I would like to simultaneously log data from 3 of the 50 Kg load sensors. After examining several of the load cell, wheatstone bridge, and scale related SparkFun web pages I believe I need to purchase the following:
It is my understanding that HX711 requires 4 wires while the 50 Kg load sensor only has 3 wires so I am hoping the load sensor combinator can be used with a single 50 Kg load sensor and output the 4 wires needed for use with HX711. I would do this 3 times for the 3 independent measurements I want. I would like help confirming that this is the case before I make the purchase. I am open to other suggestions as well but I am not the best tinkerer so simple solutions are preferred.
I want to log each 50 Kg load cell independently so I do not want to go through a Wheatstone bridge. I am focusing on the 3 wire 50 Kg load cell instead of the 4 wire button load cells or flexiforce products due to higher costs which I would like to minimize. Also, the 4 wire straight bar load cells will not work for me as they are too long. I haven’t examined the microcontroller yet but I assume an Arduino or Raspberry Pi will have sufficient number of pins to log the 3 sensors. The tutorial for HX711 has an image showing 2 digital pins are required for logging from HX711 (CLK and DAT) and I see that Arduino Uno has 8 digital pins so I think the Arduino Uno will work for me but I haven’t fully convinced myself that it will work. It is my understanding that Raspberry Pi has more capability but all Sparkfun tutorials I examined seem to use Arduino so I plan on going down that road. I also found a youtube video by iforce2D for load cells that uses Arduino as well so I assume Arduino is preferred.
I have used a lot of load cells professionally and these three wire ‘half bridge’ cells are the very cheapest of the cheap. The only time I’ve encountered them is in very inexpensive bathroom scales. Sparkfun’s supply appears to be surplus equipment from this same sort of device. I’m sure they work well enough for some kinds of projects but you’ve hit upon some of their limitations.
For a similar profile cell, search ‘planar beam load cell.’
After further review I did note there is a “Support Tips” tab on the following webpage (https://www.sparkfun.com/products/10245). In that there were links to other projects which seem to successfully use 1 or 2 load cells (specifically the ProtoPic demo on Youtube uses 2 of the strain gauges and the Robotistan google translation page shows only one of the load cells in use). The ProtoPic Youtube was demonstrated to work with applied load but the Robotistan page doesn’t have a demo. I am learning more about Wheatstone bridges on the following webpage https://www.allaboutcircuits.com/textbo … in-gauges/ and it does seem to suggest that some resistors will be needed to complete the bridge for use with HX777. The Robotistan page suggests that one of the 50 Kg load cells is actually 2 strain gauges as only 2 additional resistors are used opposed to 3. Ideally the Robotistan page solution with only 1 load cell will work for me since I won’t have to wait for the TI chip to arrive that they use. I suspect the ProtoPic solution is higher quality however but I can’t articulate why.
I obtained all the parts and performed testing but was not able to get the Robotistan approach to work. I also tried flipping wires in case the wiring wasn’t as expected but I still wasn’t able to get proper output. I then tried to see what was going on in the ProtoPic youtube video but the wiring diagram was too complicated to follow as they don’t seem to show where the all the wires from the sensor go (especially the black wires). Any help in getting the sensors to work will be appreciated. The thing I find lacking in info I see online is a lack of specification of where the 3 wires from the load cell go. A detailed wiring diagram would greatly be appreciated. Online resources also seem to suggest that 2 load cells are actually in the device but I can’t say for certain. The ProtoPic video is the only thing I have seen that actually works with applied load and they used 2 load cells instead of 1 so there is conflicting information. I was hoping that the HX711 could be used instead of the TI chip that is used in the ProtoPic video. In summary, can anyone please help with a detailed wiring diagram showing how to connect either 1 or 2 of the load cells with the HX711? Thanks in advance.
Two (3 wire) load sensors can be connected like this diagram:
For a single load sensor, you need two resistors to complete the Wheatstone bridge. R1 and R3 in the picture below should be as close to the resistance you measure between the red and black wire on our 50kg load sensor. If my memory is correct, that should be 1K.
