So, earlier I’ve searched around the Internet for information on electronic weight sensors you can connect directly to a Raspberry Pi. But apparently, I must have been too disappointed to record my findings or immediately label them with the “Raspberry Pi” tag. Well, here I come back full circle to pretty much the same findings, but this time, much more straight from the original sources, Sparkfun. Previously, I’ve found a third-party blog that had the how-to build, but not the explanation of why.
Yes, you do need to buy that weird Z-bar-shaped thing and screw it to platforms for weighing. Why the Z-bar? Because by rotating by a moment, it can sense weight more accurately than a pure compression button cell.
Please note that you do not need to use wood. Cardboard will also work, and give you the advantage of a lower tare weight. Again, feel free to experiment with other platform materials, you may even try thin sheet steel as a means of saving weight. The thing is, not only do you want a light platform material, you also want a platform material that will not deform too much under load, because all deformation is force that is imparted within the material itself and not transferred to your weight sensor.
20191220/https://learn.sparkfun.com/tutorials/load-cell-amplifier-hx711-breakout-hookup-guide
Fortunately, I can get the Sparkfun components from Digi-Key at a reasonable price, and they have competitive specifications. 500 gram-force load sensor at $11, 24-bit ADC on breakout board module at $10.
20191220/https://www.digikey.com/product-detail/en/sparkfun-electronics/SEN-14728/1568-1899-ND/9555602
20191220/https://cdn.sparkfun.com/assets/9/9/a/f/3/TAL221.pdf
20191220/https://www.digikey.com/product-detail/en/sparkfun-electronics/SEN-13879/1568-1436-ND/6202732
There’s also a 100 gram load sensor available, if you want to go even lighter. But for papers with a 24-bit ADC, the 500 gram sensor is plenty good, accuracy at 0.1% Full Scale, not to mention safer against over-loading which can happen quite easily once you get too good and fast at scanning.
20191220/https://www.digikey.com/product-detail/en/sparkfun-electronics/SEN-14727/1568-1898-ND/9555601
Sparkfun has some great “getting started” info for using the load sensor. The load sensor itself is actually the four resistor components of a Wheatstone bridge, and you connect these directly to the amplification and ADC board. That being said, you want this board as close as possible to your weight sensor, in case you are considering placing your Raspberry Pi at a distance. Use 5 V for the analog power source, and in the case of Raspberry Pi, 3.3 V for the digital supply. This will ensure the serial input/output is at 3.3 V.
20191220/https://media.digikey.com/pdf/Data%20Sheets/Sparkfun%20PDFs/Getting_Started_with_Load_Cells_Web.pdf
20191220/https://www.sparkfun.com/products/13879
The serial protocol is simple enough in itself. When data is not ready, the output is held high, and the clock should be held low. When there is data available, the output will be held low. Apply 24 positive pulses to read out the 24 bits, and then apply the 25th pulse to clear the state and bring the output back to high. Two more pulses can be provided to send configuration data to the ADC as follows:
- 25 pulses: Input channel = A, Gain = 128
- 26 pulses: Input channel = B, Gain = 32
- 27 pulses: Input channel = A, Gain = 64
Please note that if the clock input stays high for more than 60 microseconds, the chip will enter power-down state, and when the clock input goes back low, the chip with power back on and reset. This means that for connection to Raspberry Pi, either realtime threads is a must-have or one-shot pulse timing through the 555 timer chip can be used as an alternative. The PWM outputs might also work well.
20191220/https://www.mouser.com/datasheet/2/813/hx711_english-1022875.pdf
Sparkfun also has an ADC breakout board that connects via I2C. Alas, this is designed with extra headers that I would prefer not to use, soldering being my preference for my particular projects. Nevertheless, the NAU7802 chip is worth taking note of.
20191220/https://www.sparkfun.com/products/15242
Researching more, balsa wood also looks like a good choice for the scale’s platform material. It is simiarly as light cardboard, but it can be potentially stronger than it.
20191221/DuckDuckGo balsa wood density
20191221/http://www.modelaviation.com/balsa
20191221/https://en.wikipedia.org/wiki/Ochroma
How the strength compare with cardboard? Corrugated cardboard can have a good 32 pounds per square inch of compressive strength. Okay, so I found this source that gives the compressive strength in Megapascals, so I will convert the cardboard measurement to that, approximately.
32 lbs/in^2 = 14.545 kgf/in^2 = 2.255 kgf/cm^2
= 22545.5 kgf/m^2 = 221171.351 N/m^2 = 221171.351 Pa
= 0.221171351 MPa
20191221/https://en.wikipedia.org/wiki/Pascal_(unit)
See balsa wood strength specifications here:
20191221/DuckDuckGo balsa wood compressive strength
20191221/https://www.auszac.com/pdf/Balsa_wood_Properties_Guide.pdf
Wow, balsa wood is pretty strong compared to corrugated cardboard. And I thought that was already fairly strong for the sake of working with paper!
