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Topic: Load Cell Setup and Components  (Read 806 times) previous topic - next topic

Ron_Blain

@Leo
Why would the ADS1115 be a poor choice? Granted not great but why would we say a poor choice?

Ron

Wawa

#16
Jan 22, 2021, 04:13 am Last Edit: Jan 22, 2021, 06:33 am by Wawa
@Leo
Why would the ADS1115 be a poor choice? Granted not great but why would we say a poor choice?

Ron
I added "absolute" and "ratiometric" as hints.

Load cell output voltage depends on two things: load and excitation (supply) voltage.
ADS output only depends on one thing: input voltage.
Therefore you should also measure excitation voltage, and use it's variation to compensate results.

Much easier to use a ratiometric A/D with a ratiometric sensor.
If load cell supply and A/D reference are coming from the same point, then, when supply goes up load cell output increases, but A/D output decreases with the same ratio. Automatic compensation.

The regulated 4.3volt excitation voltage of the HX711 is also internally used as reference for it's A/D.
Leo..


Ron_Blain

Otay and thank you for sharing that.  Not all but most of the load cells I worked with we used 10 volt excitation and absolutely from a stable well regulated supply and even then the cells were calibrated. 

Ron

adam272000

"It has a supply voltage of 5volt" is a wrong assumption.
Load cells don't have a fixed supply voltage. The datasheet only recommends 5volt.
The HX711 works with an excitation (load cell supply) voltage of 4.3volt, which is perfectly fine.
A HX711 board does need a 5volt supply for that,
and if you're going to use a 3.3volt processor (WeMos, ESP32 etc.), then you need a Sparkfun HX711 board with separate 5volt analogue and 3.3volt digital supply connections.

@Ron_Blain
An ADS1115 (absolute A/D) is a poor choice for a load cell (ratiometric sensor).
Leo..
Thanks for your reply and everyone else's , they have been very helpful. 
I have decided on 3 sparkfun HX177 modules for my 3 load cells. However I am not sure on the amps of the load cells are as it's not on the data sheet and I don't have the cells in hand. So I can calculate the power I need. I plan to supply power for 10 hours or less. What is the purpose is a lipo booster ?  Can anyone help explain? 
Additionally the WEMOS D1 mini ESP32 requires power through a micro USB port is there a preferred power source for the WEMOS? As the circuit is to be wireless, a power bank was suggested but is this a good power source ?? Thanks 

Wawa

I am not sure on the amps of the load cells
I wouldn't worry about that. A few mA...

A cellphone powerbank should be fine. Even a small-ish 5A type could keep it running for 24H+.
Leo..

adam272000

I wouldn't worry about that. A few mA...

A cellphone powerbank should be fine. Even a small-ish 5A type could keep it running for 24H+.
Leo..
Great thanks. Just to confirm the power bank powers ony the modules and wemos? 
The load cells still require power right? Hence my original question im struggling to decide how to power the 3 load cells in how to connect them ? 

Ron_Blain

If you go with the HX711 modules they provide excitation as was covered earlier. Load cells draw, as was mentioned, a few mA of excitation current. 

Ron 

adam272000

If you go with the HX711 modules they provide excitation as was covered earlier. Load cells draw, as was mentioned, a few mA of excitation current.

Ron
Cheers, so just to confirm the load cells draw power from the HX177 modules. Hence the power bank connected to the WEMOS provides power through the modules to the load cells. So two sources of power aren't needed just one? 

Wawa

Cheers, so just to confirm the load cells draw power from the HX177 modules. Hence the power bank connected to the WEMOS provides power through the modules to the load cells. So two sources of power aren't needed just one?
Correct.
Just make sure you connect the analogue supply (VCC) of the HX711 to the 5volt pin of the WeMos, and the digital supply (VDD) of the HX711 to the 3.3volt pin of the WeMos.
Leo..

adam272000

Correct.
Just make sure you connect the analogue supply (VCC) of the HX711 to the 5volt pin of the WeMos, and the digital supply (VDD) of the HX711 to the 3.3volt pin of the WeMos.
Leo..
Perfect ! I plan on using my 10000mAh portable  charger which has a 5V, 2.4A output . 

Ron_Blain

Correct.
Just make sure you connect the analogue supply (VCC) of the HX711 to the 5volt pin of the WeMos, and the digital supply (VDD) of the HX711 to the 3.3volt pin of the WeMos.
Leo..
Yes and that is important. Make sure the HX711 modules you get allow Analog and Digital supplies. The early boards (modules) didn't and you needed to cut a run on the board for dual supply use. This was also covered previously in this thread.
Ron

adam272000

Yes and that is important. Make sure the HX711 modules you get allow Analog and Digital supplies. The early boards (modules) didn't and you needed to cut a run on the board for dual supply use. This was also covered previously in this thread.
Ron
I plan to use 3 of the Sparkfun hx177 module: https://coolcomponents.co.uk/products/load-cell-amplifier-hx711 
Which has an ADC, therefore, will suffice?

Ron_Blain

Your next problem with this is the HX711 is an I2C device and it has an address. I have only played around with a single unit. To the best of my knowledge you can't change the address. Unless someone else has some thoughts as how to use 3 of the HX711 modules or knows how to change the address you may want to think about a MUX (Multiplex) scheme, something along these lines

Ron

Wawa

...the HX711 is an I2C device and it has an address.
Wrong. Not an I2C device, no address.

There is a HX711_multi library out there that allows several devices to share the clock pin, so four common digital pins are needed for three devices.
Leo..

Ron_Blain

#29
Jan 25, 2021, 02:08 am Last Edit: Jan 25, 2021, 02:08 am by Ron_Blain
Then he is good to go. My bad as to the I2C address.

Many Thanks for the correction.
Ron

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