Steam & diesel
loco sounds

This circuit can be programmed to produce simple diesel and steam "chuff" sounds for model railways. While the sound quality is comparatively low and no one will ever be able to say, "ah, that’s an Alco", it can nevertheless produce a unique sound in a small and very low-cost package.

The same basic circuit is used for both sound variants; only the PICAXE program is changed to get the diesel or steam sound. Resistor R1 and the setting of the 10kW pot (VR1) are chosen so that the track voltage at the adc1 input (P1) of the PICAXE microcontroller is at 3.3V with maximum loco speed.

A typical value for R1 would be 15kW, with VR1 then adjusted to give the desirable "chuff" rate or diesel sound rate with the maximum track voltage to be used. Zener diode ZD1 protects the PICAXE input from inadvertent over-voltage, while the 10mF capacitor acts as a filter for pulse-type controllers.