Some while back as part of the work on the Winchester Railway Modellers Redbridge Wharf layout we decided to add smoke and steam effects to a model of a TID tug.  After a bit of research we harked upon cheapo piezo electric mister units as used in humidifiers.  These can be obtained in various sizes and with various control boards from all the normal sources.

The unit we settled on was a 16mm disc connected to a control board that takes a 5v input.

This was rigged in in an old plastic bottle, fed by a wick and supported by a 3D printed contraption underneath the baseboard.

Fed with 5 volts from the layout and set on it's random setting it gives a quite pleasing effect and certainly catches the attention at exhibitions.

Of course it was never going to stop there!  Would it be possible to do something in a loco?  Others have done it so it had to be done.  I happened to be respraying a model of the A4 Mallard to create a model of Sir Nigel Gresley as running in 1967.  An obvious candidate for running on a model of the ex-LSWR mainline to Weymouth!

Quite a lot of space in the smoke box of an A4 so with a bit 3D printing a mounting and water tank was produced and all I had to do was figure out the electrickery bits.  It's currently very much a prototype but it seems to work...

The loco is fitted with an ESU loksound micro decoder but that isn't man enough to drive the mister circuit which needs 150+ mA.  To get round that I've rigged up a rectifier and voltage regulator to provide enough 5volt oomph.  That feeds the supplied control board but with a slight twist.  

Giles of this parish produced a chuffing narrow gauge radio controlled loco and poked around in the control board mechanism.  He found that in order to get the mister to chuff reliably you need to trickle 2 volts or so to it all the time then when you give it a squirt of 5 volts it responds immediately.  Very very useful info.

In my case I have a solid state relay controlled by Aux 1 on the DCC decoder.  This function output is turned on by function 4 and is defined to trigger on every sound chuff.  The relay effectively switches the power supply to the mister control board from 2 volts to 5 volts.

The problem at the moment is that all the control gear takes up rather a lot of space...

I have my work cutout to miniaturise all this.

Cheers

Dave