Scale as variable resistor

[Nyklus]

Hello. I am currently disassembling a digital scale so that i may use it as a delay time control.

It has a 6 lb max.

but after taking a part the scale i have a few wires thatd normally go to the circuit board.

but i am not finding a variable resistance with any of the wires hooked up with my multi meter.

does this thing use voltage before weight would change this things resistance?

[PRR]

> a digital scale
> i am not finding a variable resistance

It's in there; but you won't find it with a common ohm meter.

There are TOO DARN MANY stores selling scales. I had to drill 3 pages down in Google to get an explanation essay.

https://en.wikipedia.org/wiki/Weighing_scale#Digital_bathroom_scale

"...weight is measured using a strain gauge, which is a length-sensitive electrical resistance."

So what is that?

https://en.wikipedia.org/wiki/Strain_gauge

"...common type of strain gauge ... a metallic foil pattern. .... the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain..."

Look at the picture there.

The foil is essentially a few inches of wire. At any practical size, it will be under 1 Ohm. Most bench meters do not read sub-ohm well. Further, the change of resistance at maximum load must be a very tiny fraction of the resting resistance ,sub-sub-sub-Ohm. (It must be small because we can't have the scale beam sag several inches, and the change of length of the top of the beam is much less.) {EDIT- see below}

How do they measure that??

https://en.wikipedia.org/wiki/Wheatstone_bridge

Wheatstone bridge balances four resistors so the resting voltage difference is zero. When the variable resistor changes, the "zero" difference becomes a small DC voltage.

Some of the wire-names on that sticker suggest Wheatstone Bridge terminology.

Using very best-case guesses, a 5V excitation voltage and a 1% change of strain gauge resistance will give 0.025V of output.

With modern digital bench-meters, we sure can read 0.025V. However this may be 250 pounds. A 1 pound change of weight is only 0.000,1V. Most meters will not read this with even loose precision. {EDIT- for a 6 pound scale, different weight but about the same voltage.}

What they must have is a HIGH-gain DC amplifier, bringing the small bridge voltage up to the 0.200V range that basic digital volt displays will handle well.

If you can find the split from analog amplification to digital conversion and display, you can maybe work with a not-so-small DC voltage.

Not a resistance.

And these days it is *possible* they just soup-up the digital volt converter to read 0.000,1V changes directly. That used to be unlikely, and prone to bad drift, but the chips keep getting better and a bathroom scale is (should be) "zeroed" and used immediately (no drift over time).

[PRR]

> The foil is essentially a few inches of wire. At any practical size, it will be under 1 Ohm.

Correction. 350 Ohms _IS_ a common value. (Must be very thin foil of some very poor conductor.)

See Vishay CEA-06-125UN-350 (many-many others at DigiKey).

This enormously eases the magnitude of voltage reading.

Still the rated "stretch" is about 0.005 %, the change of resistance less than 2 Ohms.

You just *might* be able to resolve that on a good DMM. That assumes a full-scale weight on the beam, and that the scale designer worked the beam at 0.005% stretch along the strain gauge face.