# Anyone made a working scale?



## Shdwdrgn (Dec 23, 2014)

I've been looking at car scales recently (see Jan 2018 issue of MRH) and think the gauntlet style adds an interesting visual aspect, but it would be even more exciting if the scales actually worked!

I've looked at basic strain gauges in the past and decided to get some components ordered to see if I can actually weight my cars. Of course it will take some work to calibrate and then I just need to figure out what one of my loaded cars "should" weight and use that to scale all of the measurements. The longest car I should run is a 50' flatcar so a 7" piece of HO track will accommodate everything but the locos (which are never supposed to be on the scales anyway!). I believe it would be best to use two strain gauges, one on each end of the track, to get a proper measurements.

So has anyone ever worked with these little devices before? It seems like I just need to start with a Wheatstone bridge and add a signal amplifier to bring to voltage up into the range that an arduino can read, and that should be it... right? I've also found some schematics for using an active gauge on each side of the Wheatstone bridge but I can't tell if that adds the gauges together (giving a total weight) or subtracts them (tell you the difference between the two). Any quick hints here?


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## Mark VerMurlen (Aug 15, 2015)

I've not worked with strain gauges, so I did a little Google searching. If I were you, I think I'd follow this example: https://tutorials-raspberrypi.com/digital-raspberry-pi-scale-weight-sensor-hx711/. It sounds like a load cell would be the right thing to use. You could build your own with strain gauges, but a prebuilt load cell is probably much easier and more accurate and won't take a lot of trial and error to get sized correctly. I'd then put the load cell under the center of your floating scale track and base.

Mark


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## Shdwdrgn (Dec 23, 2014)

Hmm interesting, I didn't find anything like that in my searches but it looks good. I found a module of the same type that only measures 1kg (2.2lbs) which should provide more than enough range for HO scale. I think I have a way to make that work with a pair of floating scale tracks so I went ahead and ordered it too. I'm thinking I could solder some rail to some brass flats to make a blade that would go below the depth of the roadbed, which would be held by a bracket that sits on the load cell.

Thanks for the link, that will probably be a lot more accurate!


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## wvgca (Jan 21, 2013)

on the wheatstone bridge, probably about 1k ohm fixed resistors should work, along with a INA125 sensor amplifier will work ...
see that all sideways travel is removed from the car during weighing, as just a little will throw off the weight


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## Shdwdrgn (Dec 23, 2014)

I figured I would also include a trimmer pot across one side to calibrate it for a zero reading. Will have to see what I can do once the parts all arrive though. If nothing else, it will give me new toys to play with.


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## wvgca (Jan 21, 2013)

yes, the third side of the bridge needs a pot to zero it


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