Some ideas that may help in building your own Topsy Organ
Article and organ constructed by Terry Pankhurst
I elected to build a John Smith Topsy Organ because I had gained valuable organ building experience on smaller organs and wanted to progress to bigger things.     The Topsy organ is a MIDI based machine that stores its music on a micro chip, thus saving the need to carry loads of paper roll or book music.    It contains 12V solenoids in the form of pallet valves, a small computer board, and a card reader to carry and read the micro chip music, all of course, powered by a 12V battery.   I used a battery from a mobility scooter.

First, plan what you want your organ to look like.   I wanted to get away from the conventional square box construction and try and add a new dimension by making the ends and the front curved.    Plus having my front pipes following that curve.     I planned the layout of the pipes leaving just enough room to get my American made organ pallet valves into the wind chest.     It’s so easy to build an oversize organ, but remember you still have to lift and move it when it’s finished.   Keep it compact.

Scale plan of the windchest layout
This is the result of the planning.
I left a row of holes along the front for building in a Glockenspiel if I choose to fit one.
In the event they are blocked off and redundant.

Each groove feeds two pipes with air, melody and celeste.
The air flow channels were cut on the surface of the wind chest with the Pipe mounting block glued and fitted above.

Provision is made for the four bass pipes to fit below the wind chest.
The wind chest lid is best sealed with frequently spaced smaller screws.

A further rank of accompaniment pipes were fitted behind the main wind chest.

The back of the accompaniment pipes showing the small wind chest. This was left with a clear plastic cover so the workings could be seen.

Accompaniment wind chest

Bellows are a single “Double acting” type, feeding a spring loaded reservoir.
Always use all the available space to fit your bellows; you can never have too much air.

I make my pipes early on in the construction, giving them plenty of time to shrink and dry out
- so they are less likely to go out of tune when finished.
I like to make my pipes about 10% overlength to start with.
This allows material for mitring the pipes to fit them into the case and some flexibility in tuning. 
But remember to voice your pipes first.

Everything fits together, so it’s ready to put in the case and mitre the pipes to fit.

Carefully and neatly wire up the solenoids to the computer board and complete the electronics to the wiring diagram.

Work out a method of supporting the pipes in the case.
You may want to hold them tight against a frame or have them free standing,
which has its problems if the organ is moved around.

Mitring the pipes to fit in the available space.
A mitred pipe will work just the same as a straight pipe, so long as the joint is sealed properly.

Voicing could be called a black art. 
Voicing is making the pipes sound correctly, 
and must done on the organ with the air pressure decided and set.
Basically the higher the pressure the louder the sound.
Between 3 and 6 inches on a water gauge should be enough.
With the bellows pumping and the reservoir full, move the top lip above the mouth of the pipe up our down in very small increments
until the pipe sounds good and, more importantly, you have a balanced sound with all the other pipes.
Too large a gap and the pipe sounds thin and use a lot of air for very little sound;
too small and the pipe will over blow and screech.
Only when they all sound good, mark the position of the lip and glue it in place carefully.
Only after voicing should you then consider tuning each pipe.

A strong mounting for the crank handle is essential as this could be taking a lot of load when in operation.
Consider what form of bearing you will use.
This organ has two ball races.

I set my handle to come to rest slightly to one side, this made sure it was out of the way of passing people and obstacles.

This is my way of opening and closing the Celeste cut off - a small bellows that acts as an air motor.
The return spring is mounted on the back of a pipe.

You will not have got this far without a lot of thought about the case.
Because John Smith does not stipulate a case design, no two organs look the same.
This is the beauty of his designs; it leaves scope to use your own imagination.
This is what I imagined.

This is me playing the finished organ at St. Albans Organ Theatre, looking, working and sounding well.
(That’s the organ, not me!).

I hope this article will give encouragement to those thinking about building an organ
and hopefully some tips and new ideas.
Terry Pankhurst.