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Steam Train Whistle & Diesel Horn Simulator There’s nothing like a steam whistle to add realism to your model railroad layout. This unit sounds just like the real thing & can be easily modified to provide a diesel horn sound. By JOHN CLARKE Mention steam trains to those who are old enough and it brings back memories of “the good old days”, the steam engine and, of course, the steam whistle. Many would wish that the days of steam were still here. However, it is perhaps fortunate that they are not. While it is now a novelty to ride in a restored steam train, it does not take long to realise that they are extremely noisy and dirty. In their favour though, steam trains do have a character which is distinctive and exciting. Part of the unique character of 54  Silicon Chip the steam engine is the whistle. The sounds from a steam whistle are unmistakable. Its well-know trademarks include the rise and fall in pitch as the train approaches and then passes the observer; the dying sound of the whistle as the train blasts into a tunnel; the warning whistle as the train is about to leave the station; and the variations in intensity heard when the train is traversing hilly country. In Australia, steam train whistles are more sonorous than their British counterparts and this is because they actually consist of several whistles, each producing a different note. The result is a distinctive sound that remains embedded in the memory of those who love steam. Although the steam whistle does create much nostalgia, its origins are rather prosaic. Because there is steam in the boiler, some of it can drive the whistle and this is done by pulling a cord which opens a steam valve. Initially, as the steam pressure builds up, the sound level rises until it reaches its maximum intensity. When the steam valve is subsequently closed, the sound level drops off abruptly. Note that, because the whistle is driven by steam, there is a significant amount of white noise evident in the steam whistle sound. The SILICON CHIP Steam Whistle/ Diesel Horn simulates all the requisite notes, noise and level changes to produce a very re­alistic effect. It uses just two ICs and the circuitry all fits on a small PC board. This PC board carries two pushbutton switches, labelled FAST and SLOW, to produce two different steam train whistle sounds. Pressing the SLOW switch simulates the effect of the engineer opening the valve slowly, while the FAST switch simulates the sound when the cord is pulled quickly. Alternatively, the whistle sound can be triggered using remote switches or by using the Level Crossing Detector described in the March 1994 issue. In this way, the whistle can be made to sound automatically as the train goes through a level crossing. Fig.1: block diagram of the Steam Train Whistle. The sound is produced by mixing the outputs of three oscillators & a white noise source together. OSCILLATOR 1 740Hz IC1a OSCILLATOR 2 525Hz IC1b ENVELOPE SHAPER MIXER OSCILLATOR 3 420Hz SLOW S1 IC1c Block diagram AMPLIFIER Q2 VOLUME VR1 IC2 FAST S2 8 WHITE NOISE GENERATOR Fig.1 shows the block diagram of the steam whistle. As shown, the whistle sound is made up by mixing the outputs from three oscillators and a white noise source. The re­sulting output from the mixer is then fed to an envelope shaper and finally to an audio amplifier via volume control VR1. The three oscillators, IC1a-IC1c, operate at 740Hz, 525Hz and 420Hz respectively. These frequency values were obtained from the NSW State Rail Archives and match those used in real steam locomotives. Note that the oscillators do not produce pure sine waves but include second harmonics extending up to 1480Hz. Q1,IC1d Typical Australian locomotives use a 5-chime whistle but we have elected to use only three oscillators. The reason we can get away with this is that some of the chime frequencies are very closely related (ie, second harmonic) and the oscillators we use are already rich in second harmonics. The envelope shaper is triggered using either S1 or S2 to provide the slow or fast rise time respectively. S1 gradually increases the volume of the mixer output over about 200ms, while S2 provides a virtually instantaneous response. For diesel horn sounds, the oscillator frequencies are altered and the Fig.2 (below): the final circuit uses op amps IC1a-IC1c as the oscillator stages, while Q1 forms the white noise source. The outputs from these stages are mixed together & fed via envelope shaper Q2 to audio output stage IC2. +12V 1.8k 100k 1.8k 100k 100k Q1 120k BC548 C 1.8k 100k 100k 100k 10k 12 100k 10 14 IC1a LM324 13 100k IC1b 9 22k .039 100k +12V 0V .056 100k 100k 6 7 IC1c +6V 27k .056 525Hz OSCILLATOR 100k 100k .033 3 10k 2 420Hz OSCILLATOR 1 IC1d 11 10 16VW 10k 4 2.2M NOISE GENERATOR 47k 10  +12V 33k D1 1N4148 1000 16VW 100k 15k 390  FAST S2 C VOLUME VR1 50k E B 22 16VW 3 2 6 IC2 LM386 4 2.2 16VW B .047 Q2 BC548 SLOW S1 14 22k 740Hz OSCILLATOR B 5 0.1 0.1 1k 5 10 16VW E C VIEWED FROM BELOW 22  10  8 .047 STEAM WHISTLE/DIESEL HORN SIMULATOR July 1994  55 EXT SWITCH C3 .056 10uF 0.1 100k 1 .047 47k IC1 LM324 10uF D1 VR1 IC2 LM386 .047 1000uF 100k 0V C1 .039 10  S2 10  1k Q2 2.2uF 390  +12V 1.8k R1 22k 100k 100k 100k 100k 100k 100k R2 22k 1.8k C2 .056 TO SPEAKER 100k 1 100k 22  S1 33k Q1 15k 120k 10k 10k 10k 2.2M 100k 100k 100k 100k R3 27k 1.8k .033 22uF EXT SWITCH Fig.3: the two pushbutton switches are shown here mounted on the board but may be mounted at some remote location if desired (eg, on the control panel of your layout). Alternatively, the circuit can be triggered using the Level Crossing Detector described in the March 1994 issue, or triggered using the optional reed switch/ monostable circuit shown in Fig.6. Fig.4: check your etched board against this full-size artwork before installing any of the parts. noise generator output is disconnected from the mixer. Again, typical Australian diesel horns have five chimes but only three are used here for the reasons discussed above. Circuit details Refer now to Fig.2 for the full circuit details. The three oscillators are Schmitt trigger types which use three of the four op amps in a quad LM324 package. The remaining op amp (IC1d) is used to amplify the white noise generated by transistor Q1. Transistor Q2 and its associated components make up the envelope shaper, while IC2 forms the audio amplifier. Since the Schmitt trigger oscillators all operate in iden­tical fashion, we’ll just consider IC1a. As shown, its non-inverting input (pin 12) is biased by two 100kΩ resistors across the 12V supply, while a 100kΩ feedback resistor is connected between pin 12 and TABLE 1 C1 C2 C3 IC1a IC1b IC1c Steam .039uF .056uF .056uF 740Hz 525Hz 420Hz 2-Car Diesel .047uF .056uF .056uF 600Hz 520Hz 420Hz 40-43, 4401-4440 Diesel 0.1uF 0.12uF .056uF 277Hz 329Hz 440Hz 422, 442, 47, 73 48126 Diesel .056uF 0.12uF .056uF 548Hz 322Hz 429Hz 56  Silicon Chip the output at pin 14. A 1.8kΩ pull-up resistor is also connected to the output and this ensures that pin 14 goes fully high (to produce a more symmetrical waveform). The oscillator action is as follows. At switch on, capaci­ tor C1 at the inverting input (pin 13) of IC1a is discharged and so the pin 14 output is high and pin 12 is at +8V. The capacitor now begins to charge via resistor R1 (22kΩ) until the voltage on pin 13 reaches 8V (the upper threshold of pin 12). At this point, pin 14 goes low and the 100kΩ feedback resistor pulls pin 12 to +4V. C1 now discharges via R1 and pin 14 until it reaches the lower threshold voltage (+4V). When this voltage is reached, pin 14 switches high again and so the process is repeated indefinite­ly while ever power is applied. The frequency of oscillation (740Hz) is determined by the values of R1 & C1. Oscillators IC1b & IC1c operate in exactly the same manner