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DIESEL SOUND GENERATOR FOR MODEL RAILROADS This Diesel Sound Generator produces the deep throbbing sound of today's modern diesel electric locos, complete with turbo boost and alternator whine. By GREG SWAIN & JOHN CLARKE While today's model railroads can look startlingly realistic, the illusion is not helped when those locos start to move off with a heavy train behind. Instead of the penetrating sound of turbo-charged diesels with wide-open throttles, all you hear is the tinny sound of miniature motors and gear mech- anisms - nothing like the real thing. If you're modelling in O scale (43:1) or HO (87:1) it is possible to build sound generator circuits into the loco or a following wagon but they always have a problem. Because they can only use the smallest of speakers, they just don't have enough grunt. They're not loud enough and their bass output is non-existent. · And if you model in the increasingly popular N-gauge (160:1), an on-board sound system is completely out of the question - the rolling stock is just too tiny. So we've had a look at the problem of sound for model railroads. Our new Diesel Sound Generator is the answer. It can be used with any scale loco and does not rely on teensy speakers built into the loco. It uses one or more speakers, as large as you like, dotted around your layout. And it incorporates circuitry to monitor where your loco is so the sound can follow it around the layout. We built the diesel sound generator into a plastic instrument case but you can dispense with the case and simply mount the PCB under your layout. The amplifier PCB is housed in the small plastic case at right. 50 SILICON CHIP The Diesel Sound Generator is designed to mate with the Railpower train controller described in April 1988 but can be used with any train controller that varies the track voltage (that leaves out Command Control systems). It requires little wiring to add it to your layout and you can build it in a simple or more complex form to suit your needs. So what sort of sound do we get from a real life diesel locomotive? The sound is really quite complex but consists mainly of lowfrequency exhaust beats and turbocharger whine, coupled with a small amount of high-frequency alternator hum. Our circuit simulates all these effects to produce a very realistic diesel loco sound indeed. But we haven't just settled for a circuit that makes realistic diesel loco sounds. That wouldn't be good enough. For starters, this new design automatically adjusts the pitch of the diesel exhaust according to the throttle setting. It does this by constantly monitoring the track voltage. The circuit also automatically adjusts the volume according to the location of the loco and can even vary the location of the sound source. It does this by using trackside sensors to monitor the location of the train on the layout. This information is then fed into the circuit to produce the appropriate response. 12VAC TRACK SOUND GENERATOR 4 DC 3 CONTROL 2 +12V GND SIGNAL SINGLE AMPLIFIER TRAIN POSITION SENSORS 1 2 4 □ osE~SOR Fig.1: this is the layout to go for if you intend using a single amplifier and loudspeaker. The sensors monitor the location of the loco on the track. This information is then used to derive a DC control voltage which sets the amplifier volume. Alternative layouts Figs.1 & 2 show the two basic layouts you can use for the Diesel Sound Simulator. In Fig.1, four trackside sensors monitor the location of the loco and this information is fed to the sound generator board to produce a DC control voltage. This voltage is then used to control the volume of a single audio amplifier. By using this simple arrangement the volume from the amplifier stage fades (up or down) to one of four different levels as the loco passes each pair of_ trackside sensors. In a practical layout, you would arrange for the sound level to increase as the loco moves towards you and then decrease as it moves away. The two halves of the photo-interruptor are mounted on opposite sides of the track. When the loco passes between them, it breaks the infrared light beam. This causes the amplifier volume to increase for that sector. But although this can produce quite realistic effects, it cannot change the apparent location of the sound source, as the loco moves around the layout. To simulate a moving sound source, you need to use the arrangement shown in Fig.2. Once again, four trackside sensors are used but each of these now controls a separate audio amplifier with its own loudspeaker. In practice, it's simply a matter of positioning the four loudspeakers driven by the amplifiers to produce DECEMBER1988 51 ~ ::c: ..... z n 0 t==: ..... 