Hijack signals from scale lcd

[kchriste]

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

 

[dion]

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

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.

 

[dion]

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.

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.

 

[kchriste]

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?

 

[dion]

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.

 

[kchriste]

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.

 

[dion]

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.

 

[kchriste]

±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

 

[dion]

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

 

[dion]

±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

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+?

 

[kchriste]

The reading on the LCD should not change more than 10% when you put the probes on S- and S+. If it changes appreciably more than 10% then the reading on the multimeter becomes proportionally meaningless due to loading.