The Nitty Gritties

A Janko keyboard using piezoelectric keys consists of four basic parts. The most important part are the keys themselves.

The keys are were the Piezo Electric elements come in. The 'Keyboard' part of a piezoelectric keyboard is completely unpowered, i.e. though any amplifier and Mux elements must have some current source fed to them the actual keyboard itself needs no current.

Piezoelectric elements are capable of generating currents of 1.4 nano farads, (tiny), but with voltages theoretically in the range of hundreds of volts, HUGH).

I personally have generated voltages in the range of 25 volts which greatly exceeded my required voltage. The catch is that with that small a current amplification of the signal is required but once again this would be separate from the keyboard it's self.

If you'd like to read about piezoelectric film I would suggest going straight to the horse's mouth at:

http://www.meas-spec.com/product/t_product.aspx?id=2478

Once you have your film the experiment with it a bit to understand the applications that it can be put to. Even without amplification a piezo film attached directly to the oscilloscope lead and  flicked lightly with a finger will produce a pronounced wave form similar to this:]


The above wave form actually would show three rapid flicks of the finger. These voltage spikes would be directly proportional in amplitude to the speed of the finger that flicked the film. This is why it is possible to make 'Velocity Sensitive' keys from a piezo film.

That's the good news, and now the bad news. Though the theory is compelling actually harnessing this action into a workable key has proven very difficult for musical instrument makers. There is also the added problems listed below:

 1) A piezo strip measure the speed of the impact and not the force of the impact. Strike a peizo film with a finger at a set speed and it looks just as if you struck the film with a sledge hammer at that same speed.

2) There is no way to tell if a piezo film once bent is still bent. Theoretically all piezo electric devices present a pulse and a counter-pulse and a release of pressure on the film can be detected a pulse of opposite polarity. In practice this is almost useless since the a slow release may produce such a small counter-pulse as to be almost undetectable from noise in the system. This means essentially no release detection.

3) Adding to the difficulty is the fact that some films send a positive pulse followed a negative and some are the reverse of that.

4) Some piezo strips are more sensitive than others. A measured amount of velocity on one film may product\ a voltage of a third that of another film.

5) The final significant problem is one of how do you control the physical force on the film without complex mechanisms as are used with a physical keyboard, (see the illustration of a standard piano key on my earlier blog page). If you think about it the mechanics of striking a  film are precisely the same as striking a string. To accomplish this task a piano key has at least a dozen major parts, we'd like to avoid that effort.

Now the problems to be solved have been outline I think you'll have an enhanced appreciation of the key design we'll discuss in the next entry.

The raw materials for each key comes to less than $2.00 a key depending on where you buy the piezo sensor film. I've seen the prices range from $6.00 down to $1.56 for the same type of film.