0 CJ':) ""NI 12VAC .033 TRACK VOLTAGE .,. IN 14 .,. 16 17 • ,.W- 100 .J,,t • 10 16VW+ 16VW+ 1our • ~,t K LED A +12v 5.6k 0.27! 4.7k .,. +12v f .,. 1° l:w:I lli] LOW FREQUENCY REPETITION 1 I INPUT PARALLEUiJ! SERIAL l16 DIESEL SOUND GENERATOR .,. OUTrr--,, IC2 4021 1.L..11LJTh 11 SERIAL 3 08 IN 3 COMP I 10 CK 12 7 _III_H 470k r------~~-...-.,_......,______ 10 .WVR2 16VW"'I"!20k THRESHOLD '14 IC1 SIG !Nit'---- 4046 vco 4.7k +12V GNU ~~ VIEWED FROM BELOW eQc 8 10kl 680k 8200 NOISE SOURCE +12Y 0.1T • .039t ALTERNATOR SOURCE 470k TRAIN POSITION SENSORS +12V .,. o.1I .,. +12V .,, .,. 10~ l13 4 AMPLFIER --0 AMPLIAER 3 .,. +12V .,, 02 03 AMPLIAER 3 04 AMPLIFIER 1.5k ,10 15 02 .,. .,. 74C14,40106 :g IC6 40175 IC4c/s\ ? 112 03 -: .,. IC411J"s t 113 04 0.1.;: CLRl 1 680k +12V 8!l SPEAKER .,. t6,,V~~ 220 AMPLIAER 4 -:- t t. 't ~ AMPLIAER 2 2.2 .J: 16VWJ rJ: 16VWJ 220 12VAC TRACK l l SOUND GENERATOR +12V GND SIGNAL AMPLIFIER 1 TRAIN POSmON SENSORS AMPLIFIER 2 AMPLIFIER 3 AMPLIFIER 4 1 2 3 4 +12V +12V TD LOUDSPEAKER TD LOUDSPEAKER TD LOUDSPEAKER 2 3 4 □ ed around the loop, from pin 11 through to pins 3 and 12, and via the XOR gate (pins 14, 3 & 2 of ICl) back to pin 11. Thus, IC2 and the XOR gate in ICl generate a pseudo random pulse train and the speed at which this is generated is controlled by the VCO. This gives a realistic "hunting" quality to the diesel sound. The output from the noise generator is taken from the Q8 output of IC2 and applied to a low pass filter consisting of a 4. 7k0 resistor and 0.27µF capacitor. From there, the signal is fed to the pin 5 input of buffer amplifier IC3d. White noise source SENSOR 4 SPE:KER[J1 □ Fig.2: this alternative track layout uses four separate audio amplifiers and four loudspeakers. The loudspeakers are mounted at appropriate locations around the track to simulate a moving sound source. the best effect as the loco moves around the track. The volume from each amplifier can be adjusted by changing a single resistor value. How it works Most of the circuitry is contained in eight low-cost ICs. Fig.3 shows the details. ICl and IC2 form a pseudorandom noise generator and together provide the low frequency exhaust beats so characteristic of a diesel locomotive. ICl is a 4046 phase locked loop (PLL) and contains a voltage controlled oscillator (VCO) and an exclusive-OR (XOR) gate which is normally used as a phase comparator. In our circuit though, ICl is not used as a phaselocked loop. We are only using the VCO and the XOR gate. VRl, the 470k0 resistor and the .033µF capacitor set the overall fre◄ Fig.3 (left): three separate noise sources are used to simulate the diesel sound. ICl & IC2 provide the low frequency exhaust beats, IC3 provides the high frequency exhaust noise, and IC3a & IC3b provide the alternator whine. quency range of the VCO, while its actual frequency is set by the DC control voltage applied to pin 9. This control voltage is derived from the track voltage via a bridge rectifier (D5-D8) and a voltage divider consisting of two 4. 7k0 resistors. In this manner, the speed of the VCO is controlled by the throttle setting on the train controller. The higher the setting, the higher the frequency of the VCO. When the throttle is closed, the control voltage on pin 9 is at a minimum and the VCO runs at its minimum value of around 300Hz, thus setting the engine idle speed. The VCO output appears at pin 4 of ICl and clocks IC2 which is a 4021 shift register. When the circuit is first switched on, pin 9 of IC2 is momentarily pulled high via a 0.lµF capacitor. This loads an 8-bit number into the shift register (as determined by the wired connections to pins 1, 4, 5, 6, 7, 13, 14 & 15). Pseudo random pulses The VCO signal is fed to pin 10 of IC2 and thereby