In this chapter, we build a weight platform using four 50kg / 110lbs rated weight cells. The platform is made out of a 40x40cm / 15x15inch plywood board with four half-bridge load cells glued to its bottom. The board is waterproofed using several layers of lacquer. The platform connects to the Hempy module with a 4 pin waterproof connector.
Or visit the Gbox420 GitHub page and download the entire project.
After cutting the plywood board to size lightly sands the surface and soften the edges with ~120 grit sandpaper. Remove the dust with a vacuum cleaner / lint-free rag before applying five layers of spray-on lacquer. Let a layer dry 30 minutes before applying the next. Before the final fifth layer lightly sand the board using fine 300-500 grit sandpaper. For a quick 3 minute overview check out Steve Ramsey's video on using spray lacquer.
PS: I used a brush to apply the lacquer and it turned out to be a huge pain. It kept leaving marks/bristles in the fresh lacquer layer and I had to wrap the brush in a rag doused in paint thinner to keep it from drying out while the layers dried. I highly recommend using the spray-can form of lacquer instead and spraying it outside or in a well-ventilated room.
PS2: I used an 80x40cm / 30x15inch solid pine board that I cut in half. Plywood would have been a much better choice here as the glued layers give the board more rigidity.
The load cells are rated at 50kg / 110lbs and they all need to be able to flex a tiny bit when a load is applied to them. To allow this flex a metal M30 DIN125 shim was glued between the weight sensor and plywood board. Use universal all-purpose glue to secure the sensor in the middle of the shim, making sure the U-shaped internal side of the sensor can freely bend.
PS: If you are lucky enough to own a 3D printer, you can print a bracket for the same purpose.
PS2: Each weight cell has its own internal resistance, this is measured in the factory, and matching cells are sold in a group of four. When building multiple platforms do not mix up the four cells that come from the factory with cells from another batch as their internal resistance might not match.
Mount the sensors at the corners of the board using the same universal glue.
From this point on do not rotate the weight platform until the wiring is done as we depend on the Top/Bottom and Left/Right position of each cell.
Pull 3 short shrink tubes over the 3 wire pairs coming from each sensor. At this point leave them loose, do not apply heat to shrink them
Solder the WHITE sensor wires between the Left and Right sides (TL to TR, BL to BR)
Solder the BLACK sensor wires between the Top and Bottom sides (TL to BL, TR to BR)
Pull 3 short pieces and 1 long piece of shrink tube over all cables bundled together at the center of the board
Solder the RED sensor wires to the Female side of the 4 pin waterproof connector in the following order:
Top left: E- (black)
Top right: A- (blue)
Bottom left: A+ (green)
Bottom right: E+ (red)
Insulate the wires from each other using duct tape and the large piece of shrink tube. Make sure to pull a little shrink tube over the 4 pin extension cable as well.
Secure the cable in your preferred direction using two zip ties, two 4mm washers, and two small wood screws.
On the male side of the connector cable solder female jumper wires using the same color codes as on the weight platform's side. This piece of cable will connect to the HX711 weight sensor.
