Load Cells ?

[Bluechip]

Has anyone hacked a set of el-cheapo digital bathroom type scales for the load cells ?
 
If so, what make ( preferably UK ).
 
Load cells often operate from 10VDC nominal, so what is the fiddle ( if any ) to get them on 3V from a coin cell which is the usual power source, or are they sourced from the 3VDC ?
 
Dave BC
 
 

[awemawson]

Dave, I once tried and gave up!

I wanted to monitor the weight of a propane cylinder remotely as it was running my (inside) foundry with the cylinder outside. They all seem to work as a bridge. I ended up getting an industrial one complete with a panel mounting display, all mains operated (good old ebay!) but then moved house and never did install it - it's on the list !

The problem that I had was that all bathroom types I played with had an 'auto timeout' programmed internally so the display shut off after about a minute which was no good for my application.

Andrew

[vtsteam]

Similar time out problem with a kitchen type digital scale I have for weighing out materials. It is mains powered but for some reason they retained the time-out feature as if it were battery powered. Very inconvenient as the period is short, and if you are trying to record something, load up gradually, or go get something else to add on the scale (say you're mixing ingredients) it shuts itself off.

If you had done a tare adjustment for the container, turning it back on doesn't help, unless you empty the container out again and re-adjust for it. Total pain in the neck.

I often switch to using my wife's spring scale, which never fails, is easy to adjust for tare with a slight twist of a knob (OMG analog controls -- how outre) and will patiently wait forever for you to do whatever you need to.

Unfortunately I got the new improved modern wonderfully accurate (at least according to the display) digital scale in order to avoid using my wife's kitchen scale. For easily understood reasons. 

[Joules]

VT, the trick here is to have weighed the bowl and have a weight labelled up as the bowl.  Then when the bloody thing quit's on you, you can at least re tare for the bowl/whatever.   The only other solution here, is weigh near the white board and note everything down so you can add it all up later.  They weren't designed by engineers, or they would allow you to record offsets.

         Joules

[awemawson]

If you're not looking for three decimal place accuracy you can make your own load cell. Bond a strain gauge to a beam of your choice with superglue and measure using a bridge circuit. Calibrate with known weights.

Andrew

[Joules]

Or in true Heath Robinson style, a spring balance with cylinder hanging off it.  Fishing line threaded back into the workshop a small pulley on the wall and a lead weight.  Mark on wall in felt pen full and empty positions...      


         Joules   

[philf]

Unfortunately I got the new improved modern wonderfully accurate (at least according to the display) digital scale .....

This is true of many digital calipers - just because they read to 0.01mm doesn't mean they are as accurate or repeatable as that. Many years ago I bought a Mitutoyo caliper and it cost me from memory £90. I had some problems making things a good fit and took it into work to our standards room. I checked the whole 150mm range against grade 1 slip gauges and was appalled at the errors - they were way outside Mitutoyo's spec. (I think as much as 0.07mm - nearly 3 thou!)

Mituyoyo replaced it without question but wouldn't refund my postage - they sent a couple of spare batteries as compensation. The replacement, although better, isn't good enough for a lot of purposes.

Now I only use the digital caliper as a rough and ready check and have a set of micrometers to cover 0-300mm.

We have an old fashioned kitchen balance with cast iron weights. It's probably 50 years old and has never needed a new battery! For heavier items we have a kitchen dial type spring balance and I can weigh up to 2cwt with an ex-butchers spring balance. Not a 'Digit' between them. The only digital scale we have is the bathroom scale which is about as much use as a chocolate fireguard - you get a huge difference by shifting your weight left to right or forwards or backwards - rubbish. That's one reason I don't use it - the other is I'm frightened to see how much weight I've put on!

Going back to the OP of how to get from 3v to 10v - no problem - there are many charge pump ICs designed to do just that sort of job. (With a few extra components.) At work we used a lot of digital(!) countdown timers powered by a single D cell (1.5v) with an audible alarm. Unfortunately the sounders couldn't be heard from any distance so we fitted 12v sounders and a small charge pump circuit to boost the 1.5v to 12v.

We used load cells to measure how much was left in barrels of chemical but I can't remember what voltage they worked on. Three of the load cells were purloined by a maintenance engineer to make a rig for weighing his microlight aircraft.

Phil.

[Bluechip]

Andrew:
 
Wasn't thinking about using the whole contraption. Just rip it apart and hopefully land up with the 'bridge & 4 wires' bits ... the rest being despatched to oblivion.
 
Some where on youtube I found a vid with someone hacking 4 of these things from a bathroom scale, can't damn well find it now .. but they are just the same principal as the big ones.
 
If that was the case then all I'd do was to shove 5V at it and squirt the output into one of the ADxxx Instrumentation Amp chips I have ... err, somewhere ... and thence to a 12-bit ADC in a PIC.
 
Did briefly think of buying one until I saw the prices at Farnell & RS ...     
 

http://uk.farnell.com/tedea-huntleigh/1022m-10m-f-106/load-cell-10kg/dp/7256152?ref=lookahead
 
That idea didn't last long ...
 
Got a 5kg and a 10kg on way from Honkers ...  £13 the pair ... not really what I wanted re: O/P ( 1mV/V rather than 10mV/V ) but if they're n.b.g. I've lost nothing much ...
 
Phil:
 
I wondered if they had a max voltage of 3V ? Not bothered about max volts as I was going to use 5V anyway. Not sure whether 5V would clobber the things ..
 
My scales don't move much if I wobble on them, just about 0.2kg ?? Posh ones from John Lewis ... the previous version was utter crap, tell you anything and refused to work at all on a carpeted floor ... Comet sh .... excreta .... when they went tits up I larfed and larfed and larfed .... serves the sods right ...
 
Anyway, thanks all for the input, I will await gifts from afar ... I hope ...
 
Dave
 
 
 
 
 
 

[Noitoen]

Load cells are passive components usually with 4 wires. The 10V is a standard voltage for industrial amplifiers but is not essential. What matters is the differential voltage increase of the bridge. The effect on using 3V is the resolution of the bridge output. With 3 times less voltage input you will have 3 times less millivolts output per weight unit and have to use higher gain on the amplifier which means more noise. I've used the AD623AN in the past to amplify a Honeywell pressure transducer bridge on a Pic18F452 with great success in the past.

I've seen bathroom scales with a 3 wire sensor on each 4 corners and suspect that they work in pairs to make the full bridge circuit. It should be easy to test.

At some Chinese shops, I've also seen a little digital scale to weigh luggage. Maybe it has a regular 4 wirw bridge sensor you can use.

[vtsteam]

Enjoyed your reply Phil! 

Maybe I can trade my modern high quality energy saving digital scale to my wife for her inaccurate old spring loaded clunker that takes too much room on her counter, is behind the times, and must be an embarrassment for her when company comes over, etc. etc.

[awemawson]

You old smoothie Steve 

[mattinker]

I suspect that will depend upon your "honeydo" account!!

Matthew

[John Hill]

How to remotely weigh propane bottle:--


Put the gas bottle in a water bath and if necessary  attach enough flotation until it floats.  Run a hose from the water bath to the monitoring position and attach to a sight glass on the wall. 

As the gas is used the bottle will float higher and the water level in the sight glass will rise as less water is being displaced.

[RussellT]

the water level in the sight glass will rise

Really?

Neat idea.

Russell