$4 strain gauge based hanging scale - Page 1
Forum section: electronics / general

author: saturation
author_rank: Super Contributor
timestamp: May 22, 2013, 01:37:34 pm
content: A hanging scale is one of the lowest cost weigh scale you can use for general purposes. It tends to have low parts count and potential high reliability: fewer moving parts if designed well, at least no tray, springs, or a large chassis. In the old days, they were just heavy duty springs with a pointer glued to one twist, and the degree of stretch was proportionate to weight.Besides being small and portable, it can also weigh items far above it size, just fashion a sling for it to hang the object to the hook.For general electronics use, it good for weighing devices for mailing or for air transport restrictions.Its very rare that scales be sensitive enough to use as a postal scale, or food scale, if its made to weigh heavier objects such as packages.This cheapo Chinese design is such a rare scale, and well designed for function based on the tear down analysis, and ingenious in its use of cheap parts. Its sensitive enough to weigh 5 grams. Its > 99% accurate to 110 lbs or 50 kg, support SI and US units, and has a built in celsius thermometer. It can tare. Electronics scales have advantages over mechanical ones: smaller, lighter, greater dynamic range and sensitivity. Cons are durability, longevity and need for batteries.As a travel scale, the cheapo construction insures its very lightweight. The heaviest item are the metal strain gauge assembly and the batteries to power it. See tear down photos.Its widely found, chances are a local eBay seller has it anywhere in the world, so you needn't buy it from China. You'll find them under many brand names, often at higher prices, but the hidden value are the brand-less package-less bulk sellers selling them for $4-6 shipped [ in local USA] as a fishing scale. If you find it listed as a 'luggage scale' the price for the same item easily >>2x .http://www.ebay.com/itm/20g-40Kg-Digital-Hanging-Fishing-Luggage-Weight-Scale-/250779372627?pt=LH_DefaultDomain_0&hash=item3a639d8c53To use the scale, you hold or tie the nylon handle while dangling the item to be weighed on the bottom hook. If set to auto lock, it will lock on to a value in 3 seconds. It will lock on items > ~ a pound or 500 gms, and below it depends on the rate of change of weight.The strain gauge, d, is made of a tied nylon handle and bolted hook. The [resistive, likely] white-blob sensor detects the degree of flex on the central metal plate. The whole weighing assembly floats in the chassis, so the chassis need not be as strong as the sensor assembly.The PCB has an SMT LED, e, blinks during measuring refracted by red diffuser on the chassis. The LCD [ white rectangle] is connected by elastomers to the PCB, f, running the unlabeled microcontroller, g. The controls a,b,c are all SMT types and the switches are unattached plastic knobs on the PCB.To check the calibration, I compared its readings to a known good scale and weight of water, i.e. 1kg ~ 1L ~ 1000ml. For gram weights I use US coins.ConsCheap thin plastic, so durability is low. Treat it delicately.Longevity? Unknown, so I have 2.Comments on other variants of this design you can find in many places:Avoid those that use watch batteries, the quiescent drain current is high enough to kill batteries left in here for months. Its also more a pain to remove and replace batteries, compared to the AAA design. With NiMH LSD cells, battery usage and replacement is inconsequential.Avoid those than have integrated plastic handles into the chassis; the worse are those where the electronic assembly doesn't float relative to the strain gauge. This increases the strain on the plastic, which will later fail and break. It also more likely to cause weigh calibration problems as the plastic ages and wears or deforms.Avoid a variant were the nylon cord is replaced with a straight through metal rod connected to the strain gauge, and a metal ring or handle at the top. This rod can act as a lever on the plastic shell, and pulled the wrong way can break the plastic shell. It also adds more weight. The nylon version is cheap yet ingenious as it will not mar soft plastic chassis, and nylon is very easy to service or replace when aged.Enjoy.

author: grenert
author_rank: Frequent Contributor
timestamp: May 22, 2013, 01:55:03 pm
content: Thanks for pointing this out, Saturation!Definitely the make vs. buy question for a simple scale goes clearly in favor of buy!Was it >99% accurate right out of the box? Can you tell if any calibration is possible with this?

author: saturation
author_rank: Super Contributor
timestamp: May 22, 2013, 02:09:20 pm
content: You're welcome. Yes, what I did was add water to 2 gallon jugs slowly and watch the scale value. Its ~ 100% accurate compared to my detecto scale, and compared against water volume using a graduated cylinder, water is 99% close to 1cc = 1gm at room temperature, its spot on out of the box.Alas, you cannot calibrate the scale should it go off, but can you zero or tare it.When I first tore it down I did not put the nylon line back properly, so it kinked; the scale read off, but consistently by 8%, adding the 8% corrected the value but you had to do it manually. If you do tear it down the nylon line must move freely in the chassis so it connects directly to the strain gauge assembly without drag. There are 2 small plastic guide rails where the nylon strap exits and it should be lined up with them, that's all, but its so tiny a rail you'll miss is at first.Quote from: grenert on May 22, 2013, 01:55:03 pm

author: robrenz
author_rank: Super Contributor
timestamp: May 22, 2013, 02:37:22 pm
content: Nice find saturation. If I didnt already have a industrial version of that I would buy one.

