Load cell question - Page 1
Forum section: electronics / beginners

author: german77
author_rank: Regular Contributor
timestamp: October 10, 2018, 04:24:55 pm
content: I need to measure the weight of some trucks. I got a proper measuring device "rice lake 720i" that allows me to print receipts to a serial printer.I can read what the serial device sends without a problem. But the employees some times take measurements without printing the receipt and keep the money for themselves.I could make the serial port to send the weighted value all the time but the printer will not work properly anymore. Then I got a brilliant idea, what if I measure with a hx711 the same loadcell. Then I will know if someone measures something and if it's not registered. I can be calibrated from the rice lake device.Then I realized that there may be a problem. A load cell works with voltage. The rice lake uses 10v and ground for E+ and E-, that makes A+ and A- to be around 5v. Also the maximum voltage that hx711 handles is 5.5v. But I'm using a raspberry so that makes it 3.3v.But since I only need to measure the difference between A+ and A- that would be way less than 1V. If I do not connect E+ and E-. I will not be referenced to those 5V. And That would mean that the hx711 will work properly without causing damage to the hx711 or the raspberry.I am right or there is something I'm missing. Or what can I do to make the hx711 to work.... I could use a buffer op-amp and a voltage divider to change the voltage to a proper level. But from previous experience that would introduce a lot of noise and temperature instability.

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: October 10, 2018, 04:30:50 pm
content: Put up some schematics , because right now , you are not making a lot of sense

author: german77
author_rank: Regular Contributor
timestamp: October 10, 2018, 05:29:48 pm
content: I feel that there is no need for schematics, as everything is a standard. But I will put them any way.

author: tooki
author_rank: Super Contributor
timestamp: October 10, 2018, 05:44:18 pm
content: Seems to me that all you need to do is to enact the same policy that other stores use which have had problems with employees pocketing the money: Post a sign for customers to see, saying "If we don't give you your receipt, it's free." This gives the customer a really good incentive to police your employees — if they don't get a receipt, then they want their money back!

author: german77
author_rank: Regular Contributor
timestamp: October 10, 2018, 06:39:58 pm
content: The receipts are only for internal administration. The ones handed to the buyers are handwrittenalso the employees are also the ones who stole the product. You will be amazed the many way's they manage to disappear 1 ton of the product. Firing them will not solve anything as 80% of the employees are not even mine. From the internal administration, loss of product it's not that big of a problem. The real problem it's when other companies say that they are missing product.The setup I'm creating is take photos and info of the trucks, before and after they are loaded. That way I will have more control of my employees to manage them to register every truck. And with other companies to show evidence that the truck left with that amount of product.I just found out. That my model has two serial ports. And they can be configured differently, that's great because I can use one for printing and another for the raspberry. But the 720i is not the only model they use. That would mean that not every model will have a second serial port.

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: October 10, 2018, 09:44:48 pm
content: I don't think these setup will work as such , because if you refer to first schematic (the app block to be more precise) you can see that HX711 has max. supply voltage set at 5.5V coming into load cell. This means that input A sees a voltage of around 2.75V with diff. input range of +-0.5V.With this on mind , voltage of 5V/+-0.5V coming into input A would be too high.(Can't find anything in datasheet regarding max. input voltage , so maybe it's still safe but since I never used HX711 , can't tell)https://www.mouser.com/ds/2/813/hx711_english-1022875.pdfYou need atenuation. Try putting 100k in series with A+ terminal , if that is too much , try 10k. That should work.ORYou can try and use Arduino directly coupled to A-/A+ (no need for HX711)

author: german77
author_rank: Regular Contributor
timestamp: October 10, 2018, 10:53:31 pm
content: If you have worked with a load cell you probably know why I use a HX711 and not directly to an arduino. The voltage difference between A+ and A- it's around 1mV that's way to low to be read accurately with the ADC of the arduino.But since I have the second serial port working. Now I will test without worrying of burning the HX711 or an arduino. The worst that can happen is to be working.

author: wraper
author_rank: Unknown
timestamp: October 10, 2018, 11:14:14 pm
content: You cannot do this with HX711 as load cell is powered from 10V and around half of that will be on its outputs. But HX711 accepts maximum AVDD-1.3 V on it's inputs. Therefore if powered from 5V, there shouldn't be more than 3.7V on inputs. You would need to use something else what can work at higher voltages.

author: german77
author_rank: Regular Contributor
timestamp: October 10, 2018, 11:54:44 pm
content: Here I'm back after testing. Just to say that it works without letting the magic smoke escape. My theory was right from the beginning, I don't need to connect E+ and E-. The only problem is that the rice lake 720i it's not calibrated any more as the HX711 introduces an offset error. Nothing to worry about. I could simply add resistors to increase the HX711 impedance. And this error usually is eliminated on the tare.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 11, 2018, 12:25:41 am
content: There are two serial ports on the 720i, why not just use the serial port not being used for the printer? You can interrogate the scale continuously via the programming interface to catch suspicious activity.EDIT: Sorry, missed that reply.There is a lot more to weighing something than just reading the voltage off the load cell. Drift tracking and zero tracking come to mind.Finally is your truck scale affected by "Legal for Trade" legislation in your country? If so you will not be allowed to connect things to the load cell.

