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[on:]

hi

i explain here in french the whole stuff im doing
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1259955512



i'm triing to build a musical instrument using piezos as knock sensors and arduino bluetooth
it has to be very sensitive as i dont hit piezos directly (but on wood they are stuck onto)



it is working well on duemilanove with threshold = 1 (/1024)
but when trying same code with bluetooth one i have a lot of continuous values from 0 to 8

i tried to plug piezo circuit on mic input to hear the problem and here it is :


piezo hits are lost into a 50hz buzz

on arduino bt only, not duemilanove
same problem occures without anything but a resistor plug in it
(continuous values to

is there a way to fix this problem ?

thx !

[Reply #1 on:]

maybe i should build this


using same 3 batteries

how would i have to plug this to arduino ?
still using 1M[ch937] resistors ?

[Reply #2 on:]

a guy just told me an electret microphone needs power to work
piezo dont

does anyone know a simple way to preamp a piezo ?

please ?

[Reply #3 on:]

a guy just told me an electret microphone needs power to work

Yes but that amplifier is providing power to the mocrophone through the 10K resistor.
use the same circuit with a piezo but don't fit R1.

Anyway are you sure that hum pickup is real? It just looks like a measurement problems or grounding problems rather than anything the BT contributes. You have got a common ground between the ardiono and the BT module?

[Reply #4 on:]

hi

thx for replying !

i did some over tests with my microphone input today and i m not sure anymore this is due to a buzz as i don't see it anymore

anyway

when pluging 1M[ch937] resistor on arduino bt between ground and any analog input, arduino still send continuously values from 1 to 8 for this input

i don't know why but i guess i can't resolve this problem as i'm not into electronical stuffs enough to get into arduino

so i think my only choice if i want to keep it wireless is to preamp piezos (with my 4.5v battery pack)

are you sure about what you said for this preamp sheme
(sorry to ask but i got to be sure as i dont own components yet)
removing the whole r1 part will do the trick ?

[Reply #5 on:]

same problem when plugin directly ground to analog input

[Reply #6 on:]

when pluging 1M[ch937] resistor on arduino bt between ground and any analog input, arduino still send continuously values from 1 to 8 for this input

Yes I wouldn't expect a 1M resistor to ground to have any effect on the input. You could try connecting a 0.1uF capacitor across the resistor to hold the peak charge a bit longer.

are you sure about what you said for this preamp sheme

I am sure it is a preamp schematic, I am not sure it will solve your problem. It is a simple self biasing common emitter amplifier and with no feedback the gain will be the gain of the transistor. The data sheet says the gain should be between 100 and 300 so that is what you will get. Whether this is enough is an other matter.

[Reply #7 on:]

The data sheet says the gain should be between 100 and 300 so that is what you will get

does this mean between 100 and 300 % or 100 and 300x ?

i believe the best would be something like 20x gain

i think i could try something like this

[Reply #8 on:]

No that will not work. You have isolated the transistors base and you will get no biasing current down it.

The gain is X100, so with a reading of 8 you might get a reading of 800.
If you want to cut it down do it on the input of the arduino with a potential divider coming fro the transistor's collector, don't mess with the front end.

[Reply #9 on:]

damn you re right

i mistake this

this might be better then

If you want to cut it down do it on the input of the arduino with a potential divider coming fro the transistor's collector, don't mess with the front end.

sorry i don't know much in electronic this is my first circuit building
do you mean that to get a lower gain i should add a resistor near c2 ?

thx i appreciate your help realy much

[Reply #10 on:]

this might be better then

yep  

do you mean that to get a lower gain i should add a resistor near c2

close ..... let C2 feed into a potential divider, the ratio of the divider will cut down the output of the amplifier.

[Reply #11 on:]

ok i've been looking to potential divider in wikipedia





i think this would divide voltage output by 2
(gain x50 would be ok to me)

do you think i risk to burn arduino input or will it stop clipping at a max value ?

[Reply #12 on:]

i think this would divide voltage output by 2

Yes spot on.  

will it stop clipping at a max value

It can't get get bigger than 5V because that is what the transistor is being fed with. So any larger output will simply just clip at 5V.

Now you will have no trouble with this because of the 100K series resistor, but when you have a capacitor coupled signal with one side of the capacitor going from 0 to 5V the other end of the capacitor can go from 5V to -5V. The 100K resistor will protect the arduino input but you might want to add some catcher diodes to the top and bottom rails, like here:-
http://www.thebox.myzen.co.uk/Tutorial/Protection.html