controls the speed at which the 8-bit pattern is shuffl- OK, so we have our low frequency exhaust beats. We now have to simulate the high frequency exhaust noise and this task is performed by transistor Ql and op amp IC3c. Ql is connected as a reverse biased diode (ie, the base-emitter junction is reverse biased) which makes it into a good "white noise" source. The resulting noise produced by Ql 's controlled avalanche breakdown is AC-coupled via a .047 µF capacitor to pin 10 of IC3c which is a non-inverting amplifier with a gain of 69. A voltage divider consisting of two lM0 resistors sets the bias at pin 10 of IC3c to half supply. The amplified output of IC3c appears at pin 8. From there, it is fed via a l0k0 resistor to pin 5 of IC3d where it is mixed with the low frequency exhaust beats. Alternator whine Unlike the previous two noise generator circuits, the alternator whine only comes into play when the track voltage reaches a predetermined level. This provides more realistic simulation, since alternator whine only becomes apparent after a diesel loco gets up speed. IC3a and IC3b provide simulation of the alternator whine. IC3a functions as an amplifier and controls Schmitt trigger oscillator IC3b via diode D9. Let's take a closer look at how the circuit works. Diodes D5-D8 provide full-wave DECEMBER 1988 53 t-1- 12VAC tTRACKt ® HH ,rh~~~® c~r-c l ~ ... ;~ 8 ®~®C¥;]' 1000uF FL 12 + v~O~R2 GNO_l...." SIGNAL- I _ ~ e ~ --mE}-l [ ~470 L~t2; ] i .l!; :g ~ ~ 09~ ~ ;! i;j liJ ~ ffi ~~ 1 -!llliJ- 10k --rrz:o..---cm::J- .i: 8 I] - --ruo,. ~ ~~ I ~ <at>) .,:e !.1:; .. ®~ /'.!'\6 ~ .l!; ~ ~~ ~ e - l3 CONTROL I TD AMPLIFIER 1 - - - - TO AMPLIFIER 4 ..mo,. A oc VOLUME 1 - T D AMPLIAER 3 I -TOAMPLIFIER2 ~ 1 K• E• K• E• K• E• K• Ee ~ ~ A• C• A• C• A• C• A• Ce OP1 __ OP2_ OP3_ OP4_ _ _ _ _ _ _ _ _ _ _ __. Fig.4: here's how to mount the parts on the sound generator PCB. Make sure that all parts are installed the right way around and use PC stakes to terminate the external wiring connections. The 12VAC supply can be derived from the train controller. rectification of the track voltage as previously described. The output of this bridge rectifier is then fed to a voltage divider, filtered by a lOµF capacitor and applied to pin 3 of IC3a. Trimpot VR2 sets the threshold at which the circuit begins to operate by applying a preset voltage to pin 2. So what does this all mean'? It means that, at low throttle settings, the voltage on pin 3 of IC3a will be less than that on pin 2. Thus, the output (pin 1] will be low and diode D9 will be forward biased. As the throttle is progressively opened, the voltage on pin 3 rises above the pin 2 threshold and IC3a functions as an amplifier with a gain of 2. When its output reaches about 0.66 of the supply rail, D9 will be reverse biased. D9 determines whether Schmitt trigger oscillator IC3b functions or not. The 470k0 feedback resistor between pins 14 and 12 sets the level of hysteresis, whilst a voltage divider consisting of two more 470k0 resistors sets the midpoint voltage on pin 12 to half supply (ie, to + 6V). 54 SILICON CHIP When D9 is reverse biased, IC3b oscillates at about 2.4kHz, with the .039µF capacitor alternately charging and discharging between 0.33Vcc and 0.66Vcc (ie, between 4V and BV). This signal simulates the alternator whine and is fed to pin 5 of IC3d via a 150k0 resistor where it is mixed with the noise signals from IC2 and IC3. However, when D9 is forward biased (ie, when the track voltage is below the threshold level], the voltage across the .039µF capacitor is clamped to 0.5Vcc. As a result, IC3b stops oscillating and the alternator whine ceases. DC volume control IC3d functions as a non-inverting buffer amplifier and is used to drive the following audio amplifier stages. For the sake of clarity, our circuit shows only one audio amplifier. It consists of DC volume control IC7, 741 op amp stage ICB, and transistors Q6 and Q7. If you want to add additional amplifiers, all you have to do is duplicate the circuit based on these components. IC7 is an MC3340P "electronic attenuator" chip from Motorola. It functions as a DC volume control. As the voltage at pin 2 is increased from 3V, the gain is reduced from + 13dB to below - 70dB