An audio trigger to CV converter?

Hey guys,
I'm trying to figure out how to make something here...

I have piexo buzzers installed in my marimba keys and currently running into a DIY "megadrum"  that converts the audio to midi.  I want to send that midi into a midi to CV converter to run modular synth stuff.  The big trick is that I need to use my own tuning table because I am using alternate tunings.  At the end of the day, I need to be able to hit a note on my marimba and have it spit out a specific voltage that I have programmed.

I've been thinking that maybe there is an easier way by getting rid of the midi part all together and going straight from the audio signal to the CV...so can anyone recommend how I would specify a voltage spike from one of the piezos going into some chip and then spitting out a predefined voltage? 

I know that what I'm asking for is more complex than what I'm asking on the surface because I need to consider things like playing fast and having the chip clock to send the voltage every time a new note is struck.

maybe arduino would be a quick and easy solution?

If you are running the piezos into the drum machine, I am assuming that  they are putting out logic high.  Would I be correct?

If so, switch an npn transistor with that. Connect the collector to +V. Put a trimpot between the emitter and 0V and take your voltage output through the wiper.  Put a diode in series and trim the voltage output to its respective note. Sum all the outputs to a single node and you are done. Make sure the maximum output is 5VDC. The diodes will prevent interaction among the outputs.

Run that straight into the analogue input of the microcontroller. Program your digital output.

Although you'll have a resolution of 0.019V in 8 bits you'll never have 256 notes. So your analogue output voltages do not need to be in that resolution. You can set them apart wide and ignore the values in between in your program.

Of course this is for playing one note at a time.

I don't know much about logic.  Here is the schematic of the megadrum that the piezos are feeding into:
http://www.megadrum.info/schematics.php

I wish I was better at this to more easily answer your question Sahib.  Thanks for taking the time!

No problem. Logic high means +5VDC. Low means 0VDC.

I checked the schematisc of the megadrum.  It seems the piezos are directly applied to it. Anyhow, we have no business with the megadrum.

Check this link.

http://www.discovercircuits.com/PDF-FILES/PIEZOSWITCH2.pdf

It has two piezo amplifiers. The one on the top sources current. Which means when the piezo is tapped the output becomes +5VDC (or near). This is logic high.

The one at the bottom sinks current,in which case the output is pulled down to 0V. This is logic low.

Great.  Thank you.  I am whipping this up.  I don't have any mosfets here like in the schematic, but I have some 2n5088s.  A couple of questions-can I just substitute the NPN for the mosfet and connect the collector to +5V. For this part "Put a trimpot between the emitter and 0V and take your voltage output through the wiper"  I have some trimpots here-what value should I use?  I've got some 10Ks here.  And  do I hook the base of the NPN to the 0V as well?  I'm asking because at this point I'm just getting around 3.5 V off the wiper without much change, but I may have made a wiring mistake...

Mbira said:

...  I'm asking because at this point I'm just getting around 3.5 V off the wiper without much change, but I may have made a wiring mistake...

Click to expand...

I am assuming this is when you hit the note. However, I am not sure where the wiper is set. If the wiper is at the top extreme or in contact with the emitter you should get the full rail voltage or very near. If you don't then what that means is that the transistor is not being turned on hard. We are using the transistor (or fet) as a switch here. So it has to be turned on hard, meaning fully or at saturation.

Edit: A note on using the transistor as a switch. The load resistor (which is the trimpot) value is the Vcc divided by the collector current or the current that will pass through the transistor. The base current is the collector current divided by the beta of the transistor. All you need is the ohm law to calculate the base resistor value. However, you'll see on the data sheet that the beta varies so you'll experiment to find the optimum resistor value (using a trimpot)

You can not replace the fet with bjt in that circuit. Also piezo is a voltage generator so you need fet.  (If you look at the megadrum schematic, the inputs run into 4051 mutiplexer which is also mosfet.) In the link I gave it says "..when you tap the piezo..." which gives me an indication that the fet is turned on hard, meaning the output (source) is at +5V or very near. So try that circuit first and see if you get +5V on the output when you hit the note.