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How To Build Exertion Instruments

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Saved by Noah Vawter
on April 29, 2010 at 1:02:50 pm

Part 2




     This page explains how to build an Exertion Instrument, by using the reference design, the Electric Eel.  This is the same instrument that was feature on the first big youtube video hit "Ievan's Polka." 




Electric Eels are comprised of the following parts:


The Generator is the most important part of the Exertion Instrument.  For your first design, we recommend using the carriage from a computer printer.  Sliding the print head back and forth backdrives the stepper motor, generating enough current to make an instrument sound!  There are more generator designs on this website.

 This keyboard is a row of 10 switches.  They can be programmed to do anything, but the standard setup is a pentatonic or major scale. 


These switches were chosen because they have a nice springy action to them, but future keyboards may use capacitive sensing (like a theremin!) for even faster action.

The Electric Eels use a chip similar to the Arduino - the Atmega32 microcontroller.  This one runs faster than the Arduino.  It also has more programmable pins for controls and sensors.




The later generations of exertion instruments, such as the Electric Eels and Kick Drum, used both phases of a stepper motor to drive individual voltage doubler circuits. 

These circuits rectify and smooth the generator's energy with diodes and capacitors.  One output supplied the CPU.  The second output supplied the amplifier.  Switching to this configuration resulted in lower necessary RPMs to supply power to the instruments and reduction in gear train noise.  There were quirks with this design, though.  While it works with a large range of component values, there are certain preferred ranges.  Preferred, in this case, because they lead to either louder sound or longer CPU Execution time.





















Keyboard * CPU * Generator Electronics * Speaker * Belt/Gear * Generator








Generator                                  Amplifier Module                 Synthesizer Module                      Speaker





























Chip                              Pros                                             Cons               Power Consumption          Price               Features               Environmental Footprint

Atmega32                    mucho I/O                  lots of pins to solder.

Atmega8                         cheap, low current?

ATTiny                             low current? 

DSPIC 33 MHz               Fast, multiplier.

DSPIC 40 MHz               surface mount version of 33 MHz

PSOC                         Reprog. Analog Sections.              Win. only?                    






What CPUs would you like to use? 


Old sound chip?  If you use an old sound chip, you will need some way to send it register messages.  You may use a second, small chip to do this.


Example sound chips:

MOS 6581 SID, AY-3-8910, Yamaha YM3812




Voice synthesizer? 




Generator Electronics



This circuit conditions the instrument's generator output so it can run the CPU and amplifier.  It is an area of constant technical improvement. 


Early Generator Electronics


In the earliest generations of Exertion Instruments, hand-cranked generators utilized plastic gears to transfer the slow-movement of a handcrank into a fast-spinning DC motor shaft.  Their generator electronics simply regulated the DC output to 5 or less volts in order to protect the CPU.  They used the familiar 7805 regulator.


Later Generator Electronics




Since the generator drives the instrument by sending current through its coils into a capacitor and the CPU, the internal resistance and inductance of the motor should be matched to the rectifier's capacitance.  This results in a charge curve which can be measured on an oscilloscope. 


In general, supplying unnecessarily high voltage to the CPU is not recommended.  If the motor/rectifier combination leads to voltage which is much higher than the CPU's normal operating voltage, try swapping the voltage doubler, for a bridge rectifier with a single output capacitor.


Currently Researched Generator Electronics


Switching generator - At very low thresholds of movement, the instrument should respond, even if is quietly.  This is analogous to a very lightly plucked string.  With a linear power supply, the minimum movement necessary to make sound is limited by the voltage the generator can provide.


Switching generators


Possible Future Generator Electronics


Self-Interrupting FET/transformer combination -  These oscillators work as low as 27mV with common parts like Junction FETs and a transformer.








This speaker was chosen because it was small and cheap.  Since then we've learned that larger speakers are usually more efficient.  This is because larger cone excursions carry the cone beyond its most efficient range of motion.  It's better to have longer magnets than voice coils, which is precisely the architecture of many woofers. 





This makes it so that a side-to-side strumming motion, like writing with crayons, spins a rotating generator.






This generator is a backdriven. two-phase stepper motor.  It provides 0-50V to drive the audio amplifier and synthesizer CPU.

When building exertion instruments, the motor/generator is the source of all electrical energy in the instrument.  Its size is roughly proportional to the amount of energy it can put out.  Bigger motors put out more energy.  Smaller motors are less efficient :(

Motors with neodymium magnets are preferred.  The stronger field results in more wattage and louder volume. 










For selecting/buying/scrounging parts, go to this page.




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