except that the frequencies are different because of the differ­ing RC values at their inverting inputs. The resulting triangle wave capacitor voltages from the three oscillator stages are mixed together via 100kΩ resistors and fed to the collector of transistor Q2. This waveshape is used instead of the square wave from the op amp output since it has a high second harmonic content, which is what we want for the whistle. The noise source is obtained by reverse connecting tran­sistor Q1, so that its base-emitter junction breaks down. This breakdown occurs at about 5V and the 120kΩ resistor limits the current into Q1 to prevent damage to the transistor. The resulting output at the collector is rich in noise and is AC-coupled into pin 3 of non-inverting amplifier stage IC1d. IC1d operates with a gain of 221, as set by the 2.2MΩ feed­back resistor and the 10kΩ resistor at pin 2. The amplifier is DC biased to 1/2Vcc via the two 10kΩ resistors across the supply and the 100kΩ resistor to pin 3. A 10µF capacitor decouples the half-supply rail. The amplified noise output appears at pin 1 of IC1d and is mixed with the oscillator signals at the collector of Q2 via a 47kΩ resistor. Envelope shaper As previously mentioned, Q2 forms the envelope shaper. Normally, Q2 is biased on via D1 which taps a voltage divider consisting of 33kΩ and 15kΩ resistors. The 1kΩ emitter resistor stabilises the bias, while the 2.2µF capacitor shunts signal to ground. Since Q2 is normally turned on, all of the signal at the collector is shunted to ground and no sound is heard from the loudspeaker. However, if switch S1 is pressed, the 22µF capacitor on Q2’s base slowly discharges via the associated 100kΩ resistor and so Q2 gradually turns off. As a result, the signal on Q2’s collector gradually increases to a maximum to produce a steam whistle sound with a slow attack time (about 200ms). When S1 is subsequently released, the 22µF capacitor quick­ ly charges via the 33kΩ/15kΩ voltage divider and diode D1. Q2 now turns on again and shunts the signal to ground, thus shutting off the steam whistle sound. The FAST switch (S2) works in virtually identical fashion to S1 except the it shunts Q2’s base voltage to ground almost immediately via the associated 390Ω resistor. This produces a whistle with a fast attack time (ie, the whistle rises to maximum volume almost immediately when the switch is pressed). The signal at Q2’s collector is AC-coupled to volume con­trol VR1 and then fed into pin 3 of IC2, an LM386 audio amplifi­er. This IC has an output power capability of about 325mW and a gain of 20 when connected as shown in Fig.2. Its output appears at pin 5 and drives an 8-ohm loudspeaker via a 10µF capacitor and a 22Ω current limiting resistor. In addition, a Zobel network comprising a series 10Ω resistor and .047µF capacitor is connect­ed PARTS LIST 1 PC board, code 09305941, 142 x 61mm 2 2-way PC-mount screw terminal blocks 2 PC-mount pushbutton click action switches (S1,S2) 4 PC stakes 1 20mm length of 0.8mm tinned copper wire (for link) 1 50kΩ horizontal trimpot (VR1) Semiconductors 1 LM324 quad op amp (IC1) 1 LM386 audio amplifier (IC2) 2 BC548 transistors (Q1,Q2) 1 1N4148, 1N914 diode (D1) Capacitors 1 1000µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 1 2.2µF 16VW PC electrolytic 1 0.1µF MKT polyester 2 .056µF 5% MKT polyester 2 .047µF MKT polyester 1 .039µF 5% MKT polyester 1 .033µF MKT polyester Resistors (0.25W, 1%) 1 2.2MΩ 1 15kΩ 1 120kΩ 3 10kΩ 14 100kΩ 3 1.8kΩ 1 47kΩ 1 390Ω 1 33kΩ 1 22Ω 1 27kΩ 2 10Ω 2 22kΩ Diesel horn parts Note: add 1 x 0.47µF, 1 x 0.1µF & 1 x 0.12µF 5% MKT polyester capacitors to include the diesel horn sounds listed in Table 1. RESISTOR COLOUR CODES ❏ No. ❏   1 ❏   1 ❏ 14 ❏  1 ❏   1 ❏   1 ❏   2 ❏   1 ❏   3 ❏   3 ❏   1 ❏   1 ❏   2 Value 2.2MΩ 120kΩ 100kΩ 47kΩ 33kΩ 27kΩ 22kΩ 15kΩ 10kΩ 1.8kΩ 390Ω 22Ω 10Ω 4-Band Code (1%) red red green brown brown red yellow brown brown