In my small business I needed to weigh medium to large items and boxes on a floor scale for shipping. Rather than pay way too much for an industrial model, I used a digital bathroom scale. I found it to be close enough for the rough accuracy I required when over 15lbs (7kg), as well it measures up to 330lbs (150kg). Way more than I can lift.
I was frustrated because the display was always hidden under the item I weighed. Sure the result locked in but only if you got lucky, blindly timing it right and quickly moving the box after.
I can also see a potential of this for persons with some physical challenges. Think about the hard of seeing reading at their feet, bad backs or spines making it difficult to bend to read the display, pregnant women who can no longer see the floor beneath them (Quit laughing before some big mama come slap you!), etc. I am just saying it could be medically difficult for some to see well to the floor level. It would be much easier for them to have the display at about face high?
So I wondered about why the display has to be stuck inside the bathroom scale and thus on the floor or under the item being weighed? I then realized that it does not when you see the inner workings of my electronic bathroom scale.
Step 1: Tools and Stuff You Will Need
I started with a very basic digital bathroom scale that I bought from my favorite Thrift Shop cheap. (I did not want to risk my own on my first convert.)
The scale I chose to do just happens to be made by EKS, it has a Silver color ABS platform over black metal base, LCD display, Capacity max 150kg (330lbs) and d = 0.1kg (0.2lbs), Auto shut off, Tap start, 9vdc battery.
Solder, Iron etc
Drill with small bit (3/32 inch)
Piece of 4-wire cable (telephone wire is perfect, look any Junk Store for <$1.)
and of course duct tape, naturally! (What is a project without it?) In this project we do not use it but it is so handy you will want some anyway.
Step 2: Open Up Your Scale
Orient the scale face up with the display away from you, as if you are about to use it. Consider the display then at north position.
Grip the sides, at the east and west and while clasping the top to bottom, flip over so only the north and south switch. Place it on workbench face down. Ok, you did not really have to clasp bottom to top yet, but get into the habit for later or you will say many bad words. I said some I did not know I knew.
Remove the battery.
This is a good time to remove the 2 screws securing the lbs/kg switch. If your scale is like mine.
The only thing holding my scale together is 4 strong springs. Unhook each, but note the orientation.
Now it is important to clasp together the top and bottom parts. Grip the sides, at the east and west and while clasping the top to bottom, flip over so only the north and south switch in one movement. Place it on workbench face up.
Step 3: The Guts
Gently lift off the top, flipping over as you do, just in case there is any thing else connecting it to bottom. There was not on mine.
Before anything gets knocked out of place take special care to learn how the beams are hung on your unit. Also notice how the stirrup hangs the beams onto the load cell (actually it is a strain gauge, but lets not get too technical here). See next step for more pix of the interior.
Also take a close look at which direction the springs are hooked into the top plate. It is important, as they will fit both ways until you try to hook them up later. If you are careful with the flipping steps, they should remain hooked.
If yours is an older scale or dirty with dust bunnies you may want to disassemble the beams and clean up a bit.
Step 4: What If It's Too Late Or You Did Not Listen
So you either have a different style scale or you jumped ahead and missed a step. Now its all a mess inside.
If you want to see what it is like to forget to flip right, Look at these pix.
I don't believe it is possible to put it together this way. But it does help to show an underside view and to understand how things work.
Step 5: Wiring The Guts
Remove the display circuit card, switch and battery holder together.
You should see the display is wired to the load cell with 4 wires. Make a drawing of which color wire goes to which connection on both the card and the load cell.
We will replace this short piece of wire with the long telephone cable. Wire color is not important but connecting the same points is.
Dress the cable to the desired length. Solder one end of the new cable to the wires of the load cell. Remembering to feed the cable out through one of the holes in the base. I used the battery holder hole. You can use duct tape to make sure the wire does not touch the beams.
Step 6: Wiring of Display
When I unsoldered the wires of the lbs/kg switch I found a nice blank place on the card for this switch to go. I had to drill the six small holes for the pins though. Even though the switch has six pins we only use two as an Single Pole Single Throw
Connect the outer end of the new cable to the display circuit card to match the wiring used at the load cell and follow the pix.
I didn't get fancy and just screwed the display on the workshop wall above the scale. Extra wire allows me to put the scale on the floor to.
Step 7: Not a Legal For Trade Scale, But....
A bathroom scale is not accurate enough for commerce, ie, when charging $ amount based on unit weight. All scales used as 'Legal for Trade' must be certified routinely to be remain legal.
But for parcel shipping or personal use, I think it is alright. In shipping the item is expected to be measured by the shipping company.
One of the first things I did was pre-weigh parcels before I went to the post office and was pleasantly surprised my results compared well to their results.
My goal was specifically the courier companies that pickup at my home repair shop. They somehow expect a person to guess what the weight will be for billing. I need to know how much the shipping charge is so I can pass the cost on to my customers. With the weight and dimentions I can get an estimate of charges on the interneet. I've never been billed extra after my shipments even when I underestimated weights lower than actual.