In this picture below, a stepper motor is fitted with a control knob.  When that knob is turned, it generates power which is rectified by the small wooden circuit board with capacitors and diodes on it.  They form two voltage doubler circuits and are connected in parallel in order to power the synthesizer in the back, the Stylophone.  The Stylophone is an inexpensive toyish synthesizer although it has been going strong for many years as a curiosity due to its simple, chromatically-tuned 1.5 octave keyboard which is played by pen.  Normally it's powered by 3x 1.5V AA batteries, but why bother with batteries?  Just put a mini-generator into the instrument and it will be usable like a traditional instrument; you'll be able to take the power for granted.

The point of this page is to convert a Stylophone into an Exertion Instrument - that is, add an electrical generator to it so that playing harder means playing louder.

To start, here is the schematic of a typical Stylophone - there are now several models!

http://www.stephenhobley.com/blog/wp-content/uploads/2010/09/Stylophone2.jpg

Voltage

In a simple test, a single stepper motor was connected with a control knob on its shaft.  Its rectifier was a double rectifier in series, for fastest voltage rise with the least movement.  The capacitors were 4x 2200uF in Series, for a total of 550uF.  Small turns of the motor could easily power the Stylophone, including its built-in speaker. 

At very small turns, an amplifier click could be heard but no sound.  With slightly larger turns, the click was followed by the oscillator's correct pitch.  It was possible to sustain this tone by moving the knob back and forth with a single hand.  This motion would be greatly simplified if the stepper generator were mounted on the body of the instrument.  It was also possible to generate a voltage so high that unsuual effects occurred.  Therefore, a Zener or other overvoltage protector should be used. 

Voltage fluctuations did influence the overall pitch, but only about as much as the vibrato switch.   Still without a regulator, it would be difficult to play without vibrato.

Also, latchup conditions were possible with transient current sources.  Perhaps a regulator would alleviate that?  or perhaps the 100uF capacitor on the input to the Stylophone needs to have a different value?  bigger?  smaller?

Amplifier

For this test, the Stylophone's built-in amplifier chip was used.  This stylophone uses an LM386 op-amp max. 400mW, but many others use single transistor amplifiers, e.g. the schematic above.  This internal amp should be bypassed, as it does not have the current carrying capacitor needed for serious outdoor-playing instruments.  For best amplifier recommendations, the oscillator output waveform voltage/current should be measured. 

For reference, here is the LM386 pinout.

Speaker

The speaker built into the Stylophone is rated for 0.5W. This speaker needs to be upgraded to something more serious for outdoor use.

Acoustics

The body of the stylophone adds a small amount of resonance to its sound.  Its acoustic vents are only visual simulation.

Keyboard

It would be nice to use analog switches to connect its keyboard to CPU control.

The keyboard ranges from A-1 to E-2, has 18 notes, so you need an 18 to 1 mux.

perhaps a mixture of a 16 to 1 and a 4 to 1 would work? 

Main Board