author: saturation
author_rank: Super Contributor
timestamp: May 22, 2013, 03:03:01 pm
content: Thanks, and your goop thread was great. Yes, even if it works now, readers should know its only 1 mo old so long term reliability is unknown, but for $4 its worth a gamble. Based on the tear down, the only thing that I forsee that can go wrong are thin wisker like events or poorly fabricated uC, leading to a semiconductor or a memory failure. Strain gauge sensors are quite reliable. So in case, for travel I carry 2, the other is a conventional spring scale, as they are both small, lightweight and $4.Quote from: robrenz on May 22, 2013, 02:37:22 pm

author: Skimask
author_rank: Super Contributor
timestamp: May 22, 2013, 04:22:12 pm
content: Nice find.Just ordered 2.

author: SeanB
author_rank: Super Contributor
timestamp: May 22, 2013, 05:33:18 pm
content: I also have two, and as I do have certified masspieces I have checked them both for linearity and for accuracy. Both actually do have very good linearity and a very good accuracy, at least reading consistently to +-10g over the 35kg mass range I use. I would not like to see how the long term creep is, as the strain gauges will drift with time and temperature, so it will be hard to see with a constant applied load. But for simple short term mass checks they are fine.

author: saturation
author_rank: Super Contributor
timestamp: May 22, 2013, 08:32:07 pm
content: This is great to know SeanB, thanks for that input, that's good testimony for these $4 scale.Here are photos from similar designs with variations I mentioned earlier, they all tend to cost a little more and IMHO raise some design concerns:Avoid those than have integrated plastic handles into the chassis; the worse are those where the electronic assembly doesn't float relative to the strain gauge. This increases the strain on the plastic, which will later fail and break. It also more likely to cause weigh calibration problems as the plastic ages and wears or deforms.Avoid a variant were the nylon cord is replaced with a straight through metal rod connected to the strain gauge, and a metal ring or handle at the top. This rod can act as a lever on the plastic shell, and pulled the wrong way can break the plastic shell particularly near its max load of 100 lbs. This model however, has better construction. If you take care not to handle the case until the load is removed, there is little problem.Quote from: SeanB on May 22, 2013, 05:33:18 pm

author: saturation
author_rank: Super Contributor
timestamp: May 23, 2013, 11:22:52 am
content: Enjoy, please review for us when you can.Quote from: Skimask on May 22, 2013, 04:22:12 pm

author: Edwin
author_rank: Newbie
timestamp: February 19, 2014, 10:04:16 am
content: Even these scale are small and cheap in price but they gave accurate results for your measurement. I have bought one in $5 which is a portable digital scale which help me ingetting accurate measurementwith other features i need.

author: saturation
author_rank: Super Contributor
timestamp: February 19, 2014, 03:16:33 pm
content: My scales are almost a year old and doing well. I bought 4, gave 2 away as presents. I've used them for international travel and they were accurate to less than 0.5 lbs compared to the airline scale. In fact, most times its right spot on with no error. Amazing.

author: David_jones
author_rank: Newbie
timestamp: July 08, 2014, 05:13:23 am
content: Waooo great manufacturing technique use in this hanging scale. My scale is totally from itMechanical scalesare different from digital scale in working and manufacturing both .I just want to know about the maximum weighting limit of this scale .

author: pickle9000
author_rank: Super Contributor
timestamp: July 08, 2014, 05:46:05 am
content: I've had a few of those over the past couple years. They hold up well but keep getting taken by the relatives. No complaints about the accuracy.

author: billthebuilder
author_rank: Contributor
timestamp: February 20, 2015, 12:12:28 am
content: I am looking for a cheap strain gauge load sensor for a project but am having trouble finding one that's less than $30 (was looking on digikey; <$10 would be ideal). If this product sells for $4 and has a strain gauge load cell in it, where can I find one that cheap?

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 22, 2015, 12:09:32 pm
content: Have a look here:http://www.robotshop.com/en/catalogsearch/result/?q=Load+Cell&order=stats_sales_order_count&dir=desc

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: February 22, 2015, 12:48:22 pm
content: 4 bucks! Nice. I paid a bit more for mine, like 60 dollars. Well, that's because it is a bit more robust. All metal enclosure, heavy duty, 300KG capacity, etc...http://www.ebay.com/itm/Heavy-Duty-Digital-Hanging-Scale-300-KG-660-LBS-Industrial-Crane-Scale-OCS-L/200873925615?rt=nc