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CircuitryThe power supply consists of rectifier bridge RT1,VR,C1,C2 and S1 supplying a regulated 12 volts DC to the system with a minimum input from the train AC power supply of 13.5 VAC connected to pos 4&5 of RT-1The speed signal is taken directly from the train speed control module or can be taken by connecting directly from the tracks to positions 6&7 of the rectifier bridge RT-2 which always provides a positive speed signal whichever direction the train is going. This voltage is fed to the Opto-Isolator IC-1 through R1 which is used to adjust any speed signal to IC-2, IC-2 inverts the signal voltage and its output is fed to a VCO IC-3. (connecting point “A” part 2of 3)
ZD-1,a 4.1v (or 4.3v) zener diode prevents the voltage to IC-2 which is used as a simple inverter from rising above 4.5v. It has been found that full speed is usually attained at about 75% of the speed signal available , R3 is used to maintain a small amount of current into the zener to keep it in the operating mode and limits its current. To simulate the diesel sound IC-3 a Voltage Controlled Oscillator(VCO) is used with digital counter IC-4. IC-3 requires a low voltage for the highest oscillation and a high voltage near the supply rail to give the lowest oscillation
At idle speed ,the signal voltage input to IC-1 is zero but the output of IC-2 is reversed and near the positive voltage supply and the VCO is barely oscillating. A fine idle adjustment is provided by adjusting R9 . R6 is used to adjust the overall gain of IC-2 depending on the speed signal available and the desired max. speed sound desired which is constant in amplitude and applied in part to the counter IC-4 at pin 14 through R11 and also directly to R12 junction through C5. Diodes D-1 to D-7 provide additional pulses different in time through C6 , the two signals are mixed through R12 filtered by C7 for a better sound input into R13 which is the volume control and fed to the input pin 3 (connecting point “B”) of IC-5 amplifier which has a maximum set gain of 200 then to a speaker of your choice having an impedance of 8 to 16 ohms. A small 8 -postion DIP switch is used at the outputs of IC-4 to program the diesel sound to your liking thus simulating a 6,8 or 12 cylinders diesel , its use is optional. The horn signal section makes use of a dual 555 timer IC-6 ,it is used to supply two signals slightly out of phase or frequency. Half of the IC produces a fixed frequency while the other one is adjusted with R18 to closely simulate the horn sound which I admit is not perfect without the use of filters and delay lines etc... both signals are mixed through C-14 fed to IC-7 which is operated in a saturated mode to further enhance its quality , the output provides a very high impedance output to prevent overloading the diesel sound signal when mixed at pin 3 of IC-5. A remote horn push button mounted on the case connect the positive supply line used to activate IC-6 for the horn output.
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