How to measure multiple objects' weights with load cells and Raspberry Pi 4?

Question

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

• Chris Stratton: 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.
• Johan Arcos: 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.
• skvery: 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.
• Johan Arcos: 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

Answer 1

Let's talk mechanics first. The typical scale is usually just a platform with four load cells used as legs in the corners. The cells are summed up electrically to produce reasonably consistent result regardless of where on a platform you place a weight.

But do you really need four load cells? Not really. You can come up with mechanical arrangement where a single cell doing the measurement. For example something like classic balance scale where counterweight is replaced with load cell. This will save you the cost of three load cells and a combinator, with slightly increased mechanical complexity.

However, the above is irrelevant to the question. I just wanted to give some options to the budget-conscious. From the electrical point, four cells combined is the same as just one cell.

Now, a single HX711 has two inputs, so it can be used to weight two objects [almost] simultaneously. There are two problems with this, however.

First, although SparkFun board does have holes for connecting to channel B, it lacks filtering components for it, so it will be a bit noisier. Second, the gain is different for different channels. For channel A it is either 128 or 64, for channel B it is 32. So, you need to adjust the results accordingly.

Nevertheless, you can use one board for two objects right away. If you need more, plan to have one HX711 per two scales. If you run out of RPi pins your next step would be to use multiplexor to connect several amplifiers to same two pins.

Note, that I am talking about multiplexing digital signals from multiple HX711 to RPi pins, not multiplexing differential analog signal from multiple load cells to a single HX711. While that is also possible the presision will most likely suffer.

The selection of the MUX depends on the number of amplifiers you want to connect. For example, the CD74HC4067 mentioned in comments is fine chip. However it is an overkill if you only want to weight just a few objects. And you need two of these MUXes to do a job.

Much better choice is dual MUX, like ADG725, which can switch DAT and SCK wires simultaneously. You can find other dual chips with different number of channels more suitable for your needs.

Answer 2

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.

• Johan Arcos: 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?
• M lab: How you set up this?
• Johan Arcos: 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.
• M lab: it just use 2 pin per loadcell. raspi have plenty of it.
• M lab: I think you can't " weight of several objects just using one hx711" find another ways.