Musical Instruments

Elastic String Bass

Makej Projects

Elastic String Bass

Written By: Len Keeler

TOOLS:

Drill (1)
Multimeter (1)
or oscilloscope
Pliers (1)
Saw(1)

or file, or Dremel tool
Soldering iron (1)
Wire cutters (1)
Wire strippers (1)

PARTS:

Phototransistor (1)

or RadioShack #276- 145, or you can

also buy with the LED as a matched

pair. RadioShack #276-142.

Resistor (1)

5-minute epoxy (1)

Capacitor (1)

1 per string. + 1

Op-amp chip (1)

LED(1)

Infrared LED (1)

RadioShack #276- 143. You can also buy

a matched pair with the phototransistor.

RadioShack #276-142.

Solid-core insulated wire (1)

Prototyping PC board (1)

DIP socket (1)

Potentiometer (1)

preferably 1M, to allow wider output

range adjustment

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Elastic String Bass

Toggle switch (1)

Female audio output jack (1)

Battery (2)

and battery snaps

Standoffs (2)

with matching screws

Plastic tubing (1)

/ used a short length of the clear tubing

that ICs are shipped in. A wooden dowel

will also do.

Electrical Tape (1)

Elastic bands (1)

preferably the black, fabric-covered

bands from office folders, but plain

rubber bands will work in a pinch

Guitar body (1)

/ used a Coleman Camp Cooker

sandwich press, which is a good size

and has a nice metal container with easy

access to the inside.

SUMMARY

Plug this rubber-band bass into a standard guitar or bass amplifier, and you can play
amazingly low freguencies and cool sounds. Each rubber band sits between a paired infrared
LED and receiver, and as it vibrates, it varies the amount of light detected. Each string's
signal is then amplified and mixed with the signals from other strings.

Rubber bands sound very different from steel or nylon strings. Their tone is rich in
harmonics, and the high freguencies damp out fast. Rubber's high elasticity also means you
can generate unusually low notes out of short lengths of band. Because the amplifier
reguires both positive and negative voltages, I power the guitar using two 9V batteries, which
are switched with a single dual-pole toggle. A red LED indicates when power is on. My
original version had 4 elastic bands, one much longer than the rest. For simplicity, this

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Elastic String Bass

article shows how to build a single-string version, which you can easily extend to
accommodate multiple strings.

Step 1 — Plan the overall layout.

• Figure out how you'll fit the circuit
board, components, and batteries
into your guitar body. My sandwich
maker's interior measured 4"x4"x1
1/4 ", so I had to trim the board a
bit. I used a saw, but you can also
score a line with a file or Dremel
and snap the board along the line.

• You have lots of options for the
guitar body. You could use a toy
guitar, a frying pan, anything that
will hide the electronics and extend
out to stretch the strings. For my
original multi-string bass, I
machined the body out of aluminum
stock. Shape, size, and strength
don't matter much because string
tension is low — a toy plastic
ukulele will generate notes more
typical of an upright bass!

• If you're building your own guitar
body, leave extra room for wires
and components; it's easy to
underestimate.

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Elastic String Bass

Step 2 — Build the circuitry.

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Elastic String Bass

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• The instrument's mixing electronics
are based on a classic op-amp
summing amplifier circuit. An input
capacitor for each string blocks DC
voltages to make sure you're
amplifying only the vibrations (AC).
Then potentiometers adjust each
string's output signal to less than
about 0.5V, for uniformity. The
adjusted outputs are added via a
shared connection and then fed into
an integrated circuit amplifier, a
741 op-amp, which boosts the
combined signal. An output
capacitor blocks any DC signals
from entering your guitar amplifier.

• Power for the op-amp comes from
switched 9V batteries. The op-amp,
capacitors, potentiometers, and
resistors all connect on the circuit
board itself, while the batteries,
switch, LEDs, detector(s), and
audio jack are outboard
components.

• Use solder and hookup wire to
assemble the mixer/amplifier
components on the circuit board,
following the dowloadable
schematics above under Files.
(The schematics show the single-
string instrument in black and
optional strings in red.) Any layout
will work, so long as the
connections are correct; I centered
the op-amp and put the capacitor-
resistor-potentiometer input

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Elastic String Bass

sequence along the left side of the
board, and the output capacitor on
the right.

• The 2 battery snaps connect in
series, with each end connecting
back to the board through one side
of a double-pole toggle switch and
with a ground lead at OV between
the 2 batteries.

• Recalling that I C pins are
numbered counterclockwise from
the dot or notch, connect the op-
amp's pins 4 and 7 to the -9V and
+9V sides of the power,
respectively. Pin 3 connects to the
negative input (ground) and pin 2
connects to the positive (signal).
The op-amp's output, pin 6,
connects through a capacitor to the
tip of the 1/4" audio jack, and the
ring of the jack connects to ground.
Solder more leads from the board
to connect out to the power
indicator LED, the infrared
transmitter(s), and the
phototransistor(s).

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Elastic String Bass

Step 3 — Test the circuitry.

