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Author Topic: Measuring weight the cheap way  (Read 18241 times)
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I'm working on my graduate thesis project and need to find a cheap way to measure the weight of the contents of a plastic bin that is part of my larger project. I need it to measure up to 25 lbs. with a circuit built directly underneath the bin. I looked at a post by killerdark (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1221027834) and tried the exact same thing to no avail.

I'm wondering if anyone might have some suggestions for building my own weight scale or strain gauge of sorts to interface with Arduino (an/or and op amp if needed) without using expensive load cells. I'm not so much concerned about accuracy as I am about cost. Do you think FSR's would to the trick? I've even thought about placing the bin over stretch sensors and reading weight that way...as you can tell, I'm definitely not a pro with anything dealing with electronics, so any expert advice or direction would be greatly appreciated.

Thanks!
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Use a liner pot and some springs to give it mechanical resistance. Liner pots can be expensive but you can also get cheap ones.
this project:-
http://www.create.ucsb.edu/~dano/matrix/
uses lots of them, you could try and search for the source.

This is the lowest cost one I could find at about £15.00 :-
http://uk.farnell.com/bi-technologies-tt-electronics/404r5kl1-0/linear-motion-control/dp/1330291
« Last Edit: January 06, 2009, 04:17:20 am by Grumpy_Mike » Logged

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I have a plan to do something like this using a cheap set of kitchen scales.  With a bit of luck (maybe a lot of luck), there'll be a way to interface them to the Arduino.  Otherwise, I'll be making a strain gauge amplifier.  The cheap scales I'm looking at are rated to 5kg.
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Linear pots with springs is an interesting approach. Definitely something I hadn't thought of!!! I'll be researching this more. Aside from my project, there are a lot of other students at my school also looking for options for integrating weight measurements in their projects. I'd love to find some good cheap ways of doing this and then post some good (and simple!) documentation on it since it seems difficult to find...

Thanks again!
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Since there's so much interest in this (see this thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1221027834), I've taken a sequence of photos of the progress so far.  To begin with, the scales:



http://www.flickr.com/photos/anachrocomputer/3176921519/

Remove the oval sticker in the centre of the underside...

http://www.flickr.com/photos/anachrocomputer/3176921525/

....to reveal the two screws that hold the top:

http://www.flickr.com/photos/anachrocomputer/3176921529/

They're tight, so be sure to use the right size screwdriver, and press hard to avoid chewing up the screw heads.

http://www.flickr.com/photos/anachrocomputer/3177778402/

Now remove the other eight small screws around the edge of the lower cover, and flip over to look at the top again:

http://www.flickr.com/photos/anachrocomputer/3177778408/

Lift off the top cover to reveal the beam and the strain gauges:

http://www.flickr.com/photos/anachrocomputer/3177778450/

Finally, the PCB with (presumably) amplifiers and A/D convertors, plus the LCD and controls:

http://www.flickr.com/photos/anachrocomputer/3177778458/

Right, time to get back to that PCB and start figuring out what it does and how we can connect it to the Arduino!
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This site has schematics and board layouts for a project using scavenged load sensors from bathroom scales.
http://www.steeman.be/?p=192

He uses these weighing sensors to measure his rocket motor thrust(!)

The site includes Eagle PCB schematics and board files.

The interesting circuit is here: http://www.steeman.be/lazy-index.php?file=post-images/Load%20cell%20amplifier/SchematicGainSection.gif

It uses a pair of AD627 instrumentation amplifiers to interface the strain gauge output with a microcontroller A/D port.
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Thanks for posting that, lwr20!  I was considering using some of the op-amps in an LM324 as an instrumentation amplifier, but the AD627 looks like it might be simpler.
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So.. Anachrocomputer... How is this project going?
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Aerocon Systems (www.aeroconsystems.com) will sell you at 20 kg load cell for $30. Then you can use a TI INA125A for an amplifier.
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To be honest, I haven't made any progress, partly due to all the stuff I've been involved in with Dorkbot, Tech Adventure and the Maker Faire!  I think the next step will be to make an instrumentation amplifier, either from an LM324 or using an AD620 or similar.
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Anachrocomputer: You might want to at least take a look at the INA125 -- it's got the excitation supply for the load cell built into the chip, which saves a bunch of parts and fiddling. And it costs half what an ad620 does.
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Sorry for bringing this back again, but do you people have any news??

I'm also interested in something similar.

Thanks!
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I've not really made any progress on this idea since posting the photos. I need to purchase an instrumentation amplifier chip (maybe the AD620 or similar) and get it wired to the strain gauge in the scale. The existing amplifier is a chip-on-board "blob" and gives us nothing to go on; a new amplifier front-end is required.
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I have built a circuit using 4 load cells to measure the weight on the wheels of a car,  I use a hydraulic motor to put torque on the engine and then measure the change in weight on the wheels.  It is to help with the setup of racing cars

I use a INA122 amplifier and send the signal to the arduino analog input where I convert the voltage to weight and then export it to excell in graph form.

I have 4 amplifiers one for each loadcell and when I test them seperatly it works perfect but when I connect more than one amplifier the output voltage goes to 5v but the input from the loadcells stay the same.

The amplifiers use the same power and ground.  Does anybody know how to connect more than one amplifier in the same circuit?
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Do you need to keep the weights seperate? You can normally connect the 4 cells into one amplifier.
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