author: wraper
author_rank: Unknown
timestamp: October 11, 2018, 12:37:58 am
content: Quote from: german77 on October 10, 2018, 11:54:44 pmThat's not how it's done and very likely you'll eventually blow up either HX711 or your scales. Currently current is sunk through input protection diodes inside of HX711.Quote, nothing to worry about except you'll have incorrect readings on both scales and HX711. And eventually something will fail.

author: german77
author_rank: Regular Contributor
timestamp: October 11, 2018, 01:33:48 am
content: There is something that you aren't taking in account. The HX711 doesn't have input protection of any kind. There is also no over voltage in the HX711 and it will not fail. For one simple thing It's not ground referenced. For example A+ pin will have 5.0003V, A- pin will have 4.9998. The only voltage that the HX711 sees is the 0.5mV.Its like touching a 5kV wire with one hand and another wire 5.001kV with the other hand while you are standing in a non-conductive floor/air. Are you going to be electrocuted to death. The answer is no. Because the voltage difference it's 1V.If the power supply weren't isolated and they shared the same ground then the HX711 would fail in a blink of an eye. The resistors solution din't work because there is not enough current on the input of the opamp as expected. I can turn down the HX711 via software and it will remove the offset on the scale. If I only take a reading really fast once in a while the scale wouldn't even notice. And I get correct readings on both devices.

author: wraper
author_rank: Unknown
timestamp: October 11, 2018, 02:00:41 am
content: Quote from: german77 on October 11, 2018, 01:33:48 amYeah, you completely don't understand what you are doing and shit you are talking about. The reason it still somewhat works is exactly due to ESD protection diodes on inputs of HX711 as they sink the current to either Vcc or GND. Otherwise there easily could be over 100V AC between GND and inputs (Y cap in PSU). Do you effin understand that inputs of HX711 are not isolated? And the fact that HX711 does or does not take readings does not matter. In negatively affects scales regardless of that.QuoteYeah, and why do you think significant current flows there to begin with?

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2018, 09:18:30 am
content: The 5 volt common mode voltage from the load cell needs to be level shifted to the roughly 2.5 volt common mode voltage that the HX711 expects. Assuming that they do not share a common, this can be done by connecting one of the excitation lines with a 2.5 volt offset however the HX711 will no longer be running ratiometricly and there is a good chance they will interfere. The HX711 also might have troubles with no excitation load.While not ideal, and instrumentation amplifier could also be used to make the 2.5 volt level shift while connecting the negative excitation lines together but the same issues still apply.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: October 11, 2018, 10:02:26 am
content: Quote from: german77 on October 10, 2018, 04:24:55 pmThat's a very capable controller. Might be easier to suck out the measurements over serial. "11.12 Alibi Tracking" in the manual suggests they are logged. And then compare with what is declared.Or just put a CCTV camera over the station.

author: german77
author_rank: Regular Contributor
timestamp: October 11, 2018, 04:03:13 pm
content: The voltages of hx711 from the ground of the arduino are 8.2mV for A+ and 7.6mV for A-. Not AC voltage that I can measure . The current going through each pin is 8uA. This is not theory any more, those are real measurements.8uA is what makes the scale to have an offset. To be more realistic if I put my fingers on one of the pins it's enough to give me very wrong measurements two orders of magnitude higher than what the hx711 introduces even when there is 10mA going through that load cell.Quote from: NivagSwerdna on October 11, 2018, 10:02:26 amI do use serial to suck out the measurements. Whatever the 720i say I know it's true. I will even don't use the HX711 in this device because I have two serial ports. Not every thing is logged as it's registered manually. Normal CCTV will not suffice since I need to record 6+ months of data in pretty high quality and will not display measurements.Actually this tread should be closed. The problem is solved by using the second serial port and not needing to use the HX711 anymore. I just still post because it's funny how they think the HX711 is under risk of failure, It has been running 12hours straight without a problem and will last much more. I'm sure it's close to the limits of the op-amp as it's very close to ground. And this is very bad practice, in reality if I connect the serial, the HX711 will fail because the serial port will connect the ground between both devices and now 5v are on my input pins.Otherwise is a good experiment to try. Next time I will use an instrumental amplifier to attenuate the signals levels to a safe level for the HX711.

author: wraper
author_rank: Unknown
timestamp: October 11, 2018, 04:07:24 pm
content: Quote from: german77 on October 11, 2018, 04:03:13 pmI doubt there is no AC at all, but here is a figure to think about: Common mode input AGND+1.2 AVDD-1.3 V. Input voltage shouldn't be below 1.2V against GND.

author: german77
author_rank: Regular Contributor
timestamp: October 11, 2018, 05:08:24 pm
content: Quote from: wraper on October 11, 2018, 04:07:24 pmI know, it shouldn't work by what the datasheet says. But the measurements are working not really sure why. I expected to do nothing when I first connected it.