Deciding when the chuff sound should be generated can be
handled by the microprocessor, based on motor speed, once
the user adjusts the chuff rate (via CV). However, this will only
be correct for the single speed where the user calibrated the
decoder to the locomotive.
How correct the chuffs are over a speed range is a big question.
Some locos and decoders play well together and, once the chuff
is adjusted at a medium speed, they will look in sync from a few
scale-MPH until the wheels are turning too fast for the eye to fol-
low. Others are in sync over a very narrow range of speeds.
The one sure way to synchronize the sound is with a switch
that tells the decoder exactly when to make a chuff sound,
based on wheel rotation. These chuff cams are usually very
tricky to install. Most folks who use them do so out of a love of
perfection. The time necessary for an after-market cam installa-
tion makes them very expensive if one is hiring the work done.
All the chuff cam does is tell the microprocessor, “now” when
it needs to initiate a chuff sound. The chuff cam wiring is just
a single wire into the microprocessor block. I left it off figure 3
for clarity.
Keep-Alive Circuitry
With sound decoders came sensitivity to power dropouts on
the track. The loco could be running down the track at 1/3
speed and hit a bit of dirt. If that power dropout was long
enough to reset the microprocessor, then when it woke up, it
thought the loco was standing still and ran through the “fire
up” sequence. Nothing is more concerting than a loco running
down the track but emanating the sound of a diesel motor
cranking over!
