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I am working on a class project that involves a raspberry pi and strain gauge load cells. basic project idea.

The goal is to measure the weight of two or more objects using a set of two load cells (In the picture it uses 4, but for this case, I think two can do the job). As we may know, a load cell needs a hx711 in order to change analog into digital. This picture is using a combinator in order to group the signals of the load cells into one output that goes to the hx711. My problem is that I am not sure how to track two or more different objects' weight only using one hx711. Would this be entirely possible, or I must use a combinator and a hx711 per two load cells? If so, is there another alternative to using the hx711?

Thank you very much for your time and help.

Resources:

  1. https://www.sparkfun.com/products/13878
  2. https://learn.sparkfun.com/tutorials/getting-started-with-load-cells/all
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    \$\begingroup\$ The connections you are showing are for when you want to total the weight on all cells as in a bathroom scale, not read them individually. It looks like the HX711 has a second input channel and on the Sparkfun board that is on pads near the chip, you would probably need software changes to set the mux for that. Beyond two you may need another chip/module, since these tend to be bit-banged putting it on alternate pins should work. \$\endgroup\$ – Chris Stratton May 5 '20 at 16:52
  • \$\begingroup\$ Thank you for your answer Chris. If I would like to measure the weights of two or more objects simultaneously, should I base my idea on a different schematic? The problem I foresee is that I will be using too many HX711, which its number is limited by the number of GPIOs. \$\endgroup\$ – Johan Arcos May 5 '20 at 21:23
  • \$\begingroup\$ The simplest would be to use a de-multiplexer en.wikipedia.org/wiki/Multiplexer. You only need three pins to select one of eight. You can connect it to the select pin, or even multiplex the analog signals if accuracy allows. \$\endgroup\$ – skvery May 6 '20 at 18:08
  • \$\begingroup\$ Thank you skvery. I was doing some research, and you are right, the best option is to use a multiplexer. For anyone with the same issue, I will be using this : sparkfun.com/products/9056 \$\endgroup\$ – Johan Arcos May 7 '20 at 16:01
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This product link use 4 Load sensor (single strain gauge) config as Wheatstone bridge. to measure weight of single object. You should use load cell (4 strain gauge, internal Wheatstone configuration) with load cell amplifier Link one by one. this will more suitable with your requirement.

Note: You always need 4 strain gauge to measure weight of 1 object.

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  • \$\begingroup\$ Thank you, makes more sense. Do you by any chance know if it is possible to measure the weight of more than two objects simultaneously? \$\endgroup\$ – Johan Arcos May 5 '20 at 21:10
  • \$\begingroup\$ How you set up this? \$\endgroup\$ – M lab May 6 '20 at 2:25
  • \$\begingroup\$ Yes, the issue is that to measure the weight of an object you need a HX711, but if I would like to measure the weight of more objects, then I need more HX711s. Obviously, this number is limited by the number of GPIO in the raspberry pi. My questions is if it would be possible to measure the weight of several objects just using one hx711? Thank you M lab, appreciate your help. \$\endgroup\$ – Johan Arcos May 6 '20 at 14:31
  • \$\begingroup\$ it just use 2 pin per loadcell. raspi have plenty of it. \$\endgroup\$ – M lab May 6 '20 at 14:50
  • \$\begingroup\$ I think you can't " weight of several objects just using one hx711" find another ways. \$\endgroup\$ – M lab May 6 '20 at 14:51

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