How to make a giant Theremin?

An artist friend of mine is making a sculpture out of 12 gauge steel.  It will be about 8 feet tall and 5 feet wide. He asked me how he could have it make sounds when people interact with it.  I immediately thought of making it into a giant Theremin. It would be a simple Theremin with pitch control, but no volume control.  If the dome is insulated from the ground, could the whole sculpture be the antenna?  Can anyone recommend a simple circuit that I could build that would work with this?  It should be able to be connected to a common audio amplifier and speakers.

It would be ideal if the whole sculpture could be an antenna, but if that wouldn't work, it might be possible for some part of the sculpture to be the antenna.

Is there another way to have the sculpture produce sound when people are near it?  I don't think that an optical Theremin would work for this as the people would be interacting with it outdoors day and night in constantly changing lighting conditions.

When I googled to see if I could find any information on making a large Theremin, I found the giant Theremin in Melbourne, Australia, which isn't really a Theremin at all.  That sculpture apparently uses video cameras and computers to detect people and produce sounds.  That is much too complicated and expensive for this project.

Thank you for your ideas and assistance!

Hello Jim,

"Is there another way to have the sculpture produce sound when people are near it?  I don't think that an optical Theremin would work for this as the people would be interacting with it outdoors day and night in constantly changing lighting conditions."

Monterey? we are almost neighbors. Your subject comes up periodically. IMHO your best approach especially with any forward field range will be a an on/off result and not a variable pitch frequency. So you're artistic approach should be to turn something on and off. The most proven method is passive infrared detection and after that is ultrasonic detection.

Look these two methods up in the field of home alarm systems. There may be something new on the market but I am not aware of it, then I am not aware of a lot of things.

Your original approach is the same as making a car sitting on four rubber tires a theremin. There are technical reasons why this could never work effectively.

Good Luck,

Christopher

Hi Jim,

A lot comes down to the size of the structure, and how much you know about electronics.. You are unlikely to find any circuit ready to do the job.. but with a bit of messing about, you may be able to make something cheaply which does the job.

The size (area) of the conductive elements defines the "background" capacitance seen .. one wants to reduce this if at all possible -  avoiding a load of horizontal conductors, particularly close to the ground, will help.. Also, the structure needs to be electrically isolated from ground.

Something like a TS555C RC oscillator would be a good starting point - if one had a couple of nF "background" capacitance, and interactions changed this by +/- 50pF, you would be able to get some variation in oscillator frequency which could be mixed with a fixed frequency from another 555 to produce an audio output..

It wont be playable, one person would probably give only marginal shift in pitch (the more people gathered 'round the structure, the higher the pitch) and you would probably want to link pitch to volume in some way so that there wasnt a continuous irritating whine.. But it should be possible.

Having a single antenna conductor probably wont work - the capacitance from the adjacent  structure will have almost the same effect as if the entire structure was an "antenna".

But it all really comes down to your skills with electronics - it certainly could be done, but will require some calculations to get you in the right "ball park" and then a lot of fiddling about with component values on the actual sculpture.

Email me and I will give you some starter ideas - BUT - ONLY if you understand how a 555 works, what capacitance is, how a theremin basically operates etc, as I cannot spare the time to teach you electronics! ;-)

Fred.

<--- my email is in my avatar.

It is also worth considering the sound produced. Without additional processing the instrument will produce a constant howl of unvarying volume or timbre that randomly slides up and down in pitch. 

Hi Jim,

If theremins did not dictate the limits on the physical distance of their response then I would recommend this proximity method developed by a genius. The volume responds to human approach; which could control the loudness of some inspirational music projected from the Art.  Here is a sample of a volume expression .mp3 antenna controlling sound but only in a 0 to 2' range. I just don't know how this would be possible next to a large metal object. I think I know how those results above were done.

Christopher

IMO, Gordon is right - Even the best tone gets extremely annoying if it continuously slides about when people walk near it or whatever.. You would need a mechanism to mute the sound when no-one is within a certain proximity at least.. Beyond that, there are all sorts of tricks to make things much more interesting..

For example, using 555 timers and logic gates, the audio can be fed into a digital counter (4040) which pulses the output.. At the bass end (furthest from antenna) one gets slow bass bursts but as the frequency increases (closer to antenna) so does the rate - this would be suitable for a dramatic sculpture, as it is not exactly soothing ;-)

A lot more data is needed about the type of sculpture and what it is trying to convey artistically before real "advice" can be given - You asked about using a theremin type scheme.. but this could be for a whale sculpture (in which case pulsing would certainly not be applicable) or it may ba a sculpture of a terminator *in which case it might be! ;-)

All I am prepared to say is that it is doable - I have seen a theremin built into a large mortar launcher (or something similar - I couldnt tell one instrument of murder from another) using its metalwork as an antenna, and even a car could be used as a theremin "antenna" if the person doing the job understands enough about electronics and capacitive sensing.. Not a theremin you are likely to be able to play any tune on, but a "pitch only" "theremin" nonetheless.