at around +5V. In this circuit, a 470k0 resistor at pin 2 sets the maximum attenuation (ie, it sets the minimum volume from the loudspeaker]. To increase the volume when the train passes a set of trackside sensors, all we have to do is to switch another resistor in parallel with the 470k0 resistor on pin 2. This job is performed by transistors Q2-Q5 but more about that later. The output from the attenuator is coupled directly to pin 3 of ICB, which is a 741 op amp. This in turn drives a push-pull transistor output stage consisting of Q6 and Q7. From there, the signal is AC-coupled via a 220µF capacitor to an BO loudspeaker. Negative feedback for the amplifier is via the 1.5k0 resistor connected between the commoned emitters of Q6 and Q7 and pin 2 of ICB. Together with the associated lkO resistor, this sets the overall gain of the amplifier to 2.5. The associated 2.2µF capacitor rolls off the amplifier low frequency response below 70Hz. Trackside sensors Four photo-interrupters, OP10P4, are used as trackside sensors to detect the position of the loco. These photo-interrupters consist of an infrared LED and an NPN Darlington phototransistor. Normally used as position sensors in printers for computers, they are cut in half and the halves are located on opposite sides of the track (see Figs.1 & 2). The infrared interrupter LEDs are supplied with current via a 4700 resistor from the 12V supply. The Darlington transistors have their collectors connected to the + 12V supply and their emitters connected to OV via 1 BOkO resistors. When under full illumination from the LED, the phototransistor conducts and its emitter is pulled high. When the light is interrupted the voltage at the emitter drops to OV. The four phototransistor outputs This view shows how we mounted the sound generator PCB inside a standard plastic instrument case. Most readers will probably prefer to mount the board out of sight, underneath the layout. The external wiring can be run using rainbow cable. are monitored by Schmitt triggers IC4a to IC4d. The outputs of the Schmitt triggers connect to the data inputs of IC6, a 40175 quad D-type flipflop, and to the inputs of quad input NOR gate IC5, a 4002. Data storage IC6 is the key component in determining which amplifier's gain is increased. For the system to work properly, the amplifier associated with a particular sector of the railroad layout must increase its gain when the train enters that sector. At the same time, the amplifier associated with the previous sector where the loco was present must fade down. In addition, the circuit must be able to cope with any change in direction that the loco might make, To simplify external connections, we terminated all the wires from the generator board on two multi-way insulated terminal blocks. These are recommended even if you don't use the plastic case. DECEMBER1988 55 DC VOLUME CONTROL GNO sa SIGNAL +12V SPEAKER Fig.5: parts layout for the DCcontrolled audio amplifier. The + 12V supply is derived from the sound generator PCB. Depending on your requirements, you can build up to four of these audio amplifiers to go with the sound generator PCB. We mounted the amplifier in a small plastic case but it could also be mounted in the same case as the sound generator board or mounted under the layout. For best sound output, the loudspeaker(s) used should have good bass frequency response. If the high frequency response is excessive, you can chop it back by reducing the 680kn feedback resistor between pins 8 & 9 of IC3. as it enters, leaves and re-enters any of the four sectors. Whenever the loco passes between one of the trackside sensors, the output of the associated Schmitt trigger (in IC4) goes high. Normally, unless the train stops light transmission across one of the four interrupters, all four Schmitt triggers will be low and the output of the quad input NOR gate IC5 will be high. When one of the Schmitt trigger 56 SILICON CHIP outputs does go high the output of IC5 will go low. This low signal is delayed by the 15k0 resistor and 0.1µ,F capacitor and then passed to the input of Schmitt trigger IC4e. The output of IC4e then goes high and clocks the four D-type flipflops. Let's say that the output of IC4b has gone