At this point I am more concerned about the method than accuracy or precision. So whatever is going to ease your explanation of converting a analog signal from the load cell to a digital equivalent on my pc which I can display on the screen using any of the RAD IDEs available, would be fine with me.

Thanks again.

 

If by RAD you mean Rapid Application Development, then you'll have to decide which RAD IDE you are going to use first. The only thing like that which I am familiar with is LabView. For LabView you can get analog signal acquisition modules which come with LabView specific drivers. Something more generic like Visual Basic may work also but you'll need something standard, like a USB virtual COM port and microcontroller, to interface to the computer.
By the sounds of things, you need a prebuilt solution, which is very RAD specific.
EDIT: you could search for an analog data logger project to interface with your scale.

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Inside every little problem, is a big problem trying to get out.

 

I am sure thst the displaying of digital values on a pc screen is not too big a deal no matter which RAD is used.

If you have the time, can we divide this task into steps so that I can research to better understand any advice/referrals and direction fom yourself, for eg, what is the next step in reaching the end goal: amplification (how) of the signal from the 4 wires so that a pic can detect the signal? Then write pic(c) software to interpret the signal? Then push the massaged info to pc through an rs232 as numbers?

I hope I make sense.

Thanks.

 

For the next step, amplification, we need to know:
What are the voltages on E+ S+ and S- relative to E- (ground)?
How much does the voltage between S+ and S- change between nothing on the scale and the heaviest object on the scale? You'll need a voltmeter that can read in the millivolts for this.
From this info we can design an amplifier to boost the signal to a 0-5V range suitable for a PIC's ADC input.

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I eventually want to port the whole project back to a pic based system with lcd and keyboard. My understanding of pic programming is not advanced enough yet to do this, but I am working on a few simple tutorials. Electronics is also fairly new to me.

 

I just Googled USB scale. It looks like there is already such a thing available:
NorthShore Care Supply--Pelouze Digital Rate Computing Scales

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I am actually looking forward to learning about how a scale works, and the exitement of getting the envisaged project to work. Although I do realise that time is money, I would greatly appreciate any advice and time.

Dion.

 

I am using a DT830B digital multimeter( I found this out about it - The DT-830B can measure DCV (0,1mV - 1000V), ... ). I moved the selector dial to DCV 2000m, and got the following readings on while touching the the E- wire with one of the probes:-
On zero weight
E+ = 699
S- = 700
S+ = 1492

I got the same results regardless of weight applied. I tried swopping the probes around, but got the same result.

dion.

 

Is the scale still operational? ie: Does the LCD reading change with different weights while the voltages on the E+ E- S+ and S- leads do not?

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Yes, the scale is still operational. But I have noticed that the when one of the probes from the multimeter is touching E- and the other E+, the reading on the LCD does not change, but when it touches either S+ or S-, the value on the LCD does not reflect the weight of the item on the scale.

 

It sounds like the DT830B is loading the circuit too much. I did a quick search for the specs on that meter and it shows an input impedance of 1MΩ on the VOLTS ranges. Usually digital meters have a higher input impedance (10-30MΩ) and don't load the circuit much. Try measuring between S+ and S-. As long as it doesn't create a measurement error of more than 10% on the LCD, the reading you get should be good enough at this point.

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With a weight f 1.499kg on the scale, the reading I get on the LCD(of the scale) is 322g, and the reading on the multimeter is 700 when S+ is connected. This represents an error of > 10%? When I connect S- with the same spec above the reading is 2.6kg. Both seem > 10%. btw what does E&S stand for.

 

±10% of 1.499kg would mean that the LCD should show between 1.349kg to 1.649kg which it obviously does not.

+Excitation
-Excitation
+Signal
-Signal

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Inside every little problem, is a big problem trying to get out.

 

I will try and get another multimeter to do the measurement with.

 

Should the reading on the multimeter be within 10% of the reading on the scale's LCD, or within 10% of the changed reading on the LCD once the multimeter probes(leads) touch S- and S+?