The working Piezoelectric key.

Here is a diagram of a working piezoelectric key.


The design appears very simple because it is and it took a lot of thought to get it that way. Once you realize that a working keyboard needs at least 88 keys and to be fully Janko compliant it needs 264 keys, (the one I demonstrate on my first page has an intermediate 176 keys), you'll begin to realize the simplicity and economy are the name of the game.

If I count every wire in the design there are 7 parts to this key practically none of which need be machined or looked upon as 'moving parts'.

(Note: Some equipment such as an Oscilloscope and Volt/Ohm Meter  will make your life easier but are not actually needed for constructing a key, more on that later).

The list of parts are:
  1) 1 Piezoelectric strip, ($1.80 at windworld.com)
  2) 2 Mini dominoes, (28 for $2.00 at GameWorld on the Internet)
  3) 1 1.5 x .5 x .125 strip of "Headboard liner cloth purchased from any upholstery store.
  4) 1 bead of Hot Glue for making the beak at the end of the top tile.
  5) 1 Roll of sticky plastic film wrap, (Saran Wrap).
  6) Wire for attaching to the Piezoelectric film's leads.

Making a key.
    Step #1:  Solder a measured length of wire to each of the piezo films leads. (I found out too late that actually checking the polarity of the pulse and soldering the same color wire to the lead that actually produced the positive pulse would have been helpful bu not required.)

    Step #2: Wrap the piezo strip tightly in a small strip of plastic cling wrap. The piezo strip is destroyed by hot glue which tends to pull the conductive ink from the surface of the strip under force.
 
    Step #3: Wrap the cling film wrapped piezo strip in a strip of headboard liner cloth tightly to approximately 1/3 to 1/2 it's length.

   
Step #4: Placed the assembled sensor on the smooth surface of a mini domino and glue it into place. Pay attention to, if at all possible, assure that the leads at the back of the piezo strip are as flush as with the back of the domino due to the fact that they're metal and therefore prone to damage.

Step #5: Using your hot glue gun add a small bead of hot glue to the surface of the second domino approximately 1/8 inch from the leading edge. The tiny drop of glue serves as a 'beak' and transmit force to the outer edge of the piezo strip creating a more defined and sensitive response from the strip when struck.

Step #6: The final step is to glue the top and bottom halves of the key together creating your finished key. Alignment is Critical and you may want to spend some time developing various jigs to hold the pieces to together correctly as they're assembled. I found the clamp end of a 12'' crescent wrench to be most helpful.

And there you have it the finished key. In the next segment I begin to discuss the circuitry needed to make these keys sing. For those of you who are Engineers or savvy Hobbyist you need go no further to create your piezoelectric keyboard but if you are like me when I started, and don't know a diode from a hole in the ground the next discussions may be useful.

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.