Very simple pocket theremin

I'm just asking about using of spray, because I'm heard it's very easy to lost tape's magnetic properties.

If you say, that tape is better, I can you it.

The range of sound depends on resistance of the tape, right? (not lenght?)

Gordon posted:

"(Incidentally, the whammy bar is a volume control.)"

If you are referring to the vibrato bar on an electric guitar, I beg to differ. That is a mechanical device for altering the tension, and thus the pitch, of the strings. Push down on it and you drop the pitch of all 6 strings, pull up and you raise the pitch of all 6 strings.

So that device is NOT a volume control.

Tremelo (mislabeled as "vibrato" on many a guitar amp!), on the other hand, IS a sort of volume control. A low frequency oscillator (2-13Hz) varies the gain (usually by means of a light-dependent resistor & LED device) at a slow rate.

You can do true vibrato electronically with digital signal processing. You will see that effect on a lot of guitar pedals these days.

Don

Don - Uh, no - I was referring to the volume control on the theremin cello in the video linked to in the posting preceding mine. As the author, Inq, is a guitarist I figured he would understand what I meant if I referred to it as "the whammy bar." Sorry if that caused confusion.

Pitch bend would be superfluous on a theremin cello or other fingerboard synth, but some sort of volume control is pretty much a necessity.

Inq - yes, kind of. How much current flows through the tape depends on how resistive the material is to current flow, how wide the tape is and how long it is.

I think you will understand these things better if you do some simple experiments. You could readily put a prototype ribbon controller together and use it to vary the current sent to a torch light-bulb, for example, before considering using it to control something more complex. You could try different lengths and widths of tape to see what happens, and similarly with spray.

Sorry. Guess I should have read a bit more closely about the "whammy bar" volume control on the theremin cello.

"theremin cello" is a difficult thing for me to wrap my head around anyway! What would you bowing if the fingerboard is a conductive ribbon?

Don

(maybe) Last question: how do I solder the wire to the tape or spray strap? Won't it melt/burn in contact with soldering tool?

It will melt!

You should solder the wire on an eye and fix it later when it's cold with a screw on the tape.

Don - no bowing involved. "Theremin cello" is a misnomer - it's better referred to by its less common name - "fingerboard theremin" - but we seem to be stuck with "theremin cello" because it bears a passing resemblance to a cello, or possibly because Lev was a cellist.

Here's a clip of Pamelia Kurstin playing one.

http://www.veoh.com/videos/e14328

Thanks for that clip, Gordon! I see why this guy wants to build one!

(NO, NO, not ANOTHER project ... my theremin's still not done!!) LOL

Don

Just looked at that ribbon controller post. They show the "resistive side" (guessing that would be the "dull side") of the tape facing down.

The wires are just wrapped around the nylon screws and are held up against the video tape by nylon nuts. Brass eyelets would be a better choice. Solder the wires around the outer surface of the eyelets (the cylindrical part) before assembling them onto the nylon screws.

The larger diameter eyelet surface will make better contact with the video tape than the raw wire. You should be able to find all this stuff at a Lowes or Home Depot. The "eyelets" will be called "grommets" and will typically be found in the location where they have staples for electric staple guns. These consist of a eyelet with cylindrical part attached and a washer. They are indended to be used to put grommets in fabric. You only need the eyelet with cylinder attached part.

You could use some scraps of un-etched PC boards for the "conductive metallic surface glued to plastic base". You will want to tin-plate them or coat them with a surface of solder because untreated copper will turn green pretty fast and will not make a good conductor at that point.

Next is what to do with this ribbon controller. Since you will end up with something that creates a different voltage for different finger positions, this lends itself to controlling an optocoupler. Vactrols are best ... they are already designed to do what we want. But you could make your own.

The photocells in night lights are fairly well suited to serve as a light-dependent-resistor for audio. They respond best to yellow LEDs, so put the photocell at one end of a cardboard tube or piece of PVC pipe and the yellow LED at the other. Use electrical tape hold them in place and seal off any light leaks. Connect a 100 Ohm resistor between the copper PC board and the anode side (+) of the LED. Connect the cathode (-)side of the LED to ground.

You could use this creation as a volume control by having the photocell in between an audio source (a theremin for example :) ) and the inverting input of an opamp. Connect a 100k Ohm or 1M Ohm resistor between the opamp's inverting input and its output. Connect the opamp's noninverting input to ground. Voila, you have a ribbon volume control.

To use the thing as a theremin-cello, you are going to want the photocell to be part of the frequency-determining network of an opamp oscillator circuit. Go do some searching, you'll find a bunch of opamp oscillator circuits to try.

You could even use one of the CMOS gate "simple theremin" circuits. The photocell would go in place of one of the resistors that set the time constant.

The optoisolator circuit just makes it a bit easier to have a controllable circuit. There's ways you could tie the ribbon controller in directly, but you'll still have to do some design work. The control voltage features on some existing theremins allow the theremin to control something else -- not something else to control the theremin (they are CV OUTPUTs, not inputs).

Sorry for the lengthy post, but I thought this might best answer your questions.

Don