8 load cells - xh711


I decided to try and make a bed occupancy sensor (which is just a bathroom scale using a bed as the plate really). There's a lot of examples online on how to wire 4 load cells to a hx711, but I could only find 50 Kg cells at a reasonable price and 4x50 is only 200 Kg, which is really not enough for 2 people + a heavy king size bed.

I figured I can just put 2 cell under each leg and bring that up to 400 Kg which is more than enough, my question is about wiring that to the hx711. Wikipedia says you can just put all your load cell in parallel, but most posts about 4 cells don't show that. For example arduino uno - How to get weight data from glass electronic bathroom scale sensors? - Arduino Stack Exchange describes a wheatstone bridge.
What would be the best way to wire them ? I was thinking of maybe doing that whetstone bridge configuration, but with 2 cells in parallel on each of the 4 sides. Would that be valid ?

Also how do I test that ? I tried 4 of them in that configuration just now and my multimeter reading is constantly moving, even though I'm just weighing a big book that isn't moving. I haven't received my header pins yet and nothing is soldered so that could just be my wires moving while I try to measure, but I'm not even sure of the setting to use.

Once that's working I'll move on to programming the cc2530 chip to report the reading over zigbee, which is more up my alley !

Thanks !

I've never used load cells, but.. If all you want is occupancy (do you rent the room by the hour?), then why won't the 50kg cells work?

Actually, I am interested in the project because we want to know when Mom is not in bed when she should be.

Make another search for higher rated load cells. Putting 2 cells under each leg would not work unless You add a kind of balancing arm distributing the load of the leg fairly equal to both sensors.

I don't really mind if the result isn't precise, I just want to be able to detect one or two people in, even if the weight is off by 50 Kg that's fine.
It's just for home automations

If those two cells under one leg does not share the load one sensor will be overloaded and most likely damaged.
Using a balancing arm dividing the load in two equal parts one of them can be replaced with a solud block if any kind! The sensors will measure half the weight. Only multiply the sensor value by 2.

There is indeed a way to use four load cells which works excellently well, but it is not extensible for eight AFAIK. I have mentioned it in another posting here. Arguably you could use two load cells under each leg if they are at each end of a balance arm which does not need to be too precise as one way or another the two load cells will see the total weight. And if absolute precision is not required, a balance arm with equal sides and a dummy block on one side (of the same shape as a load cell).

If you use eight load cells, you simply need two HX711 and add the result in code. That does work. :grinning:

I suppose the hx711 is cheap, I could get another one. And I would then just have two wheatstone bridge, with each one load cell under each leg ?

Wikipedia says this "Load cells can be connected in parallel; in that case, all the corresponding signals are connected together (Ex+ to Ex+, S+ to S+, ...), and the resulting signal is the average of the signals from all the sensing elements. This is often used in e.g. personal scales, or other multipoint weight sensors." so I was just thinking of doing that, putting all the cells in parallel and see if I get something remotely correct.

What I want really is to be able to tell if 0, 1 or 2 people are in the bed, so as long as the resulting measurement is within 80Kg of reality it'll do fine. I'll try with 4 for now, and order another hx711 to try with 4x2 then, thanks !

Your quoting of Wikipedia is the most fantastic argument I've read here.
It handles the electrical output of some suitable sensor, not the mechanical aspect.
If You persist, go ahead. I'm out.

Do you have to use load cells?
I see online that most bed occupancy systems work with a long FSR (Force Sensitive Resistor) under the mattress.

@Railroader: I'm just saying what I saw on wikipedia, not trying to win any argument, I know nothing about any of this or I wouldn't be here asking. If that quote is wrong, or if I'm just misunderstanding fine, I'm just trying to understand.

@Wawa: I haven't tried those because I've seen a few people say that this isn't working great when you move at night, it apparently tends to give a lot of false "out of bed" signals. But that's just what I saw on reddit, I don't really know.
I have everything to make the weight scale so I'm trying that, worse case I'm out a few euros.

I made a medical bed occupancy sensor using an FSR under just one leg and it worked great.

@Wawa: I haven't tried those because I've seen a few people say that this isn't working great when you move at night, it apparently tends to give a lot of false "out of bed" signals. But that's just what I saw on reddit, I don't really know.

Maybe that's why they use an extra long one, to make sure at least some part of the FSR is loaded.

Don't confuse a pressure sensor with a flex sensor.