black yellow brown yellow violet orange brown orange orange orange brown red violet orange brown red red orange brown brown green orange brown brown black orange brown brown grey red brown orange white brown brown red red black brown brown black black brown 5-Band Code (1%) red red black yellow brown brown red black orange brown brown black black orange brown yellow violet black red brown orange orange black red brown red violet black red brown red red black red brown brown green black red brown brown black black red brown brown grey black brown brown orange white black black brown red red black gold brown brown black black gold brown July 1994  57 Make sure that all polarised parts are correctly oriented when installing them on the PC board & don’t forget the wire link. A small 8-ohm loudspeaker hidden underneath the layout can be used to provide the sound. This should be mounted near a level crossing or some other appropriate place. across the output to maintain high frequency stability. Power for the circuit can be derived from a 12V DC plugpack supply or from the train controller itself. A 10Ω resistor and a 1000µF capacitor provide supply decoupling and filtering. followed by the diode and the capacitors. Note that the values shown for C1, C2 & C3 are for the steam whistle simulation. If you want a diesel horn sound, these capacitors will have to be selected from Table 1. There are three different diesel horn sounds to Construction choose from, to suit your locomotive. The Steam Train Whistle circuit is In addition, the noise generator must built on a PC board coded 09305941 be disabled by omitting the 47k# mixand measuring 142 x 61mm. Fig.3 ing resistor at pin 1 of IC1 Fig.3 shows switches S1 and S2 shows the parts layout on the board. Begin construction by inserting the mounted on the board and you can PC stakes (for the external switches) do the same if you wish. In most apand the wire link. This done, install plications, however, the switches will the ICs, making sure that they are ori- be mounted separately from the board (eg, on the control panel) or some other ented correctly. The resistors are installed next, triggering device will be used. The main point to watch here +12V is that the switches are cor­rectly D1 oriented (ie, the flat section on 33k 1N4148 each switch body goes towards the adjacent transistor). If you orient 5 6 the switches incorrectly, the whis100k Q1 IC4c tle will sound permanently when BC548 C 10k B 4 power is applied. 390  15k Finally, install the transistors E (Q1 & Q2), VR1 and the PC-mount FAST S2 screw terminals. The unit should LEVEL CROSSING STEAM now be carefully checked to DETECTOR WHISTLE ensure that all parts are in their Fig.5: this diagram shows how the Level correct locations and that all poCrossing Detector can be used to trigger larised parts are correctly oriented. the Steam Whistle circuit. The output The circuit is designed to be from the Level Crossing Detector simply powered from a regulated +12V takes the place of switch S1 (or switch S2 supply. Our Railpower Walk­ if you want a fast attack time). 58  Silicon Chip around Throttle for Model Railroads (April 1988 and May 1988) and the Infrared Remote Control for Model Railroads (April, May and June 1992) have suit­able supply rails or, as previously mentioned, you can use a 12V DC plugpack supply. To test the unit, set VR1 to mid-position, connect the loudspeaker and apply power. The steam whistle (or diesel horn) should now sound when either S1 or S2 is pressed. Adjust VR1 so that the unit produces the desired volume. A basic installation would simply involve mounting the switches in a convenient position on the main control panel of your layout. Leads could then be run back to the PC board, which could be hidden under the layout along with the loudspeaker. The best place to mount the loudspeaker would probably be near a level crossing or near a station or