In this chapter we build a 25kg / 55lbs rated weight scale to monitor the weight of a Hempy bucket. The scale is made out of two 35cm / 14inch round cutting boards with a full bridge load cell sandwiched between them. The cutting boards have several layers of lacquer for water proofing and anti-slip sticker on both sides to keep the bucket stable. The scale connects to the Hempy module with a 4 pin waterproof connector.
The weight scale is still compatible with the Hempy module, but the Weight platform has double load capacity and is easier to build.
One Hempy module supports two Hempy Buckets
Five layers of lacquer and anti-slip pads
Anti-slip pad at center, two countersunk screws holding the load cell (under the pad)
Three anti-slip pads, two countersunk screws for holding the load cell
Load cell mounted with 2x2 screws and 3mm thick washers.
The 4 pin waterproof connector is held in place with two screws, two washers and two zip ties
As the first step you need to measure the distances between the load cell mounting points. This will be needed to determine the 2 - 2 drill points on the boards. The 25kg / 55lbs load cell used for this build needs two 5mm / 3/16inch holes on the bottom side and two 4mm / 1/8inch holes on the top. On both sides the holes should be 20mm and 35mm from the center point of the board.
Two 4mm diameter holes, 20mm and 35mm from the center
Two 5mm diameter holes, 20mm and 35mm from the center
Cutting boards are not always perfectly round, simply measuring and drilling the load cell mounting holes could result in misaligned top and bottom boards. This process will help to counter-measure this problem:
Finding the center point of both boards
Measure out 20mm and 35mm from the center in one direction on the top board
Drill both of the 4mm holes on the top board
Temporary attach the top board to the load cell with two screws
On the bottom side measure and drill the 5mm diameter hole 20mm from the center
Temporary attach the top board to the load cell with one screw
Align the top and bottom boards and tightly fasten the single screw holding the bottom board
Remove the two screws holding the top board to expose the load cell
Mark the spot for the second hole using a small diameter drill bit (4mm bit should fit the load cell's screw hole) and lightly tap it with a hammer to leave a mark in the wood
Remove the screw holding the bottom part and remove the load cell
Drill the 5mm hole on the bottom board where you left the mark at step 9
The cutting boards are waterproofed using lacquer finishing. Before applying the lacquer make sure the boards have a smooth surface. Cutting boards are usually sanded so you can skip this step if the boards feel smooth to the touch. If not, sand them down and remove the dust with a vacuum cleaner / lint-free rag. Once the boards are clean apply five layers of spray-on lacquer and let the boards dry 30 minutes between each layer. Before applying the final fifth layer lightly sand the board using fine 300-500 grit sand paper. 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 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 in a well ventilated room.
Use a 120-200 grit paper if the surface feels rough to the touch.
Once done, remove all dust from the surface
Lightly sand the boards to get a smooth finish
Lacquer will darken the wood instantly, apply in an even layer and let it dry for 30 minutes between layers
After applying five layers the boards are ready for assembly
A load cell has 4 wires:
Red : Excitation + , connected to E+ pin on the HX711 load cell amplifier
Black: Excitation - , connected to E- pin on the HX711 load cell amplifier
Gray: Signal - , connected to A- pin on the HX711 load cell amplifier
Green: Signal + , connected to A+ pin on the HX711 load cell amplifier
At this point just connect the load cell to one side of the 4 pin connector using a soldering iron and shrink tube insulation, and note down which color wire from the load cell is connected to which wire in the connector.
After the lacquer finishing is dry it is time to assemble the weight platforms. For this you will need:
2pcs - 20mm internal diameter washers, 3mm thick
2pcs - M4 screws, length equal to the board + washer + load cell thickness - screw head size (countersunk into the board)
In the sample build this is: 15 + 3 + 13 - 2mm , meaning a 30mm screw is ideal. The screw should not touch the board on the other side as that prevents the load cell from flexing. The head of the screw will be countersunk into the board.
2pcs - M5 screws, length equal to the board + washer + load cell thickness - screw head size (countersunk into the board)
The washers are installed on both sides of the load cell to keep them from touching the boards and allow the load cell to flex.
The 4 pin connector is secured to the board using two screws with a washer and zip tie.
Place the second washer on top of the load cell and the screws into the top board
Make sure the washers are centered around the screws before pulling them tight
Calibrate the scales using the Gbox420 weight scale calibration sketch.
