Chuff triggers and wiring

In sound systems, on steam locomotives, it's nice to have the sounds of the chuffing synchronized with the actual movement of the pistons.

The first "step" is to get the "speed" right. The next refinement is to match the piston position.

Another related issue is the ability of the sound card to handle different chuff speeds (will expand on this)


(discuss how this works, how it senses speed (voltage or bemf like QSI)

Disadvantage is that you cannot time to the actual position of the pistons, only to the right "frequency".

Electrical triggers

The earliest way was mechanical contacts, usually a metal wiper on a drum or on the back of one of the drivers.

A drum was normally placed on an axle that did not have a drive gear, and then contact strips put on it, 2 or 4. A wiper could then make contact for the chuff timing.

Another way is with contacts placed on the back of one driver and one or 2 wipers to make the contact.

These are simple methods, but "electrically noisy" resulting in poor performance. Also the contacts get dirty, and wear and get out of adjustment.

Next was magnets on the wheels with a reed switch.

This was a vast improvement. The reed switch is unaffected by moisture and dirt and does not require adjustment.

This is the method most used today. Unfortunately, reed switches cannot work very fast, so high speeds may not work well.

This also takes pretty consistent spacing between the wheel with the magnets and the reed switch, which can be a problem in a model with a lot of side to side movement in drivers. Often the setup is put on the first or last driver and then that driver is restricted from moving with washers on the axle, leaving the other drivers to conform to curves.


A further refinement was magnets with a hall effect sensor

The usual type is a sensor that reacts to any type of magnetism. This allows a small "disc" type magnet to be used and the hall effect device mounted on the chassis to pick up the magnetic pulses. This was first used by LGB I believe and is very effective. There are no "kits" that I know of to retrofit a locomotive.

Recently Victor Spear mentioned a "latching" type of hall effect sensor on a forum. This particular one is a Melexis US1881. This sensor latches on or off by virtue of the last magnetic field's polarity. Apparently this works very well in harsh or noisy environents.

You do need to present the north and south poles alternately to the sensor, as opposed to how a disc magnet and an "ordinary" hall effect sensor or reed swtich needs only "one polarity".

He has used 1/8" cubic magnets.



Optical circuits have been used too, by Bachmann for example



Places to buy magnets:



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