tunnel. If you do elect to use this approach, make sure that the switch wiring is correct (see previous warning). A more complex arrangement would involve using the Level Crossing Detector (SILICON CHIP, March 1994) to trigger the unit. All you have to do is connect the output from the Level Crossing Detec­ tor across one set of switch terminals – see Fig.5. That way, the steam whistle will automatically sound each time the train goes through the level crossing. The whistle will sound for as long as it takes the train to pass through the section between the detec­tion magnets. A third option is to trigger the steam A Simple Timer Circuit For The Steam Train Whistle +12V This simple interface 10 circuit will enable you to 16VW 10k Q1 10k BC548 trigger the Steam Train 4 8 C OUTPUT TO 7 3 10k B D1 Whistle from either the D1 STEAMWHISTLE TIME 10k 10k 10k 10k 1N4148 1N4148 SWITCH ADJUST IC1 Level Crossing Detector TO E VR1 7555 100  LEVEL CROSSING 5 6 or from a separate reed 100k DETECTOR OR .01 REED SWITCH switch, and have it sound 2 1 0.1 for a preset time (adjustable 47 from 0.5 to 5.5 seconds). N B INPUT The circuit is simply a E C S mono­stable which, when VIEWED FROM REED triggered, provides a low BELOW SWITCH STEAM WHISTLE TIMER output signal of between 0.5 seconds and 5.5 secFig.6: the circuit for the Steam Whistle Timer uses monostable IC1 to drive onds, depending on the switching transistor Q1. VR1 adjusts the period. setting of trimpot VR1. This low output can be used to capacitor decouples the supply so that the magnet will close the simulate the closing of a switch. for IC1, while the 0.1µF capacitor contacts. Fig.6 shows the circuit details. at pin 5 decouples the internal IC1 is a 7555 timer which is con66% resistive divider across the PARTS LIST nected as a monostable. Initially, its supply. 1 PC board, code 05207941, pin 2 input is high, the pin 3 outConstruction of the circuit in62 x 39mm put is low and transistor Q1 is off. volves assembling the parts onto 1 7555, LMC555CN, TLC555 When a low going signal is applied a PC board coded 05207941 (62 x CMOS timer (IC1) to the input, pin 2 is pulled low via 39mm) – see Fig.7. Follow the over1 BC548 NPN transistor (Q1) a .01µF capacitor. As a result, pin lay diagram when installing the 1 1N4148, 1N914 diode (D1) 3 now goes high and turns on Q1 parts on the board and make sure 1 100kΩ horizontal trimpot which in turn triggers the Steam that D1, IC1 and the electrolytic 4 10kΩ 0.25W 1% resistors Train Whistle. capacitors are oriented correctly. 1 100Ω 0.25W 1% resistor The pin 3 output remains high The circuit can be tested by ap6 PC stakes or 1 x 4-way & 1 until the 47µF capacitor at pins 6 plying power and shorting the inx 2-way PC-mount screw and 7 charges to 66% of the supply put terminals to trigger IC1. When terminals voltage. This period is set by the this is done, the steam whistle value of VR1 and its series 10kΩ should sound. Capacitors resistor. In practice, VR1 is adjustThe reed switch can be laid 1 47µF 16VW PC electrolytic ed to set the required duration of inside the track and triggered by 1 10µF 16VW PC electrolytic the whistle. a permanent magnet in a similar 1 0.1µF MKT polyester Power for the circuit is derived manner to the Level Crossing De1 .01µF MKT polyester from the +12V rail used to power tector. Note that the reed switch the Steam Train Whistle. A 10µF will need to be oriented correctly Fig.7: the parts layout for the timer circuit. whistle using the monostable circuit shown in Fig.6. This option allows you to set the duration of the whistle to between 0.5 and 5.5 seconds and Fig.8: the full-size PC board pattern. will give a more realistic effect. Naturally, the loudspeaker should be mounted near to where the train will be when the whistle blows, to ensure maximum realism. If you want the whistle to sound at different locations on the track, just add additional SC circuits. July 1994  59