Build a scale that has a display, tare function and be able to fire a relay to switch a valve

How much weight? How accurate does the trip weight need to be? How fast does the weight increase?

Doug Has all the right questions here.

I have had a luck with Arduino, A load cell and Amplifier HX711 and this Library.

https://github.com/bogde/HX711

For more reading check out the article from our friends at Hackster

https://www.hackster.io/MOHAN_CHANDALURU/hx711-load-cell-amplifier-interface-with-arduino-fa47f3

Hey Doug,

We are looking at a delta of between 507 to 512 grams on our liquid fill.  So the idea is that we tare the scale with the first empty bottle and then using a foot pedal, open the valve to fill and then the scale hits it's "weight limit" then fires a signal to a relay that can flip the valve to a different position.

Thanks.

Maybe you could just have a capacitive switch on the outside of the bottle. When the liquid reaches the level of the switch , the switch shuts off the fill valve. You might need to shield the capacitive sensor from stray capacitance, but not much other circuitry needed....

Hi Scott,

It might be easier to use a stepper motor and a peristaltic pump so you can meter an exact amount of liquid that would be equivalent to the weight desired. The only sensor you would need would be to sense the placement of the bottle and then use a micro processor to trigger the dispense cycle and then wait for the bottle to be removed and another put on the target.

https://en.wikipedia.org/wiki/Peristaltic_pump

John

jw0752, that mechanism doesn't have feedback, I think. To have that work reliable all conditions have to be perfect.

What is the purpose of the exercise ?

If the filled bottles are to be sold there are very strict regulations in most developed countries about the requirements for accuracy and repeatability.

Do you know how much the bottles weigh and how much they vary ?

You can use a single point load cell and suitable amplifier with an ADC. A 12 bit ADC would give you a resolution of about 0.25g which is just about good enough.

A 16 bit ADC is still quite cheap.

You will need a processor to control the system and you will need to start turning the valve off before you reach the target weight (since the flow will not stop instantly).

It is a good idea to record the amount actually put in each bottle so you can do statistics to check the quality of your process.

You can buy cheap load cells from lots of places including Aliexpress - you can buy good ones (for a lot more money)  from Farnell, HBM (who actually make them) and several other sources.

Here are some that look OK:

https://www.loadcells.com/product-cat/load-cells/platform-load-cells/

MK

Could perhaps use an interim fixed measuring cylinder rather than trying to rely on measuring the bottle itself. Fill valve fills the measure until it hits the target then stops and arms the drain valve. Operator then opens the drain valve with the foot peddle once the empty bottle is in the desired position. Once the cylinder is empty, the drain valve closes and the fill valve opens to refill the measure while the now full bottle is removed and replaced with the next empty.

There are many possible technologies and methodologies to perform this function that have been successfully employed in industry, including all the techniques mentioned so far. They all require significant knowledge to implement well, but the issues have been solved and the information is available. The learning curve cannot be avoided when designing a system but the real question is which method should be investigated, researched and learned? Because learning them all is a much bigger task.

• peristaltic pump - does not contaminate the fluid
• weight - can be a simple spring with a switch or strain gauges or a load cell with lots of circuitry and possibly software
• timed valve (volume)
• mass flow measurement
• volume flow measurement
• volumetric cut-off
• fill level cut-off
• Presssure measurement
• float switch
• spring switch
• ultrasonic level measurement
• acoustic resonance
• conductive switch (level sensor)
• video inspection
• overflow switch
• optical reflection
• optical attenuation
• radar level measurement
• depth gauge
• balance switch (when the balance starts to move, an optical switch is triggered)

And there are many more - these are just the ones that immediately come to mind that I have used or researched. Each method could have a whole book full of theory, datasheets, app notes and examples. The best choice cannot be made without knowing the fluid and bottle characteristics and what is riding on the system performance.

It may be possible to eliminate some of the learning curve by selecting subsystems that meet the requirements - for example it is difficult to design a strain gauge solution because the mechanics and electronics are tricky. A load cell is a ready-made strain gauge solution that includes some of the mechanical design but it is still tricky to make an accurate system. A set of scales with electronic output includes most of the tricky mechanical design and the scales can even have a tare function built-in. Of course as you get more complete systems, the price goes up and the device becomes less specifically designed for the application.

Now this... this... is a true community post.

One question, Lots of Advice and alternative options!

Thank you to everyone involved.