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Elastic String Bass 

Makej Projects 

Elastic String Bass 

Written By: Len Keeler 


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

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


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) 


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. 


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 

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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 

• 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 

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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. 

W m 








• 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: 

© Make Projects 

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Elastic String Bass 

How-To: Two-String Paddle Bass 0/07. .. 

Bass String Winding with Antique Machinery 

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

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