The idea of a super affordable smart pen was born when I was playing with the earphone volume slider while listening to music. I realized my force was applying to the volume slider and music was changing accordingly. This force (pressure) could be measured by the iPad if:

1. A housing is holding the slider vertically
2. A sliding rod is linking the nib and the little handle on the slider
3. A standard waveform is played instead of music
4. The volume slider modified waveform is sent back to the iPad through the MIC channel, for a comparison

As a proof of concept, the volume slider is the perfect pressure sensor. Despite the Taiwan volume slider factory is so helpful and made me batches of free sample. I have to switch to a Force Sensing Resistor, FSR because volume slider has a tolerance that is too big to be compensated by the driver and the resolution of a volume slider is NOT fine enough to reflect the light stroke performed by an artist.  

Although the made in USA FSR costs 30 times more than the slider, its performance is professional. It’s worth it. 

Volume slider provided by the Taiwan factory

Proof of concept 3D printed housing and drawing