Theremin 2 – The Opto-Theremin

In my previous post Theremin 1, I described an optical theremin circuit which was ultra-simple – and worked!  However, I described how once I had finished it and played with it for a while, I realised there were some slight problems in being able to play it effectively; and also began to wonder how I could make it sound more interesting and varied.

This post describes what I did to finish it off.

The solution I came up with to make it sound more interesting was based on something I’d seen on a couple of Lunetta-related websites: a 4040 divider outputting several octaves at once, which could be individually selected and mixed together to create the final waveform.

This was the original optical theremin circuit:

Optical Theremin circuit

I reduced the value of the timing capacitor in the original circuit from 22n to 15n to make the basic pitch a little higher and connected the 4040 between the output of the oscillator – pin 2 of the 4049 – and LDR2.  Together with the direct output from the oscillator, I used the first 7 outputs from the 4040, covering 8 octaves overall – a rather excessive range, but the pitch variation obtained using LDR1 was very wide, and I figured the highest and lowest notes might be needed at some point – contributing to the tone, even if not the main pitch.

The 8 different octave outputs were connected together via SPST switches, and various combinations of octaves did produce a surprising variety of tones.

Just one more thing was needed, I thought, to maximise the availability of square waves at 8 octaves, and that was a filter.  I’ve written elsewhere in the blog about Ray Wilson’s simple-but-effective 741-based low-pass filter from the Music From Outer Space website, which I liked so much I built two of them: one of them as a stand-alone unit (Active low-pass filter); and the other one here in the Optical Theremin.  The only difference was that in this case I replaced the cut-off frequency control with another LDR (LDR3).

I discovered afterwards that I hadn’t used the most up-to-date version of the circuit, but adding the extra cut-off frequency fine adjustment and resonance control (the extra features of the revised version) would have been too much.  As it was, I already had 3 LDRs, and only two hands to operate them with.  I added switches to select between the two volume and filter LDRs and two potentiometers, which would enable one of them to remain constant while the other one was manually controlled.

The filter circuit went between the outputs from the 4040 and the input to the Volume LDR (LDR2).  And that was it for the design and construction of the circuit, which now looked like this:

The reason for passing the output of the 4049 oscillator (at pin 2) through two more stages (spare ones in the same 4049) was to give it the same power as the outputs from the 4040.  Using the signal from pin 2 as an output as well as feeding the input of the 4040 seemed to be too much for it, and it wouldn’t work.

Also – as I had done with a couple of recent instruments (for example, the Cracklephone) – I added a pair of banana sockets, so a larger external speaker could be used.  Not shown in the diagram is a switch to cut out the internal speaker when these sockets are in use; and a similar arrangement to the Cracklephone, two 3.5mm sockets where a small goose-neck microphone can be attached.  This is not part of the Theremin circuit, and is the nearest it gets to having a line-out.

This what the inside of the case looked like just before I put it back together.  I don’t recommend you try and stuff so much inside a small case, as I always seem to be doing.

Optotheremin inside IMG_0960 lge

As for the physical construction of the instrument this presented one or two problems.

The one of most interest concerns the LDRs.  Electrically, these seemed to work perfectly; but the problem with them is they’re so small, and it’s very difficult to make subtle changes to the amount of light falling on them with a large human hand.

This is a perennial problem with these small-sized LDRs, and what I decided I needed was something like a torch or spotlight has to widen the spread of light – a dish or reflector, which would effectively increase the area the hand would have to cover in order to restrict the light falling on the LDR.

Having thought of torches and spotlights, I reasoned that I could use MR16-type spotlights, remove the original bulbs, and fit the LDRs inside.  These MR16 (or, with a different fitting, GU10) bulbs can be expensive, but my local Tesco’s was selling a pack of 8 for £1.25, so I bought those, and got to work on them with a hammer.

I’m not saying I did a neat job – and I damaged a few of the 8 in the process! – but I ended up with three reflector housings for my three LDRs.  Knocking off the fittings was easy, but getting the bulb out was not: these things are evidently put together before the glass front is attached, as the bulb is considerably larger than the hole through which the electrical connections pass.  At first I tried knocking the bulb through into the body of the reflector and breaking it up – a procedure not dissimilar from that of the Egyptians, removing internal organs through tiny holes before the process of mummification – but that proved impossible to achieve without breaking the glass front; so in the end I just broke open a hole large enough for the bulb to come out of.

My original intention was to drill holes in the Theremin case large enough to insert the end of the reflectors, and superglue them in place, but in the end the hole would have to have been an inch (25mm) wide, and there wasn’t enough space inside the case to insert something this big.

So instead I mounted the reflectors on the surface.  I built up a sleeve for each one using 25mm inside diameter O-rings.  These were about 3mm deep, so 4 of them superglued one on top of another were enough to support a reflector.  More superglue ensured that the reflectors stayed in position, and that the LDRs, passed through a hole under each reflector,  were in the right place.

Optotheremin front IMG_0973 lge

Optotheremin top IMG_0972 lge

This picture of the back of the instrument shows an arrangement I’ve used in quite a few cases where there wasn’t enough room for the battery inside.  Close to the power socket I’ve stuck a square of velcro, and the battery holder sticks to this.

Optotheremin back IMG_0975 lge

Velcro IMG_0970 lge

Having a battery stuck to the bottom wasn’t  a problem, as the instrument wouldn’t be resting on it: the  1/4″ Whitworth nut, glued over the hole through which the Beatbox tuning pot was accessed, is the attachment for a mini silver tripod which I managed to get hold of for 99p on eBay.  This would allow the theremin to be raised from the table top and set at the best playing angle.

Optotheremin tripod side IMG_0978 lge

I think the tripod gives it a futuristic look – or at least what was considered futuristic in about 1940 . . .

IMG_0979 edited

I adjusted the original tripod between these last two pictures to allow the feet to spread out a bit wider, as it was a little unstable, especially with the battery velcroed to the outside.  This just involved a little sawing and cutting and it now stands much better without being in danger of toppling over.

It sounds like this:

2 Responses to “Theremin 2 – The Opto-Theremin”

  1. February 22, 2022 at 8:28 pm

    Can you help me where to find the wiring diagram for the original beatbox?
    Mine has no voice.

    • March 5, 2022 at 6:38 pm

      I don’t think circuit diagrams for the new stylophones are generally available, Zsolt. I found them for the old ones from the 60’s and 70’s, but relied on circuit-bending practices – trial-and-error – to work on the newer ones.

Post a Comment

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.


June 2012

Enter your email address to follow this blog and receive notifications of new posts by email.

%d bloggers like this: