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Topic: 3-Wire Strain Gauge (Read 9705 times) previous topic - next topic



I have decided to make by next project, and need some assistance...  I picked up a cheap digital scale from XScargo ($9.99).  After tearing into it, I discovered 4 strain gauges, one per corner.  During the course of my searches on how to interface them to the UC I could not come up with any explanations on how to wire up a strain gauge that has 3 wires, only 4 wires.  My electronics knowledge is entirely self taught, so there is a VERY large chance that I've just missed something simple.

Any ideas?  (i need roughly 70lbs and accuracy isn't all that vital; its to measure the level of propane in a heater)


Normally a 3 wire sensor is just wired up like a potentiometer, with a center tap and two ends. You wire the ends to a supply + & - and the sensor output is the center tap. You usually put this into one input of a differential amplifier with a simple potential divider into the other input.
Think of it as half a Wheatstone bridge.


Nov 09, 2010, 12:41 am Last Edit: Nov 09, 2010, 12:46 am by nickvd Reason: 1
So then it is possible that my multimeter and (cheap diy scope) are just not sensitive enough to detect the changes... I knew about the amp requirement, but just assumed that I would at least be able to check via multimeter prior to building an amp... (i tested it with both 3 and 5v since the scale is run from a single CR2032).

I will likely have a few chips laying around that could do it... Any ideas which? :D (heh, I have no idea what I have).

That being said, there is a ready made pcb inside that ties all 4 of the gauges to a single 4 pin daughter board (using a single 1x4 header which is soldered onto the main board), so I'm anticipating a very easy hook up :)


After further inspection, it seems as though the daughter board only connects the red wires (red/black/white) to the main board, the black and white wires are tied together in pairs of like colours (as seen below).  As I have said before, I am still pretty much a noob when it comes to electronics, but I cannot see how they would make that work in the scale, where is the power coming from? and if it's the red wire, how are they reading the gauges?

I also tested the multimeter again and did notice a predictable change in resistance when test the black/red wires, the resistance goes from 1001 to 1002 when pressed and 1003 when pressed as hard as my fingers would let me...


Nov 09, 2010, 03:49 am Last Edit: Nov 09, 2010, 03:52 am by nickvd Reason: 1
All the lines trace back to the big black blob :(

the bottom left is the connection point for the daughter board pictured above.

The resistances are as follows

white/red: 1002 ohm
black/red: 1001 ohm
white/black: 2000 ohm


Nov 09, 2010, 04:59 am Last Edit: Nov 09, 2010, 05:16 am by nickvd Reason: 1
from the opposite end of the '100K' marking (edit: heh, just noticed two 100k markings... 1 is on the top)

1 goes through a 33k res then blob-town
2 is ground
3 goes to a 51k res AND to blob-town, other end of res goes to another res/cap  (104 marking, to ground) AND blob-town
4 goes to a cap (marked 104, to ground) AND blob-town

heh... blob-town...

I drew it out to the best of my mspaint ability ;)

You'll want to load the larger version for sure....


Tested the voltage between black and red while applying 3 and 5v across white/black.  Couldnt see a predictable change in reading when using 3 or 5v using my el-cheapo meter ($10), however when i cranked my power supply as low as it could go (read 100mV at red/black) it would change, but only slightly (as expected)... makes me wish i had an analog meter... or a better digi....


As far as the amp is concerned, I came across a similar schematic that used an LM324.  Seeing as I don't have any, I found a couple LM358's  and well... since I know nothing, I am only assuming they will work the same...  Using his schematic, I more or less copied it, leaving out an led or two...  Will this work suitably?


Nov 10, 2010, 03:32 am Last Edit: Nov 10, 2010, 04:32 am by nickvd Reason: 1
Ok, I more or less understand that, (trying to rework it in my head :))... what opamp would you recommend, or are you saying the 358's would do... (5v preferably)..

That being said, I'll be trying the one posted since i have two sitting here, and if i release the smoke then, well, all in good fun I say ;)

edit: yeah.... nuthin... I'm still not entirely sure how to wire in the strain gauge.  I read .5v on the output of the amp circuit i posted, and it jumped to 3.06 after connecting the white/black to the input (black also tied to ground -- afterthought: possibly my mistake) with red going to +5v.  No change in the reading (via output/ground)..

Or am I just way off the mark? :)


Ok, so I've been playing around with it with not much in the way of a result.

I re-soldered the 4 strain gauges back onto the daughter board pictured below:

and checked resistances between the 4 outputs (1-4 from left to right as pictured)

1/2 and 3/4 are both 1500ohm
1/4 and 2/3 are also both 1500ohm
2/4 and 1/3 are both 2000ohm

I read somewhere that the input voltage usually goes to the higher resistance of the pairs, so with +5v running into 1/3 (+/- respectively)
I did measure between 0.2mV with no load and 1.4-1.7mV with about 175lbs on it... I was expecting a bit more of a change...

I wired it into a opamp (lm3900n to be precise), and while it did amplify the voltage, I could no longer detect a change when under load.  Not knowing much about electronics I searched for some schematics and wound up using something similar to this:

Depending on how I re-jiggered things I would read either ~100mV or ~4.5V with no change under load.

Here's the current setup:

I have a bunch of random IC's I picked up for free, I searched through my inventory spreadsheet and came up with the following (searched for description contains 'amp')

LM386      Low Voltage Audio Power Amplifier
TL072CP      Dual Low-Noise JFET-Input General-Purpose Operational Amplifier
LF357N      Series Monolithic JFET Input Operational Amplifiers
LF398N      Sample-and-hold amplifiers
LM358N      Low Power Dual Operational Amplifier
TL441CN      Logarithmic Amplifier 16-PDIP 0 to 70
LF356J      Series Monolithic JFET Input Operational Amplifiers
CA3130E      15MHz, BiMOS Operational Amplifier with MOSFET Input/CMOS Output
NE5534AP      Low-Noise Operational Amplifier
TL072CP      Dual Low-Noise JFET-Input General-Purpose Operational Amplifier
LF411CN      Low Offset, Low Drift JFET Input Operational Amplifier

Any thing here ring a bell?  I can only assuming (with my knowledge) that at least one of them would work for my purposes (trying to run at 5v, but I'll budge if I have to).

Thanks for the help!


Nov 14, 2010, 05:46 am Last Edit: Nov 14, 2010, 05:47 am by nickvd Reason: 1
There are no resistors on the board itself.  Just the solder pads for some surface mounts..

Here's a closeup:


Also, I've switched out the lm3900 for a lm358 with the same (as in none) results (r1/rf = 100/10k [also tried 10k/1m])

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