Pages: [1]   Go Down
Author Topic: Using Low votage audio amplifier with a piezo electric sensor.  (Read 832 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Newbie
*
Karma: 0
Posts: 14
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I am trying to hack a scale so that I can read the data onto my Arduino. There are four piezo sensors, one at each foot, in the scale. Obviously I can't just read straight off of the sensors, so I bought some LM386N 400 mW low Audio Voltage Amplifiers.

I spent some time online looking for simple instructions on using, alas I could not find any.

Thanks in advance to anyone who can help!
Logged

Anaheim CA.
Offline Offline
Faraday Member
**
Karma: 47
Posts: 2928
...
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

LM386's are just audio amplifiers, what you want are low noise, low drift Op-Amps... Something that LM386's could never be. beyond tat I cannot advise you except to say that most scales use "Load Cells" which are resistive sensors... Not piezoelectric elements as Piezoelectric elements are prone to drift and calibration errors.

Doc
Logged

--> WA7EMS <--
“The solution of every problem is another problem.” -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

Offline Offline
Newbie
*
Karma: 0
Posts: 14
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

It seems I was miss informed! I knew about the load cells, I just thought they were a form of piezoelectric sensors.

Any Amplifiers you would suggest? I am going to head over to Radio shack and pick up parts soon.
Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 46
Krazatchu Design Systems
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

An instrumentation amp is a good start, the AD623 comes to mind, unlikely to find that at radio shack...
Also there are complete chips that include AC excitation for the load cell bridge if you want to get fancy...

Here's an example from 2008:
http://krazatchu.ca/2008/12/31/data-logging-weigh-scale/


Logged

Michael
----------------------
http://www.krazatchu.ca

Offline Offline
Newbie
*
Karma: 0
Posts: 14
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Hey thanks, nice project! I am ordering a few of those. Your project is exactly what I am trying to do, aside from sending the info from the computer.

How exactly did you hook up the AD623?
Did you have more then one (for each leg)?
Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 46
Krazatchu Design Systems
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

IIRC, all the legs were in series.
For hookup, you will want to research Wheatstone bridge, a couple of links:

http://www.maxim-ic.com/app-notes/index.mvp/id/3426
http://en.wikipedia.org/wiki/Wheatstone_bridge

The sensors should have four wires, with two active elements and two passive.
Essentially it is two resistive dividers where one is reference and the other changes.

The output of these two dividers is treated as a differential signal going into the AD623...
The instrumentation amp gives an output that is the difference between the two inputs...
Logged

Michael
----------------------
http://www.krazatchu.ca

Offline Offline
Newbie
*
Karma: 0
Posts: 14
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Hmm, thanks for the info and help.

The sensors in my scale have three wires black, red, and white.
Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 46
Krazatchu Design Systems
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Sounds like the load cell is only half a Wheatstone bridge, a resistive divider, you can verify this with a multimeter..
It would be wise to add a divider for reference, that way small fluctuations in supply will be canceled...

Logged

Michael
----------------------
http://www.krazatchu.ca

Pages: [1]   Go Up
Jump to: