Using Low votage audio amplifier with a piezo electric sensor.

[Chorvath]

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!

[Docedison]

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

[Chorvath]

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.

[krazatchu]

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/

[Chorvath]

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)?

[krazatchu]

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...

[Chorvath]

Hmm, thanks for the info and help.

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

[krazatchu]

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...