[sdiy] Photoelectric String

[Michael Boracci]

Hey Robotboy,
 I am answering this because I have some experience building a similar
thing. Mine is actually working now. Still have some work to do on the
signal processing, but otherwise it is making a noise.

I used a laminated KOHO hockey stick for mine. It is very rigid in one
direction and great for a simple neck. My unit has two strings. It's also
good for fending off drunken audience members.

I sawed a flat guitar bridge in thirds and it works great . I used guitar
tuning machines at the head end. A piece of brass rod for the nut embedded
in the neck.

Since I am a bassist I have used bass strings, but they are too fat for this
type of thing, so I went to nylon coated beading wire for now, ( nine strand
stainless steel cable )  which is really strong and great on the fingers.
DiAddario ( hope that is spelled right ) string company is very close to my
house so I was thinking of going over there and asking them for a really
long guitar string. : ) You don't have to use metallic strings with photo
pickups either, so you can try Weedwacker string which is fat and light.

Well as far as the photo pickups go you have to be sure that you collimate
the output of the LEDs. Meaning the light should not spread too much or the
bulk of the intensity modulation will take place outside the diameter of the
phototransistor. So you can mount the LED in a tube of some sort to prevent
the beam from spreading too much. I am using flat top LEDs from Radio Shack.
I re-polished the tops so that they were extra flat and they work really
well when placed in a tube. In addition to that you have to offset the
phototransistor slightly from the LED. I am using two phototransistors per
LED. One for the bottom half of the string ( mounted below the string ) and
one for the upper half ( mounted below the string ). This way I get the
variations that occur on the top half and the bottom half of the string
movement. I need to figure out the phase requirements yet, because I know
that there has to be an inversion in there somewhere to get a relevant
signal. The sound is pretty buzzy at this point, but it is working.

LED                                         U
String                                       |
Phototransistors                      HH

The pickup must be placed at the peak of one of the string nodes otherwise
you may end up with little or no signal.  When a string vibrates it divides
itself up into nodal sections which are equal to half of the string length
( also other mathematical relationships to string length, but they are much
smaller and insignificant in this case). So on a guitar there are three
points where the string movement is minimum. The nut, the twelfth fret and
the bridge. Therefore your pickup should be placed at a point one quarter
the distance from end to end for max amplitude. The commercial versions of
this type of thing have these pickups integrated into the bridge. Mine did
not work well that way so I moved it. They may be using a reflected laser or
something which would make sense in that case. Mine is picking up the string
interrupting the LEDs beam directly.

I am working on feeding a portion of the signal back into the stick to
create a drone. This was suggested by another list member . He suggested
using a thingamajig that basically vibrates your car like a subwoofer
without actually being a speaker. I forgot the name exactly. For me that is
too big, so I am trying to figure out what I want to use at this point. It
may end up being a dual coil which effects the string directly  (that was
also suggested), sort of like an Ebow. Or if I go to Weedwacker string then
I will use a motor type device ( Like a solenoid ).

Hope this rambling helps you out.

Michael



  -----Original Message-----
  From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Robotboy8 at aol.com
  Sent: Sunday, November 16, 2003 10:30 PM
  To: synth-diy at dropmix.xs4all.nl
  Subject: [sdiy] Photoelectric String


  I posted about this a long while back, before Ireally got my ideas
together, but now I'm asking again and with someslightly different
questions, as the design has changed a LOT.
  I'm thinking of building a single-string... thing... for very lowtones.
Something of a cross between an additive synthesizer, electricbass, and
string bass. Allow me to explain further before you beginpelting rocks at
me: it'll be a single string (any reccomendations onwhat to use? bass guitar
strings? garage door cable? picture-hangingwire twisted around itself?)
running the length of a piece of wood (4or 5 feet). Towards the bottom it'll
have a set of three infrared LED'sabove and in line with the string. These
can be turned on and offindependently. Beneath the string (and directly
beneath the LED's) willbe a total of 3 photoresistors and 3 phototransistors
(2 components perLED). These are varying a current being passed through them
separatelyand then mixed together before running to the output. The 'volume'
ofeach can be changed by knob. Phototransistors will output square
waveswhile photoresistors will put out slightly more sine-ish waves, andnone
of them will be quite in phase with one another, so it shouldn'tbe just a
mushy square output.
  Okay, so now for the questions.
  1. Am i crazy/stupid for even wasting anyone's time asking this?
  2. Any suggestions on what string type/length to use? (it'll be
tuned/tunable but fretless).
  3. Any general construction tips?
  4. Any clue what the decay time will be like?
  5. Anything I've forgotten?
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