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Elastic String Bass

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• It's prudent to test the circuitry
before assembling it onto the body.
If you're building a multi-string
instrument, test 1
transmitter/receiver pair with a
rubber band before forging ahead
with rest. I spread the circuit out on
a table and taped the transmitter
and receiver down so they sat
slightly above the surface and
pointed directly toward each other
about 1" apart.

• Plug in the batteries, stretch an
elastic band between the emitter
and receiver, and test for output
from the jack using a voltmeter on
an AC setting or an oscilloscope.
The output should jump a few
tenths of a volt when you pluck the
band. If you see signal, you can
hear it by plugging into
headphones, an amp (begin with
the power turned down), or a cheap
set of powered computer speakers.

• If you don't hear an amplified tone
from the string, try turning up your
amp, but if you go past 5,
something else is probably wrong.
Check that you haven't
inadvertently swapped the
transmitter and receiver, which
look similar, or reversed the
polarity of either. Test the detector
by illuminating it with a bright
incandescent source, such as a
60-watt bulb or old-fashioned

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Elastic String Bass

flashlight, instead of the IR
transmitter. Direct the light into the
receiver, and pluck the rubber band
close to and in front of the receiver.
• If you still get no signal, test the
voltage between the
phototransistor's emitter and
ground, across the 1kQ resistor.
If the DC voltage there is zero,
chances are the phototransistor is
backward. The AC voltage at this
point should also increase when
you pluck the rubber band. If the
detector generates signal at the
1 kQ resistor but the amplifier
output still doesn't work, double-
check the connections and solder
joints on the board.

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Elastic String Bass

Step 4 — Assemble the body.

• Once the circuit is working, it's time to mount the components onto the body. First comes
some drilling. Conveniently, my Coleman Camp Cooker was made of soft metal and its 2
halves come apart easily, making it easy to work with. I put holes in the case for the power
switch, power indicator LED, output jack, and a standoff that holds the circuit board away
from the case, to prevent short circuits.

• For the photodetector, I drilled another hole centered on the body, just below the guitar's
neck. At one of the neighboring corners I drilled 2 more holes for the emitter, one for the 1"
standoff and the other for the wires. Then I cut a short length of IC shipping tube to use as
an arm that cantilevers the emitter over the detector.

• I drilled 2 holes in the arm, one to attach to the standoff, and the other to mount the
transmitter. Take care to position the holes such that the transmitter is directly above and
aimed at the receiver. All of the LED-style components — the indicator light, emitter, and
receiver — press-fit easily into holes drilled with a #9 (0.196") bit.

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Elastic String Bass

Step 5 — Add the string, bridge, and tuning head.

• I tied one end of the elastic band to the cooker's hanging hole, opposite the handles. For
the bridge, I used a piece of IC tubing with a notch in it to prevent the elastic from sliding
from side to side.

• To hold the other end of the elastic, I made a movable "tuning head" out of more tubing. I
drilled 2 holes in the plastic for the cooker's handles to pass through, and another hole
higher up to tie the elastic to. This arrangement lets you easily slide the head back and
forth to adjust the tension in the elastic, while the torque against the handle prevents the
head from sliding on its own.

• The guitar is ready to play! If you have multiple strings, the potentiometers let you even
out the volumes (the signal level increases for very low notes), and otherwise protect your
amp if the gain is high. The other thing to play with is the alignment of the emitter and
detector. Rotating the emitter's mounting arm may increase the signal level.

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Elastic String Bass

Step 6 — Play the fantastic elastic.

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• Due to the low tension, you can play incredibly low notes with just a short length of elastic.
It also makes this instrument sensitive; depending on how you pluck or strike the strings,
their tuning might change. Rather than play the elastic bass like a guitar, changing the
notes by fretting against the neck, try tugging on the elastic at the neck, like with a
washtub bass, or squeeze down on it behind the bridge.

• You can also control the tone using your fingernail as a sliding fret, lifting the string just
enough to give it a new vibrating length. Apply just slight pressure.

• Notes also have a different character depending on whether you pluck them hard or soft,
with the "hard" notes containing more high-frequency components. It's easy to make a lot
of cool sounds with this, but challenging to play a song. The best way I found to keep a
consistent tone was to play the multiple-string instrument and gently hammer on its strings
with chopsticks rather than pluck them.

• Finally, remember these are optical pickups, so you can experiment with almost anything!
Plucking the tines of a plastic comb held between the sensors produces a really creepy
sound. Even tapping on the base of a wineglass can be amplified. For my next
experimental instrument, I plan on optically amplifying the motion of glass rod.

• Schematics plus videos and audio recordings:
http://www.makezine.eom/1 7/diymusic_elas...

This project originally appeared in MAKE Magazine Volume 17 .
Related Posts on Make: Online:

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Elastic String Bass

How-To: Two-String Paddle Bass

http://blog.makezine.com/archive/201 0/07. ..

Bass String Winding with Antique Machinery

http://blog.makezine.com/archive/2009/05...

last generated on 2012-11-03 02:10:04 AM.

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