Load Cell Setup and Components

[adam272000]

Hi , I'm looking to set up 3 of the FX1901-0001-0100-L load cell (https://uk.rs-online.com/web/p/strain-gauges/8937398/), It has a supply volatge of 5V and a 20mV Output.

I am stuck on all the exact components I need. My plan is to use an Arduino nano and wirelessly transmit data to my pc. If anyone can help to outline the electrical components needed including the 3 load cells, battery power, modules board, Arduino nano and leads etc.
Thanks

[missdrew]

So you are looking for someone to design the whole circuit?

[adam272000]

No, just some components suggestion specially for power and wireless transmission. I believe I will need a hx711 module to integrate between the load cells and board and for wireless transmission I would need a another board but I'm not sure which one is best?

[missdrew]

Without more details, any suggestions would be guesses at best.

How far do you need to transmit? How long do you need the batteries to last? How often are you polling the load cell?

[adam272000]

My bad I would need to transmit across a maximum of 2.5m. The battery possibly would need to power the 3 load cells so I don't need it to last longer than an 30minutes - 1hr or more to test them. 
They are compressive load cells and the force will be applied while testing would be  roughly every 10 seconds or so. 

[missdrew]

BLuetooth HC series for the transmitter, and a lipo with a 1 Amp boost converter.

[MarkT]

Link in OP is broken - here's the actual datasheet: https://docs.rs-online.com/2859/0900766b8142cdcb.pdf

And indeed the HX711 is a popular choice for digital instrumentation amp for strain guages / load cells

[adam272000]

Link in OP is broken - here's the actual datasheet: https://docs.rs-online.com/2859/0900766b8142cdcb.pdf

And indeed the HX711 is a popular choice for digital instrumentation amp for strain guages / load cells

Great thanks. What would you suggest for power and wireless transmission? I have seen WiFi transmitters over Bluetooth transmitters being used? Is one better than the other ? 

[Ron_Blain]

The sensitivity of the load cell is 20 mV/V so with 5 Volts excitation the full scale output for whatever range you have will be 5 Volts X 20 mV = 100 mV or 0.1 Volt.  Consider that if you go with a HX711 module. 

As to wireless you can consider an ESP 32 module or similar, a Google of wireless communication Arduino should get you several examples of possibilities. You mentioned three load cells so you will need three analog channels keeping things basic. Also consider Bluetooth or WiFi which most connection schemes offer both. 

I would look to power everything using 5 Volts and you want a stable 5 volts. 

There are several ways to go about what you are looking to do. Which method you choose and the choice of hardware pretty much rest with you.

Ron

[adam272000]

I'm struggling to calculate how much power I need!

I will be using 3 Load cells (https://docs.rs-online.com/2859/0900766b8142cdcb.pdf) which has a 5V supply, I have concluded I need three HX177 board to integrate these to the microcontroller? and a WebMOS D1 mini for wifi transmission to pc.

I had a great suggestion of a 1amp lipo booster but I am not sure what battery size I would need to connect to this and the resultant circuit. 

Can anybody help explain if this circuit will work and battery size I would need to power hence what battery is of the best choice? The load cells and components will need to be powered for a short testing range of a couple of hours.  

Thanks

[Ron_Blain]

Again your link is no good, you included the last bracket as part of the link. 

Breadboard your setup and note the current consumption. Use buck/boost converters or whatever works for you. Then allow maybe a 20% overhead. Then calculate the mAH which will get you to your battery. If I have a 1 amp load and I want to run it for 10 hours at 12 volts I will need at least a 12 amp hour 12 volt battery. The load will vary depending on what things are doing at any given time. I would just take the max you see and run with it. 

I also assume you mean a WEMOS and not WEBMOS?

If you plan on running 3 load cells you will need 3 HX711 modules if you go that route so keep that in mind when you find your HX711 modules as they are not all created equally. Most WEMOS modules offer a single AI (Analog Input) and the nicer HX711 newer modules communicate over the I2C bus. I suggest you read up on them.

Ron

[adam272000]

Again your link is no good, you included the last bracket as part of the link.

Breadboard your setup and note the current consumption. Use buck/boost converters or whatever works for you. Then allow maybe a 20% overhead. Then calculate the mAH which will get you to your battery. If I have a 1 amp load and I want to run it for 10 hours at 12 volts I will need at least a 12 amp hour 12 volt battery. The load will vary depending on what things are doing at any given time. I would just take the max you see and run with it.

I also assume you mean a WEMOS and not WEBMOS?

If you plan on running 3 load cells you will need 3 HX711 modules if you go that route so keep that in mind when you find your HX711 modules as they are not all created equally. Most WEMOS modules offer a single AI (Analog Input) and the nicer HX711 newer modules communicate over the I2C bus. I suggest you read up on them.

Ron

https://docs.rs-online.com/2859/0900766b8142cdcb.pdf
Thanks ! My apologies correct link is above for the load cell . it has a Supply voltage of 5V. 

Yes I meant wemos, the wemos Of choice is :
https://shop.rasp.io/products/wemos-d1-mini-esp8266

And the HX711 I have in mind is the spark fun hx711:
https://www.robotshop.com/media/files/pdf/hx711-load-cell-amplifier-datasheet.pdf

I'll definitely have a read up on them thanks

[Ron_Blain]

OK if you look at the data sheet for your load cell (a button type compression load cell)  the recommended excitation voltage is 5 volts. That is not a supply voltage but an excitation voltage. I covered this earlier in the thread. Load cells have an excitation voltage which can vary. Load cells also have a sensitivity or output scale and yours is 20mV/V so with a full load applied you get sensitivity 0.020 V times the applied excitation 5.000 V = 0.100 V or 100 mV at full load. That's with 5.000 Volt excitation. Also note that with load cells the excitation voltage needs to be extremely stable and constant. 

The HX711 provides a stable excitation but not 5 volts of excitation. This is why it has been suggested you read up on HX711 modules and how they work. 

If you choose to use 5.000 volt excitation and get 100 mV out for full scale of span I would then consider an ADS1115 16 bit programmable gain module out front which has a gain of as high as 16: 

• GAIN_SIXTEEN (for an input range of +/-0.256V)

It gives you four single ended inputs and you have three load cells I believe you mentioned. 

Ron
 

[DaveEvans]

By the way, if you use the HX711, then you'll be able to read most of the load cell's range, but not all of it.

To get the widest range, your sketch will need to use HX711 Channel B (whereas most example sketches use Channel A).

With the approximately 4.3 v excitation from the HX711 (E+ to E-), your 20 mV/V load cell will output about 86 mV at full load, whereas Channel B clips at about 67 mV.  (Channel A clips at 17 mV or 34 mV, depending on the gain setting, so if you're only using a third or less of the load cell capacity, you could use Channel A.)

One advantage of the HX711 over the ADS1116 is that it (the HX711) has an stable in-built excitation source for the load cell, so the ADC output is not sensitive to power supply voltage changes (it's "ratiometric").  The ADS has a stable reference voltage, but that voltage can't be used to excite the load cell; to avoid errors due to excitation voltage fluctuations, the load cell needs a very stable excitation source (something other than from USB or wall wart).

[Wawa]

"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..