This was developed to use the 16 ohm resistor in the stock MTH smoke unit, so I needed to drop the voltage a bit to avoid excessive current. Also, I did a version for diesels that I allowed some configuration so you could use several different smoke units and jumper to suit. Since there is no Super-Chuffer determining if you're moving or stopped, the diesel design monitors the motor voltage to determine motion. In the diesel version below, I used surface mount components to reduce the size a bit and also added a jumper field so you could select the drops or short out one or two of the bridges if you didn't want the drops. For diesels, I typically don't want huge volumes of smoke, so I like to drop it off from max. For idle, I can tune how much more the volume will be reduced. This board is .75" x 1.15", and I fabricate them on .031" board stock to reduce the thickness as much as possible.
Here's what the diesel board looks like, and the schematic is below. Note the red arrow, that's the configuration jumper field.
