Digital Piano keypad design

[Yamin]

Hi everyone, I was hoping that some one could explain/or point me to some resource which has the explanation on how the keypad of digital pianos work - with all the zener diodes and multiplexing and demultiplexing. I sort of understand how the velocity of the keys are detected. I couldn't find a good schematic either.

Thanks in advance.

[ataradov]

The first article from google http://blog.komar.be/how-to-make-a-keyboard-the-matrix/

Velocity detection in most keyboards works by having two buttons (sets of contacts) for each key and measuring the time between the first and the second button being pressed.

[tooki]

Some expensive digital pianos (like the Yamaha AvantGrand series) use an actual honest-to-goodness piano key mechanism, with hammers and all, except instead of strings, uses fiber-optic photointerrupters (i think it's like 3 or 4 channels per key) to sense the hammer movement. (And then, just as ataradov explained, uses the timing to determine velocity, force, etc.)

[Doctorandus_P]

Try to do some research yourself and then come back with more specific questions.
From the text you're typing we can only guess where the holes in your knowledge are.
If you want to pay someone some serious money to design such a keyboard for you that's probably also possible.

Multiplexed Keyboard matrices work just fine without diodes, but when multiple keys are pressed at the same time a diode on each key is one of the easy ways to make sure the comuter does not get confused by all the "shorts" in the keyboard.
As long as only one key is pressed at any thime or it is acceptable to read faulty keys (or ignore keys) when multiple keys are pressed the diodes are not needed. Neither seems acceptable for a piano.

[Yamin]

Thanks everyone, most of the articles I find online are  about computer keyboard. I know its similar. I have worked on repairing couple of digital pianos (nothing fancy just some cleaning ). All of them had zener diodes connected to the pads and upto three demulitplexer chip like 74Ls138. Trying to figure out how it works.

[GopherT]

I've also seen photo-diode/IR emitter pairs detect movement of a small plastic strip on each key - the plastic strip had very finely printed rows of black lines. The black lines interrupt the IR beam and the rate of the interruptions determine the velocity of the key press.

[SiliconWizard]

The lower-end musical keyboards usually are just based on rubber domes (just like the cheap computer keyboards are).
Most often, there are two domes (switches) per key, with a certain spacing along the key's axis, to detect velocity (but this is really all there is to it, the rest is similar to a computer keyboard). And each key is just using one or two springs for its action. Just like NKRO keyboards, multiplexed keyboards require a diode per key, which is the simplest topology overall. But even on cheap-ass musical keyboards, the diodes are really the least of your concern cost-wise, as most of the cost will come from the mechanical parts.

Good piano-like keyboards are a lot more sophisticated as tooki mentioned. They can also detect other parameters than just velocity.