Sonic Sheep feeling constructive

The Silicon Chip Theremin Build

I have fancied building a theremin for years but, until recently, never quite got determined enough. Then, quite independantly, a friend asked if I could make one for him. This was the excuse I needed and set about figuring out how best to go about this task. Events conspired against me time-wise and I ended up opting for the 'soft option' of using a kit, I managed to find a kit put together by Jaycar of a Silicon Chip magazine design that was well documented and looked reasonable - the only problem being availability, it is a discontinued line. I found one on EBay and set about looking for a better case.

  1- The SC Theremin

  2- The DEC TapeDrive

This case contains a switched-mode power supply, giving +5v and +12v, the theremin kit already has a 6v regulator on the PCB, so it will happily run from 12v. The only problem with this supply is that it runs from front to back of the case, denying me one side for aerial mounting unless I put the aerials at the front of the case where the volume aerial will have to be rather close to the mains switch. Other options considered were to have one aerial on the front panel (which is plastic) and one on the rear - leaving little room for controls; or to put both sockets on the underside

  3- Test fitting

I wanted the aerials to be removable to make storage easier so sockets would be required. PL259 chassis sockets have a 4 bolt mounting flange, ideal for the thin steel case I'm working with. Plugs designed for URM67 co-axial cable have an internal thread of 7/16ths UNC, this just happens to be a reasonable (if slightly odd) thread to cut on 10mm stainless tell tube using a die nut.

In order to get a decent distance between the aerials I thought I would make them with screened 'extender' sections, this would allow the aerials to be about 18 inches apart and make their fixing into the PL239 plugs so much easier.
A short length of 10mm stainless steel tube with a 1mm wall thickness would allow 6mm tube to thread through it with a 1mm radial clearance thus effectively creating a rigid co-axial cable. By binding the 6mm tube with PTFE thread tape I can guarantee its concentricity without compromising insulation. A small amount of epoxy resin secures and seals both ends. A 0.75mm (20AWG) stranded wire easily pushes into the 4mm bore of the 6mm tube, soldering to stainless isn't feasable so I stripped a good amount (maybe 18 inches), folded this bare section back on itself and, having the still sleeved end of this wire sticking out where the socket would be, pulled the stripped wire into the tube and rammed it tight, giving a connection as good as any crimped terminal. The plug could then be screwed onto the 10mm tube and the wire soldered to the plug's centre pin.

  4- Aerial construction

So having all the key bits its time to try it out and see if a metal box is OK (have you noticed how theremins are always in wooden or plastic boxes?) and find out if the aerial construction results in too high a capacitance to earth.

  5- Old and New

NOTE: In the above picture you can see the aerial sockets are wired with cheap hook-up wire and the only control is the standard volume pot, floating on short flying wires, also I am still using the supplied tiny loudspeaker.

  6- Final Wiring

Final wiring complete, co-axial wire for aerial sockets, a better loudspeaker and volume and seperate 'zero' pots for volume and pitch

  7- Testing

Cover refitted and time for a final check, all works well - better than expected for the aerials' experimental construction! The screened section of both aerials are totally blind to hand movement and the exposed sections are far enough apart to prevent cross-interference in use.

A second set of aerials could easily be made with new shapes and rapid interchanging is easy.

  8- Rear View

Navigation

Max Baars excellent article on modifying the SC theremin 

Celia Sheen and her Theremin on BBC Radio3