Fred.

One more thought - But this needs a reasonable degree of competence if you were to play with it..

If you built a simple RC (TS555C) oscillator based theremin, and connected it to a seperate antenna insulated from the structure - running at about 100kHz (or certainly under 200kHz) - you could then connect a voltage follower with good power output (an IC power amp capable of 100kHz unity gain into a large capacitive load, or a transistor push-pull output stage) to the antenna.. (as in, the high Z input to this buffer / follower / power amplifier is connected to the antenna, and the buffer must be capable of outputting exactly the voltage seen on the antenna)

The output from this power stage would connect to the sculpture, and this SHOULD make the sculpture "dissapear" in terms of its capacitive loading .. You should be able to place the antenna on the sculpture, or even incorporate it in the sculpture, providing it is not obscured by any other metal from the structure

.. as in, the antenna must face the person/s interacting with it - but its position with regard to the other metal would be unimportant as long as its field towards whoever was "playing" it was unobstructed.

The reason I specify RC and TS555C is that with any LC oscillator, voltages on the antenna are likely to be a lot higher and less predicatable - with the 555 you know the antenna voltage will go from 1/3 of the supply to 2/3 the supply - so if you make the supply 12V, there will be a triangle wave on the antenna from 4V to 8V - if you use the same supply for your buffer, you have a 4V headroom either side of the maximum and minimum voltage which needs to be followed, and this does make life a lot easier!

the 4V triangle wave is easy to deal with - its big enough that minor errors (a few 10's of mV) from the follower wont have a significant effect - One could use a CMOS schmidt gate like a 4093, but the antenna waveform is a lot smaller, and follower errors become much more significant.

Fred.

ps - I have not done the above with anything larger than an antenna "shield" so do not know how well it will work - some radiated signal from the sculpture could well couple to the "player" and thereby reduce sensitivity - there would need to be good grounding in the vicinity of the "player" (a grounded metal plate or whatever under where they were standing, fo rexample) - effectively, the player would need better coupling to ground than to the sculpture.

An example of what can be done with an inexpensive pitch only theremin.

Gordon, that is a nice demonstration of what the OP is hoping for, I think. Problem is the bollard is a plastic tube over a pitch antenna. I exchanged idea's with David Young of the Uk on this particular project two years ago. Notice how the sensitivity range away from the bollard is limited. He may have went with the $100 Theremaniac pitch board using the Lev Antenna.

"An artist friend of mine is making a sculpture out of 12 gauge steel.  It will be about 8 feet tall and 5 feet wide." - Jim

Jim is trying to ignite a metal object the size of a car. I also exchanged ideas for last year's Burning Man event in Nevada and they settled for an Optical approach. They wanted to ignite the wood center pole of a large tent.

A big antenna just does not conjure up a big response, it more swallows it.

I enjoy these projects in the theremin community because it forces us to move out of the comfort zone to create and "demonstrate" new ideas.

Christopher

" a sculpture out of 12 gauge steel.  It will be about 8 feet tall and 5 feet wide. " -Jim

In terms of ground coupling, the primary concern is the steel parallel to the ground, 12 guage steel is about 0.1" thick (2.6mm) so if these are steel panels standing vertical, the direct capacitive coupling to ground will not be astronomical.. 8' x 5' does give a massive "antenna" area - and the plate will see a persons entire area as a parallel plate, so one could easily get 50pF change in the capacitive value of the sculpture as a concequence of a person approaching it.

I imagine that the background capacitance will be about 1nF for a structure of this size - so I actually think a direct, simple "make the structure the antenna" approach has a possibility of working.

But, if it doesnt work (if, for example, there is much depth to the sculpture and more metalwork parallel to ground or I have underestimated the bulk capacitance - which is quite likely) then eliminating the capacitance of the sculpture (by driving it with a shielding signal), and fixing one or more antennas to the public -facing side of the sculpture (all antennas connected together) will give as good a response as any RC based theremin can - as in, possibly usable within a range of about 50cm, and detectable to about 70 cm, and perhaps even playable for a couple of octaves 'round the 30cm position.

The shielding scheme can be used with LC oscillators, giving far greater range - but this is really getting into an area where great care needs to be taken - the whole sculpture would need to be driven at the same voltage as seen on the antenna - say 100V RMS, and this would be quite low Z to overcome the capacitance - not a simple job!

One thing is certain - with a large "antenna" or when driving a large shield (the sculpture) there is absolutely no place for high frequencies - You need to keep frequencies down as low as you can get away with.. perhaps even as low as 80kHz.. There is no point in putting anything over 200kHz on, it just wont work!

Fred.

the bottom line though is this..

IT CAN BE DONE!

The question is..

Is it worth doing?