high. When IC4e goes high, this high signal will be "latched" at the Q2 output of IC6, pin 7. This will turn on transistor Q3 which will then pull down its callee- tor resistor. This increases the gain of amplifier 2. If the loco then moves to interrupter OP3, IC4c's output will go high, and IC6 will "latch" the high signal through to pin 10. This will turn on Q4 which will increase the gain of amplifier 3. In the meantime, Q3 will have turned off and the gain of amplifier 2 will have been reduced. The "CLR" (clear) input at pin 1 makes sure that IC6 is properly set to zero from the start. At switch-on the 0.1µ,F capacitor has the effect of pulling pin 1 low which sets the Q outputs (pins 2, 7, 10 and 15) low. The 680k0 resistor then charges the 0.1µ,F capacitor to +12V so that pin 1 is high and IC6 can work normally. This means that until a loco passes a photo interruptor, all amplifiers will be at minimum volume. To control the gain of IC7 (or the MC3340 associated with each separate audio output stage), transistors Q2 to Q5 switch in a resistor to ground. The 4.7k0 resistor at the collector of Q2 sets the volume at maximum, while the 15k0 resistor at Q3 sets the volume at a minimum. The 220µ,F capacitor on pin 2 of IC7 ensures that the gain changes are not too abrupt. If you want even slower fading up and down of gain, increase this capacitor further. Power supply Power for the circuit will normally be supplied from the low voltage transformer which feeds the train controller. Normally, this transformer will deliver between 12 and 20V AC. This is fed to the bridge rectifier consisting of diodes Dl to D4. The bridge feeds a 1000µF filter capacitor and 7812 3-terminal regulator to provide + 12V. A l0µF capacitor at the output of the 7812 regulator ensures its stability. Another 100µF capacitor across the supply but remote from the regulator provides further decoupling of the supply for the ICs. Construction The Diesel Sound Generator and the amplifier are made up on separate printed circuit boards. These measure 117 x 130mm (code SC09-1-0988-2) for the diesel generator and 60 x 47mm (code SC09-l-0988-1) for the amplifier board. For a minimum set-up you will need one generator board and one amplifier board. For the most complex set-up you will need one generator board and four amplifier boards, together with associated loudspeakers. Most readers will probably prefer to mount the printed boards beneath their track layout. However, both boards will fit inside standard plastic cases, as shown in the photos. The sound generator board fits into an instrument case measuring 200 x 158 x 68mm (Altronics Cat. H-0480) while the amplifier fits in a zippy case measuring 83 x 54 x 28mm (Altronics Cat. H-0101). Assembly of the two PCBs is quite straightforward just follow Figs.4 & 5. Take care to correctly connect the polarised components such as the diodes, ICs, transistors, the 3-terminal regulator, and the electrolytic capacitors. Testing To simplify the connection of wiring, we terminated all the wires from the generator board on two multi-way insulated terminal strips. For the initial test, do not connect the DC volume control line(s) from the generator board to the amplifier. Instead, connect a 4. 7k0 PARTS LIST Diesel Sound PCB 1 PCB, code SC09-1-0988-2, 117 x 130mm 1 plastic instrument case, 205 x 158 x 68mm (optional) 1 1 6-way insulated terminal block 1 12-way insulated terminal block Semiconductors 1 40175, 7 4C175 quad D flipflop 1 40106, 7 4C14 hex Schmitt trigger 1 4046 phase lock loop 1 4021 8-stage static shift register 1 4002 dual 4-input NOR gate 1 LM324 quad op amp 1 1 N914, 1 N4148 signal diode 4 1 N4002 1 A diodes 1 7 81 2 3-terminal regulator 4 BC337 NPN transistors 1 BC548 NPN transistor 4 STIN3101 or equivalent photo-coupled interrupters Capacitors 1 1 000µF 25VW PC electrolytic 1 1 00µF 16VW PC electrolytic 2 1 0µF 16VW PC electrolytic 1 0.27 µF metallised polyester 4 0 . 1µF metallised polyester 1 .04 7 µF metallised polyester 1 .039µF metallised polyester 1 .033µF metallised polyester Resistors (0 .25W, 5%) 2 x 1 MO, 3 x 680k0, 6 x 470k0, 1 X 330k0, 4 X 180k0, 1 x 150k0, 2 x 15k0, 9 x 1 OkO, 1 x 6.8k0, 1 X 5.6k0, 4 X 4. 