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Topic: Block negative voltages (Read 902 times) previous topic - next topic

tauro0221

Hi,
One of the members asked you why the use of the piezo electric for the input of  the sound? why not use a microphone that you can adjust the output sound? Like the attached link.?  It will make it easy for you to
interface since it is made for Arduino.

http://www.ebay.com/itm/10pcs-Lot-3-3V-5V-Microphone-Sound-Module-for-Arduino/161705019747?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D41376%26meid%3D5a733d3c35744d0781b88ffd5d101843%26pid%3D100033%26rk%3D2%26rkt%3D2%26sd%3D201496655971


 

KenK

#16
May 10, 2017, 04:52 pm Last Edit: May 10, 2017, 04:58 pm by KenK Reason: Add comment
I understand what you are trying to do but you haven't stated how you intend to sense the breathing.  

Are you trying to measure the air movement or are you trying to measure the chest movement?


If you are trying to measure the air flow, I would use a "Hot Wire Anemometer".  


If you are trying to measure the chest movement, I would use a strain gauge system.  Either way you could easily make either system.  

You could go to "Digikey" and look up strain gauge sensors and pick out one or two that would soot your needs.  You would then incorporate the sensors into a strap wrapped around the chest


With respect to a Hot wire anemometer, I would make one using small gauge nichrome wire(26 or 28 ga).  You would have to make four sensors with the same resistance and put them into a "Wheatstone Bridge".  Two of the sensors would be in the air flow and two in the body of the sensor.  The two elements in the air flow would be affected by the moving air and the two in the body of the sensor would account for ambient temperature changes.  By the way, you don't have to heat the wires such that they are hot, just warm (100 degrees F or so).


As many commenters have mentioned,  a piezo sensor is no really the best option.  As far as using a microphone, most microphones are piezo-electric devises.  Carbon mics vent out of favor many years ago.  


With respect to voltage limiting, use a 5.1 volt zerner diode, tied to the Arduino's analog input.  Hook the diodes cathode to the input and the anode to ground. 

The zener diode will clamp the positive voltage to 5.1 volts and -0.6 volts in the negative direction.

diogotec

I am trying to use the chest movement. I am using a piezoeletric sensor as it is the common practice in the biomedical instrumentation area (for example biopac and plux).

diogotec

Can please someone recommend me an high impedance op-amp?
The TLV2771 is perfect but... it is in the 8-SOIC package and I want to implement this in a breadboard..

Wawa

With respect to voltage limiting, use a 5.1 volt zerner diode, tied to the Arduino's analog input.  Hook the diodes cathode to the input and the anode to ground.
Generally bad advice.
A zener leaks, and doesn't protect when the Arduino is off.
(max pin voltage is NOT 5volt, it's VCC +/-0.5volt)
Schottky clamping diodes to VCC and ground would be better.
If... there would be a need for them.

A piezo can generate high voltages with small movements. I doubt amplifying is needed.
High impedance is needed if low frequencies have to be detected.
It was suggested a month ago to connect piezo ground to a mid-voltage point made by two 10k resistors.
Leo..

MarkT

A piezo is basically a voltage source through a series capacitor.
Well no, its a charge-source really - physical distortion directly pushs charge around.

You can view that as a current source with a high-pass characteristic, but its not a voltage source
at all.  The voltage is set by the load resistor.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

polymorph

MarkT, very true. Thank you.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - tinyurl.com/q7uqnvn
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

diogotec

Hi all again,
I've tried to make what u advised me - connecting the piezoeletric ground to the virtual ground (either a 2.5V with 10k resistors, or directly to 5V), but when I watch the signal in the osciloscope, it is at 0V and there is no offeset.
I have connected it the following way: the ground from the sensor goes to the offset voltage, the other sensor cable goes to o-scope and the osciloscope ground it at ground (reference to the 5V).

Can anyone help me?

tauro0221

Hi,
I have another suggestion for you to consider it. The suggestion it is the use of an strip of strain gauge. They can be use as stretch or compression. Attached it is a link explaining it. There are Arduino modules that allow you to read strain gauge.


http://www.ni.com/white-paper/3642/en/ 

diogotec

Hi tauro0221,
I haven't, at all, discarded your suggestion! But first and by now I will not quit of my piezoeletric sensor.
Can please someone help me ofsetting the signal? Thanks

tauro0221

Hi,
Attached it is an information from TI about piezo electric amplifiers that maybe help you in your project.
 

Wawa

I've tried to make what u advised me - connecting the piezoeletric ground to the virtual ground (either a 2.5V with 10k resistors, or directly to 5V), but when I watch the signal in the osciloscope, it is at 0V and there is no offeset.
Because the scope probe is a 1Megohm resistor to ground.

Just look at the A/D values.

You might have to add a very high resistor (>=10Meg) across the piezo, to get an idle value near 512.
Leo..

diogotec

Hi Leo,

Can you please explain me better your suggestion? I didn't get it!
Ps. I was just looking first in the osciloscope because I wanted to test the signal was offset'ed before inserting it in the arduino - the montage I described works at the arduino?

Wawa

A scope has a 1Megohm impedance. A DMM has a 10Megohm impedance.
You have to know your test instruments, because they can influence the circuit you're measuring.

Try to measure a 9volt battery with your DMM trough a 10Megohm resistor.
The 10Megohm resistor and your 10Megohm DMM are a voltage divider.
You most likely will measure 4.5volt.
Leo..

polymorph

Don't connect the piezo ground to the offset voltage! It just acts like a capacitor.

Tell me if you can see this file:
https://easyeda.com/editor#id=27e1f76fedea4cd194453728732a2554
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - tinyurl.com/q7uqnvn
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

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