Well, I guess I have my word, then. If cardboard is strong enough and light enough, I can just work with that. Besides, it’s more abundant and cheaper, too, so why not? If I don’t have a real need for balsa wood, yeah although it may be nice to play with, it is more practical and looks better on my bill of materials to specify cardboard for projects.
Okay, mounting this sensor is a bit tricky because only two of the four screw holes are tapped, and I want to use cardboard as the platform material. But, with proper care, this is feasible.
First of all, I need to have 3 mm standoffs, of course, in order to get proper Z-bar mounting. I can use unthreaded standoffs at the threaded screw holes, but I probably want to use threaded standoffs for the unthreaded screw holes to give me more that the screws can hang onto. Then comes the question of screwing into base versus platform. Sure, I can use a heavier base than platform, but if I do, I have to be careful not to make it too heavy, because otherwise if you pick up and set down your scale upside down, you could damage the sensor from overforce. So, although I might screw in the platform screws heads up, I might also want to screw in the base screws heads up. Plus, if I end up using the unthreaded screws for the base, which I likely will, I may also need to use additional standoffs if the base screws are too long and can’t go all the way down.
Finally, there is another problem with screwing to cardboard. How is that ever going to work, won’t the screws put too much pressure on too small of a space and never work reliably? If they are used directly, yes, but there are again ways around this issue. First and foremost is to buy washers to spread out the force exerted by the screw. This way, you won’t have as much risk of cardboard crushing, flexing, and things like that from overforce. Then, if you are using corrugated cardboard, you can use “pressed” cardboard sheets (i.e. from cereal boxes) to spread out the force even further to avoid deforming the corrugated cardboard. These two methods together should provide a pretty good foothold between your screws, your sensor, and your cardboard platform.
Since we want to make a precision scale, you’re also going to want to opt to use metal screws, standoffs, and washers for the sake of precision measurement. Yeah, nylon can be nice, but maybe this is not a game for nylon to play. This way, you also have a backup plan: if the cardboard platform turns out to be a failure, you can always switch to also using a metal platform too.
Alas, regarding the washers, looks like the metal washers have only limited choices, so I may concede with using a nylon washer here simply for the sake of getting a larger spread connection. The primary purpose was to avoid crushing and compressing cardboard, although this may come at a bit of expense of compressing nylon. Also, the nylon washers are generally thicker than the metal washers.
So, let’s review my choices:
3 mm unthreaded aluminum standoffs:
20191221/https://www.digikey.com/product-detail/en/harwin-inc/R30-6200314/952-2365-ND/3906352
3.61 mm threaded steel standoffs:
20191221/https://www.digikey.com/product-detail/en/w-rth-elektronik/9775026360R/732-10909-1-ND/6644315
Runner up: 9.53 mm nylon washer:
20191221/https://www.digikey.com/product-detail/en/keystone-electronics/3158/36-3158-ND/59626
Runner up: 7.95 mm stainless steel washer:
20191221/https://www.digikey.com/product-detail/en/b-f-fastener-supply/FWSS-004/H734-ND/274945
Unfortunately, it turns out I can’t use any washers! It won’t work, it’s too big, the screws are too close together, 6 mm center-to-center distance. With 5.4 mm screw head diameters, that leaves only 0.6 mm spacing between the two screw heads. Use pressed cardboard in place of washers.
EXCEPT, I can use washers only for the bottom side of the cardboard that meeets with the standoffs. And I only need it for the unthreaded standoffs because the threaded standoffs have a “washer” of their own.
20191222/https://www.digikey.com/product-detail/en/keystone-electronics/7235-1/36-7235-1-ND/4499421
Now, for screws. Runner up is “self-sealing” and thus has an additional gasket near the top of the screw.
20191221/https://www.digikey.com/product-detail/en/apm-hexseal/RM3X12MM-2701/335-1157-ND/2063201
Runner-up, I was considering a deeper screw, but no, I’m not going to have a 6 mm deep base, I’ll buy more screws later if I really need to reconsider.
20191221/https://www.digikey.com/product-detail/en/keystone-electronics/29316/36-29316-ND/1532210
Now, my screw pick. They satisfy my desired material and length, but they are slotted rather than Phillips heads. Two of each, one 12 mm length shaft, the other 16 mm length shaft. The longer one is extra length just in case I want more than 3 mm mounting on the other board side, i.e. for a thicker base board. In case I don’t need a thicker base board, I can switch things around and they’ll work okay because of the standoffs.
20191221/https://www.digikey.com/product-detail/en/keystone-electronics/29314/36-29314-ND/1532209
Good info on metric screw sizes:
20191221/DuckDuckGo M3 screw diameter
20191221/https://www.engineersedge.com/hardware/_metric_socket_head_cap_screws_14054.htm
20191221/https://en.wikipedia.org/wiki/ISO_metric_screw_thread