Step #1: Materials and ToolsPrevNext
Kitchen Scale (preferably one that doesn't zero itself when turned on)
Four Long Wires
Small Plastic Bowl
Fine Toothed Saw or Rotary Cutting Tool
Hot Glue Gun
Step #2: Background: How a Digital Scale WorksPrevNext
- Inside a digital scale you will find two main parts. There is a load cell that senses the weight of the object that is being measured and there are the various circuits that process the data from the load cell.
- A load cell is typically a metal bar with a sensor glued to the side of it. On older scales this sensor would be a simple strain gauge that changes its resistance as the bar is deformed. The circuit measures the change in resistance and converts that to a weight measurement. Newer scales often have multiple sensors with built-in circuitry. These sensors output serial data that is then processed by the external circuit.
- Most digital scales have a tare button that resets the reading to zero. This is very useful for calibrating the scale. However, some scales automatically reset themselves every time that they turn on. This feature is very problematic for this design. This would require the scale to be constantly on in order to measure any new snowfall. If you have this kind of scale, I would recommend using an Arduino to monitor the output of the load cell instead of using the original monitoring circuit. This option is discussed more in step 11.
Step #3: Open the Housing of the Scale to See How All the Parts Are Laid OutPrevNext
- The first thing that you need to do is open up the housing of the scale to see where the parts are located and how they are connected to each other. The plate will typically twist off. After removing the plate, you should be able to unscrew the rest of the parts. Keep in mind that screws are sometimes hidden behind stickers or the rubber pads on the bottom.
- On this scale the load cell takes up half the housing and the monitoring circuit takes up the other half. The load cell is connected circuit board with a single 4-wire cable. The load cell and the circuit board are both attached to the housing with screws.
Step #5: Cut the Housing into Two SectionsPrevNext
- You now have several options as to how to mount the load cell and the circuit board. You can create a new housing for either the load cell, or the circuit board or both. Another option is to just cut the current housing in half and then mount each part back in its original location. This is the easiest option because the two parts were separated by a reasonable gap in the housing. So a single straight cut would separate the two sections.
- Once all the internal parts had been removed, I fit the two pieces of the housing back together and I marked a line between each section. Then I used a fine toothed saw to slowly cut through the whole housing.
Step #6: Connect the Load Cell to the Circuit Board With Extension WiresPrevNext
- We need a way to connect the wires from the load cell to the pins on the circuit board. The ideal way is do this is to use the same type of connectors that are used on the load cell cable. This is a standard four pin header cable. If possible match up the colors of each cable to help avoid confusion. If you don't have this type of connector, then just solder the wires directly to the pins and cover them up with heat shrink tubing.
- monitoring circuit. I wanted to set up the monitoring circuit in my kitchen window and the sensor a few feet out in the yard. So I used about ten feet of wire to connect the two parts.