Tag Archives: Geometry

Dog Weight Scale, part 1: cutting the circular base

I want to learn how to use Load Sensors to continuously weigh stuff with an Arduino, so I thought it would be fun to continuously weigh our dog, Pippa, while she sleeps in her bed each night.  The project is a little like Nate Seidle’s Beehive scale, but simpler.

The idea is to turn Pippa’s bed into a scale. Pippa’s in fine shape right now, but it’s always good to keep an eye on your dog’s weight, and a custom-made scale is a great way to do it.

My plan is to wire 4 Load Sensors into a Sparkfun Load Sensor Combinator board and Load Cell Amplifier board, then to an Arduino 101, and from there, send the results via the 101’s built in BLE (Bluetooth Low Energy) radio.  From there I’ll need to build a gateway to relay the data to a server, but that’s another project.

The ongoing project files, including a project diary and Bill of Materials (parts list) are on my CurieBLEWeightMonitor Github repository.

It Begins

I measured Pippa’s bed and bought a pair of 4’x4’x3/4″ plywood sheets, to cut into circles: one for the bottom of the scale and one for the top.

Cutting a plywood circle is a lot of fun, because you can use geometry (I admit it: I’m a geometry nerd).

First, (assuming the plywood sheet is roughly square), draw two lines, each one connecting opposite corners.  Their intersection will be close to the center of the plywood sheet.

Finding the center of the plywood
Finding the center of the plywood

(Ignore the circle on the plywood for now: I reenacted drawing the cross-lines after I’d drawn the circle)

Next (I love this part), draw a circle centered on that intersection, using a beam compass.  A beam compass is a lovely thing for drawing very large circles and arcs: instead of having two arms like a normal pair of compasses, a beam compass has a point at one end, and a pencil that slides along the beam.  I bought a beam compass kit that attaches to a yardstick to form a beam compass, and I love it.

A beam compass kit
A beam compass kit

Here’s me drawing a 41″ diameter circle centered on the plywood sheet.

Drawing a circle using a beam compass
Drawing a circle using a beam compass

Once the circle is drawn, you can cut it out with a jig saw.

Cutting the circle with a jig saw
Cutting the circle with a jig saw

Ta Da!  Now I have a nice, circular base for my Pippa-Weight scale.

What a lovely circle of plywood
What a lovely circle of plywood

In the next blog, I’ll be soldering the electronic parts together.