keyboard with pressure sensors

[julestriko]

Hi,

I try to make a keyboard with FSR ( force sensor resistor), it modulate the resistor from 1Mohm between 1 kohm according to the fingers pressure.

the idea is remplace the "velocity" by the "force".

does someone have any idea for coding such function ?

It must work like the mpc pads.

[Just Phil]

are you trying to emulate "Polyphonic Pressure", or "Polyphonic Aftertouch", or just "Aftertouch"?

There is some confusion relating to the term pressure sensitive, with some using it as a synonym for velocity sensitive. To avoid this confusion, pressure sensitivity is sometimes called aftertouch.

Both velocity and (true) pressure sensitivity are supported independently by the MIDI standard.

Many keyboards already have this implemented...

[DragonMaster]

Hi,

Jules has a thread about this "keyboard" 's development in the French forums.

What he needs is polyphonic aftertouch. He's going to use a core with AIN modules to monitor the sensors. So far, he got the sensors to give an input voltage of 0 to 4.5V to the ADCs, but the problem is the software part.

The code would ideally monitor every sensors, send a note on/off signal over/under a certain ADC input value, and output individual aftertouch signals for each notes while they're pressed.

Many keyboards already have this implemented...

The fact is that it's an unusual keyboard design : It's made to be able to play chords with two fingers.

[stryd_one]

What he needs is polyphonic aftertouch.

The fact is that it's an unusual keyboard design : It's made to be able to play chords with two fingers.

Poly aftertouch means a sensor for every single key...could be tricky! Also, with the 1M resistance, if more than one key is used they could draw a lot of current and maybe damage the AINs, and also introduce noise which could make false triggers. I don't know enough about this to offer much more advice I'm afraid :/

I'm not sure how pressure sensitivity allows you to play chords with two fingers?

The code would ideally monitor every sensors, send a note on/off signal over/under a certain ADC input value, and output individual aftertouch signals for each notes while they're pressed.

Sounds like you already answered this question :)

[DragonMaster]

if more than one key is used they could draw a lot of current and maybe damage the AINs,

They could maybe damage the PSU IMO, but not the AIN. An ADC draws a bit of current to measure voltage, it doesn't provide power to the sensors, which are arranged in a voltage divider fashion and pull power directly from the PSU. The hardware side of the project is working, but the remaining part is the software.

I'm not sure how pressure sensitivity allows you to play chords with two fingers?

Pressure is not related to this, but the way the keys/FSRs are placed allow you to do this. What I mean is that he can't just buy a keyboard with polyphonic aftertouch as the key layout isn't the same at all.

(See attachment here : http://www.midibox.org/forum/index.php/topic,10271.msg78167.html#msg78167 for a basic view of the key disposition. )

[Jaicen]

Personally, I can't see how having a large current limiter is going to draw more current!! As the resistance decreases, the device will be able to sink more current, but you can limit that easily with a transistor, and a proper rectifier on the power line will shut down well before you damage anything anyway. If you're getting the right voltage range, with everything appropriately buffered, I see no problem at all.