Redesigned by Laurier Gendron (Aug 2006 )

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Train Circuitry Horn circuit

New Design

After many comments by interested hobbyists not being able to obtain parts like the LM566 (VCO) and others I decided to redesign and bench test new circuitry for the Diesel train and the Horn sounds . Without using a computer chip this analog design is still a challenging task and of course it still lacks that realistic sound created by the natural effect elements of reverbaration ,sustain and doppler effects that we love to hear at night.

All these effects could be created by additional circuits which of course would increase the complexity of this project and deter many would be hobbyists .

Diesel and Horn Circuit .

The main power supply to the system must be a regulated 12 volts DC with a minimum input from the train control AC or DC power supply of 13.5 VAC connected to pos 3 and 4 of the rectifier bridge . The ground bus of the regulated 12 volts supply must be connected to the system ground ,
The independent speed reference voltage is taken directly from the train speed control module or can be taken by connecting directly from the tracks to positions 5 and 6 of the rectifier bridge .The output of this bridge will always be a positive speed voltage signal whichever direction the train is going.

The 555 timer is really a poor replacement for the LM566 as a VCO (Voltage controlled oscillator ) although it is linear in function a negative voltage range is needed to activate the timer and produce the RPM to relate to the actual engine speed thus the op-amp is used to invert the track positive voltage .

The system , How it works !

The op-amp used is a FET input type ( TL071) for minimum offset voltage without having to use a zero offset voltage circuit . The offset voltage for the 741 was found to be much too high at 2.3v while the TL071 average offset of 1.15 volts was ideal to maintain an acceptable idling speed at zero volt reference input from the rails .
This voltage is fed through R1 to pin 2 of the TL071 op-amp which is used as an inverter.Its inverted voltage output from pin 6 is connected to pin 5 of the LM555 timer . C3 the 47uF capacitor across the op-amp is used to ramp the rise and fall time of the voltge applied to the 555 ,it can be omitted or its value changed as desired .
R1 and R2 form a voltage divider to set the maximum needed voltage input to obtain the inverted voltage reference range to the timer while R5 set the gain within this reference range . In order to obtain an inverted output the op-amp must powered by a +- supply R3 and R4 are used to split the supply and set a zero common reference point at pin 3 . This allows the op-amp to fully invert its output below zero volts to reflect its positive input range .

Resistors R6 , R7 and capacitor C4 determine the oscillator range output for a given voltage reference at pin 5 of the timer .When the reference voltage at the input of the op-amp is at maximum , the voltage reference voltage at pin 5 of the timer is minimum and as mentioned above the op-amp offset voltage of 1.5 volts is reflected at the input pin 5 ,this voltage is used to produce the idling rpm (minimum )as set by C4 ..
To simulate the diesel sound the 555 IC output pulses at pin 3 are used to trigger the input pin 14 of the 4017 digital counter . This IC is configured after much trials and errors to provide for the best sound ( to my ears ) which is filtered with the tone circuit made of C5,RV1,R6,RV2 and C7 which are all interacting and the result is fed to the LM386 amplifier . C10 is 500uF/16v which I recommend to amplify these large low frequencies rumble to be heard .
The amplifier is programmed for a gain of 200 with C8 ( 10uF ) in series with R10 (500 ohms ) changing the time constant and gain of C8 feed back ,this contributes to further filtering of certain pulses . For best result at least a 5"speaker rated at 5 watts or better in an enclosure should be used sitting on a large wooden surface ( Table) helped produce the diesel familiar rumble sound .

Tuning the Diesel

The output of the 4017 is a complex form of pulses some of wich cannot be heard until RV1,RV2 are set . The choice of C7 value further alter the result .
The PCB provides for the use of trimmer for RV1 and RV2 but by bringing out leads , potentiometers and a multiple position switch can be used . Please take note that RV2 not only set the amplifier gain but will also alter the sounds as well so that many different sound settings can be had. For testing purposes a 10K pot or trimmer can be used to simulate the track voltage reference . See drawing set-up below .


1 - C5,C6 are non polarized capacitors (NP) and my prefered valued for C7 is .47uF .
2- 555 IC is a CMOS type used for minimum current and effeciency
3 - solder the jumper ( red trace) before soldering in the IC socket .
4 - The LM386 is rated at 250mw if operated at maximum gain it will get hot and should be cooled with a small makeshift heatsink attached to itsd top with a dab of cement .
5 - The follwing changes can be made to suit .
(a) Reduce R9 to 270 ohms or any values in between but with close attention to amplifier current limit and IC overheating .
(b) change C4 from .22uF to any values up to .33uF
(c) C7 , any values up to 1uF , keep an eye on amp current .
(d) Do not exceed 250 ma amplifier current . Use heat sink .
6 - C3 value is 47uF

Diesel Sound Samples

For all sound samples ,C4 =.27uF and R10 value is changed as indicated to show the amplifier current used which also influences the diesel sound . A 4"size enclosed speaker rated at 6 watts was used and sound samples were taken with a microphone positioned 6" from the speaker face .

Idle-1 R10 is set at 390 ohms ,maximum current 230ma.

Idle-2 R10 is set at 390 ohms ,maximum current 230ma. RV1 at different setting .

Idle-3 R10 is set at 390 ohms ,maximum current 230ma.RV1 at different setting .

Idle-4 R10 is set at 500 ohms ,maximum current 120ma.RV1 at different setting .

Idle-5 R10 is set at 270 ohms ,maximum current 230ma.RV1 at different setting .

Ramping Ramping up and down and different RPM ,R10 is set at 390 ohms ,maximum current 230ma.C3 =47uF , RV1 at different setting and voltage speed references input.

High Power

For those who would like thunder ,you will find below the schematic for a high power amplifier , required current input is 500ma and a heatsink .

Train Circuitry Horn circuit