Ultrasonic humidifier as factory smoke generator

I was in a doctor's office when I noticed a small desktop humidifier that is typically used for burning scented oils. I realized that it would make a perfect smoke generator for a smoke stack on a train layout. I found that these units are not very expensive and one need not place any scented oils in them. They produce a small amount of water vapor that is visible but is insufficient to wet anything nearby.

I set about converting one so that I could switch it on and off from the layout software and also install it in such a way that I could easily add (distilled) water as needed. I am pleased with the result:

I bought "InnoGear 2nd Version Aromatherapy Essential Oil Diffuser" from Amazon for $16.

Below I describe the steps I took to modify it and install it in the layout.

Modification of the humidifier

The aim of the modification was to enable it to be switched externally. This whole step is not needed if one is happy to switch it on and off manually, but since I control everything on my layout from my software I wanted to be able to switch it on and off digitally also.

Note that, of course, modifying the product will surely void your warranty.

The modification took about 30 minutes.

The first step was to check that the unit was working, so I added some water and switched it on. It produced water vapor.

It has two mist modes. The first press of the 'mist' button turns on mode #1 which allows it to run for 30 seconds and then switches off for 30 seconds, then back one again...  The second actuation of the switch sets it to mode #2 which makes it run continuously.

I poured the water out, disconnected the 24 V DC power supply,  and turned it over and unscrewed the three screws that hold the base on.

Once those were out, I used a small screwdriver to disconnect the three cables inside.

I could then lift the ring out of the base

Which releases the little switch panel.

I soldered two wires to the solder pads on the left side of the rightmost switch

I tied a restraining knot in the two wires and then threaded them through an existing hole in the base
and reassembled the parts. It is easiest to hold the switch circuit board in its holders on the ring and then set the ring into the base, ensuring that the two plastic buttons are fully extended. This allows the switches to slide in perfectly behind the buttons.
I reconnected the three cables (the plugs are labelled) and tucked them all back in...

and screwed the base back on after aligning the holes with the posts in the base.

Since I planned on connecting the two wires to a digital relay switch, I attached two small plugs that fit my system.

To test the modification, I reconnected the power supply, and touched the two plugs together.

Bingo, the light came on indicating the switching was working!

I could then test it with my digital relay, in this case a Viessmann 5213 (= Märklin k84)

Now, it is important to note that the switching needs to be momentary. (Simulating a press of the button.) i.e. when one presses the switch manually it turns a circuit on, and when you let go, it switches that circuit off. I therefore have to do the same when switching it digitally. Switching the relay to 'red' connects the two wires, = depressing the button. Switching to 'green' = disconnects the two wires = releasing the button.  I therefore programmed that address to do the following:

  • Switch to red
  • Delay 1 second
  • Switch to green
  • Delay 1 second
  • Switch to red
  • Delay 1 second
  • Switch to green

This essentially produces two switch impulses, moving the unit from 'off', directly to mode #2 - continuous running.

The unit will switch off if the button is held on for 3 seconds. So, to switch it off I issue the following commands:
  • Switch to red
  • Delay 3 seconds
  • Switch to green
All this switching is encapsulated into a single event that I can toggle via a checkbox or by voice or from a browser.

So, that concluded the modification and testing of the unit, which then led to the second phase of the project, installing it under the layout so that the 'smoke' would come out of a chimney.

Installation on the layout

Around 1977 I assembled a Faller B-265 factory kit. It has a nice chimney and was designed to take an oil burning smoke unit. I had in 1989 decided to burn incense in a metal tin below the layout to generate smoke and had cut an additional hole below the chimney to facilitate that. (The incense worked well but irritated some viewers.) It was thus ready for this smoke generator.

I looked about for a suitable diameter adaptor to capture the water vapor from the unit. I found a small plastic spice container and a plastic tube that would fit into the chimney. I drilled a hole in the base of the jar and glued the tube into it.

I then cut the bottom of the spice jar off

and did a test fitting to the chimney.

There is a brushless fan in the base of the unit that forces the water vapor out of the unit, and it is sufficient to push it up the chimney.

Here is some detail showing the tube coming up through the chimney base.

The next task was to position the factory on the layout in a spot where there was a good space below for the humidifier. I found a spot that did not get in the way of the tracks below with sufficient space to hold the humidifier and have room for the spice bottle and tube to be lifted to free the unit for refilling. (I thought I was going to have to build a shelf somewhere but I happened to have a perfect spot close to where I planned on having the factory!)

I then cut the tube so that, when lowered onto the unit, it protrudes as high as it can in the tapered chimney.

I mounted the decoder nearby, and routed the DC power cable to a power source, leaving sufficient length in the cables to be able to pull the unit out for refilling.


It produces a very pleasing and realistic looking smoke.


  • A different model that a friend owns has a capacitative switch that uses the charge of the human body to actuate the switching of the unit. This is harder to modify but I had some success by switching the DC power ground to the switch sensor inside. It works most of the time. Using a unit that uses an actual switch is better. 
  • Using a unit that comes on automatically when power is applied would be optimal as one could then simply switch its external power on as needed.
  • Yes, one could add drops of scented oils to the unit too, but of course the oils available are scents such as lavender etc. When I see "coal" or "sulfur" scents available it will be interesting to try.
  • I plan on only using distilled water. This will reduce the chances of any scale building up in the unit over time.
  • The unit will automatically switch off if the water runs out.
  • The top of the unit has a bayonet fastening. I made a mark on the side to allow easy alignment when replacing the top.
  • The original button on the unit is still enabled, allowing manual switching if needed.
  • There is a second button that makes the unit light up in various colors. I don't bother with that.