7k0, 1 X 8200, 4 X 4700, 1 X 200k0 miniature vertical trimpot, 1. x 20k0 miniature vertical trimpot Audio Amplifier 1 plastic zippy box, 83 x 54 x 28mm 1 PCB, code SC09-1 -0988-1 , . 60 x 47mm Semiconductors 1 MC3340P DC attenuator 1 741 op amp 1 BC337 NPN transistor 1 BC327 PNP transistor Capacitors 3 220µF 16VW PC electrolytic 1 2.2µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 680pF ceramic Resistors (0 .25W, 5%) 1 x 470k0, 1 x 1.5k0, 1 x 1 k0 Miscellaneous Loudspeakers, hookup wire, tinned copper wire, solder etc. The LED section of the photointerruptor has a small diode symbol moulded into the plastic housing. The photo-interruptor must be cut in half and the two halves mounted on opposite .sides of the track. resistor between the DC volume control input to the amplifier board and the 0V line. With all connections made, switch on the power and check that the 7812 regulator has + 12V at its output. If all is operating correctly, the diesel sound should be emanating from the loudspeaker. The pitch of the diesel sound should increase as the train controller's output is increased. You can set the maximum pitch, to coincide with the maximum throttle setting, using VR1. When setting VRl remember that DECEMBER 1988 57 I ~ -~ ~, I 0 . SC09-1-0988-2 L '7°1 l. '7'7 58 SILICON CHIP L.= Fig.7: actual size artwork for the amplifier PCB. The actual volume level is dependent upon the resistor connected to the DC volume control input. A 4. 7k0 resistor gives maximum volume while a value of 15k0 gives the minimum volume. The values shown were used in our circuit but can be altered to suit a different track layout. Options Fig.6: here is an actual size artwork for the sound generator PCB. Ready etched boards are available from the usual sources (see hack page). the large diesels in locomotives usually run at no more than about 1100 RPM when at full power. So don't make it sound like a racing car at flat-chat. Adjust VR2 so that the hum cuts in soon after the diesel sound passes idle speed and as the loco begins to move. The remainder of the circuit can now be tested by connecting up the optical sensors and DC volume control lines to the amplifier(s). The track sensor is made by cutting a STIN-3101 photo-interrupter in half. We obtained our sensor from Geoff Wood Electronics but equivalents are readily available elswhere. Alternatively, if you wish, you can use a separate infrared LED (LD271, CQY89A) and infrared photodiode (BPW50, BP104R) or NPN phototransistor (TIL81). The advantage of the interrupters is that they are easy to ...~. mount and conceal as trackside structures. Cut the interrupter in half between the light emitting diode and the phototransistor. Use a finebladed hacksaw for this purpose and file the cut edges to provide a smooth finish. The resulting separate devices are mounted directly opposite each other on each side of the railway line. They must be mounted on the same plane and should be square on to ensure maximum sensitivity. Once the sensors are located and wired to the PCB, the circuitry is ready to be tested. When power is first applied, the diesel sound should be muted. For each interruption of a sensor, the volume should adjust to another volume. When used with separate amplifiers, a different amplifier should operate for each sensor and with a different volume. Besides the two optional set-ups we have shown in Figs.1 and 2, a number of other variations can be added. For best sound output, the loudspeaker should have a good bass frequency response. If you are using properly baffled speakers you can obtain even better bass response by modifying the amplifier board. As it stands, the amplifier circuit cuts off at about 120Hz. To lower this to below 40Hz, increase the 220µF output coupling capcitor to 470µF, the 2.2µF feedback capacitor to lOµF and the lµF input capacitor to 2.2µF. Note that this is only worth doing if your speaker(s) have useable bass response to 40Hz. It is not necessary to use speakers with good high frequency response. If you do so, you will probably find that the high frequency noise output from the circuit is too obtrusive. You can chop it back by reducing the 680k0 resistor between pins 8 and 9 of IC3c. To make a noticeable reduction, try a value of 100k0. ~