selection of the value, you can get the circuit to work in exact unison with the loco. Rb is selected so that the maximum chuffing rate suitably matches the top speed of the locomotive. We'll dis- Capacitors 1 100µF 16VW electrolytic 1 47µF 16VW PC or tantalum electrolytic 1 33µF 16VW PC or tantalum electrolytic 1 2.2µF 16VW PC or tantalum electrolytic 9 0.1µF 5mm-pitch metallised polyester or monolithic 1 150pF ceramic Resistors (Philips MRS16T, 0.4W) 1 560kQ 2 10kQ 1 330kQ 1 5.6kQ 2 270kQ 1 4. 7kQ 2 180kQ 1 2.7kQ 1 150kQ 1 1.5kQ 3 100kQ 1 220Q 3 68kQ 1 10Q 1 47kQ Note: resistors Ra and Rb need to be selected, as described in the text. Miscellaneous Solder, hookup wire, PC pins, etc. cuss how the resistors are selected later in this article. Construction All components except for the loudspeaker are mounted on the new PC board. This is coded SC09109911 and measures 114 x 29mm. Before you start assembly of the board, carefully check the tracks for shorts or breaks. Any faults should be corrected at this stage. You have two approaches to the assembly of the board. Assuming that OCT0BER1991 35 The miniature loudspeaker is shown here sitting on top of the tender's slope sheet & could be concealed with a simulated load of coal. Despite it's small size, the loudspeaker pumps out lots of steam sound. you are going to cut the board in half, you can either cut it in half before it is assembled or after the event. We prefer the idea of cutting the board before it is assembled Special resistors We assembled our prototype boards using Philips MRS16T 0.4W resistors. These have bodies only 3.7mm long (almost half the length of a normally available resistor) and so can fit right down onto the board instead of sitting "end-on". You can assemble the board with normally available resistors but as you can see from the photos, the Philips MRS16Ts give a much more compact board. Our prototype used Wima 0. lµF capacitors which have a fixed lead spacing of 5mm. However, you can also use the even smaller O. lµF monolithics (sometimes referred to as "Skycaps" because of their blue colouring.) Don't use greencaps - they are just too big and bulky. Similarly, you can substitute tantalum capacitors for the conventional PC mount electrolytics if there is a size advantage in doing so. Make sure you follow the wiring diagram carefully because removing wrongly installed components is quite a trial, since they are so small. Use your multimeter to check the value of each resistor before it is soldered into place. After you've installed the resistors and capacitors, wire in the signal and power diodes. Make sure that the correct type is used at each position and that they are installed the right way around. Next, install the four transistors and the 78105 regulator. The pin-out diagrams, which are viewed from the underside, are on the circuit schematic (Fig.2). Finally, install the LM324 op amp IC. You can identify pin 1 of the IC by the adjacent notch (or dot) in the plastic body. Don't use a socket for the IC as it will make the PC board too bulky. Assuming that you have cut the board in half, you will need to wire the two sections together with short lengths of thin insulated hookup wire. The wire lengths should be long enough so that you can stack the two CAPACITORS 0 0 0 Value IEC Code EIA Code 0.1µF 150pF 100n 150p 104 151 RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 36 No. Value 4-Band Code (5%) 5-Band Code (1%) 1 1 560k.Q 330k.Q 270k.Q 180k.Q 150k.Q 100k.Q 68k.Q 47k.Q 1oi<n 5.6k.Q 4.7k.Q 2.7k.Q 1.5k.Q 220.Q 10.Q green blue yellow gold orange orange yellow gold red violet yellow gold brown grey yellow gold brown green yellow gold brown black yellow gold blue grey orange gold yellow violet orange gold brown black orange gold green blue red gold yellow violet red gold red violet red gold brown gr.een red gold red red brown gold brown black black gold green blue black orange brown orange orange black orange brown red violet black orange brown brown grey black orange brown brown green black orange brown brown black black orange brown blue grey black red brown yellow violet black red brown brown 'black black red brown green blue black brown brown yellow violet black brown brown red violet black brown brown brown green black brown brown red red black black brown brown black black gold brown 2 2 1 3 3 1 2 1 1 1 1 SILICON CHIP board halves together. With a little trial and error you will find a stacking position for two boards whereby they overlap but stack into a height of no more than 10mm. By the way, don't use single strand hookup wire (such as that used in telephone lines). It is too fragile and too easily broken. When you've finished, check the board carefully for solder splashes and dry joints. Testing & installation Before you can test and install the system, you need to select the values of Ra and Rb. To do this you will need two pots, one with a value of up to 1MQ for Ra and one between 50kQ to 100kQ for Rb. Wire the pots as variable resistors (ie, two wire connections, one to the wiper and one to an outside lug) into the positions for Ra and Rb. 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 offimmediately and check your wiring for a short or an incorrectly installed component. If all is well, the circuit will probably make a continuous sound that simulates the noise of escaping steam. Now place your locomotive on the track and increase the throttle setting to the point where the loco just begins to move. Now adjust the Ra pot so that, when the loco begins to move, the circuit begins to chuff. You will need several tries at this until the setting of the pot is correct. Now crank open the throttle to operate the loco at the maximum desired speed. This is an important point Using the SteamSound Simulator with other train controllers OK, can you use the SteamSound SimL1lator with train controllers that simply vary the track voltage? The answer is yes but you do have to make a couple of minor modifications to the circuit. Because of the way in which simple (ie, non-pulsed) controllers work, you will not be able to power the Steam Sound Simulator directly from the rails. Instead, it will have to be powered from a separate DC supply. That in turn means that the project can no longer be mounted inside a carriage or locomotive tender, 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 D5 from the bridge rectifier and connect it instead to the positive output because with many model locos, the speed achievable with 12V DC across the track may be unrealistically high - as much as 300km/h scale speed or more in some cases. So with your loco If space is a problem, you can use tantalum capacitors instead of electrolytics. Take care with component polarity & check all part numbers carefully. of the external DC supply. This could be a 12V DC plugpack supply (which will give an output of about 16V when lightly loaded). Alternatively, you could use the supply rail to the train controller itself, provided it is in the range from 12-16V DC. (2). Delete diode D6 from the circuit. (3). Chose Ra & Rb by initially substituting pots, exactly as before. Adjust Ra so that the circuit begins to chuff when the loco starts to move, then adjust Rb for a realistic chuff rate when the loco is at maximum speed. Finally, measure the pots & substitute fixed value resistors. Note that you still must connect the SteamSound Simulator to the track via the bridge rectifier (D1-D4) to derive the necessary speed information. operating at your preferred maximum speed, adjust the Rb pot for a realistic chuff rate. In practice, the chuff modulation will be extremely rapid but still discernible. When you are happy with the pot settings for Ra and Rb, measure their resistance values with your multimeter and install the equivalent value of resistor. In some instances, you will have to install parallel combinations of resistors, with one resistor above and one resistor below the board. With tl)e resistors for Ra & Rb installed, re-check the Simulator board to make sure that it works as it should with your loco. You are now ready to install the two board halves inside the loco's tender. If you have a plastic tender, this is a relatively straightforward task although you will probably have to remove a steel weight and any internal locating lugs. SC OCT0BER1991 37