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Topic: 3-wire strain gauge - HX711 Issue (Read 509 times) previous topic - next topic

zqillini4

Hello,

       I have two, three-wire load sensors (https://cdn.sparkfun.com//assets/parts/4/5/9/5/10245-01.jpg) hooked up in a half wheatstone bridge (I think), with two 1K resistors.  Using a multimeter, I am reading small voltage changes when applying pressure to either strain gauge, when measured on between the pins A+ and A- on the HX711 ADC amp, however my Arduino serial output shows all 0's when using a variety of code examples (for example, this one:  http://www.dfrobot.com/wiki/index.php/Weight_Sensor_Module_V1#Sample_code).

My concern is that my circuit is not setup correctly as I am fairly novice, however everything I have researched about wheatstone bridges, and half wheatstone bridges seems fairly straightforward, and seems to match my circuit.  Any guidance/advice on the attached breadboard circuit diagram would be extremely helpful.

Thanks!!

MarkT

Why not use the sensors as intended, with all 3 wires, so you get maximum sensitivity and
minimal thermal drift?

What 1k resistors are you using? 1% or better would be a start (carbon comp resistors are
not suitable).
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

zqillini4

I'll have to lookup the type of resistors, they came with the arduino starter kit I got.  Regarding the strain gauges, I'm not sure how to wire it up using all three.  From what I could tell the resistance between red/white was 1K, red/black was 1K, and black/white wss 2K.

zqillini4

Any idea how/where to incorporate all three wires?  And does the bridge circuit and inputs to the hx711 look right?  Sorry, just been struggling with this seemingly trivial circuit for waaayyy too long.


Chagrin

One white and one black (from different strain gauges) should be E+ and then the remaining white and black to E-; your two red wires should go to A+ and A-. There's no need for those resistors.


Paul__B

One white and one black (from different strain gauges) should be E+ and then the remaining white and black to E-; your two red wires should go to A+ and A-. There's no need for those resistors.
Exactly right, you must connect it this way - it forms the proper bridge to give compensation and sensitivity (but you can exchange the E+and E- with the A+ and A- for exactly the same effect).  And there is a clever way to connect four sensors as well.

zqillini4

So would it be like this?  If so, I just tried it and still can't get the serial monitor to output anything (all 0's).


Paul__B



Well, it might almost be, except that for some bizarre reason, you seem to have the E+ pin tied to Vcc which would seem to interfere with the regulator on that board.

I think the idea is that use the four pins on that side of the board to connect the bridge with no other connections.  But as best I can see from the https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf,  that is probably not the problem.

Does your sketch match the data pins you have chosen to connect?

RBR

Hi,

I am also having the same issue.but not getting anything from the load cell. Did the issue resolved?

Paul__B

I do believe I covered it adequately.

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