In my previous post I changed the uploader app to run when the Raspberry Pi turns on, and installed the scale under Pippa’s dog bed. In this post, I get interesting data from the scale.
The scale has been running for a little over a week now, and has been surprisingly reliable for a first version. There is some sort of bug in which, every few days, the scale stops supplying new data BLE notifications to the gateway. I plan to refactor the scale and gateway to avoid that, but that’s another post.
In my previous post, I wrote the Raspberry Pi Node.js code to upload data from Pippa’s dog bed scale to data.sparkfun.com. This post covers how to make a Node.js program run automatically when the Pi is turned on. Oh, and at the end I installed the finished scale under Pippa’s bed.
In my previous post, I did a little woodworking on the scale. In this post, I start designing a 3D printed part that will keep the top of the scale centered on the bottom.
Ever since I measured the center of gravity of the top plywood circle, I’ve been puzzling through how to make sure that center of gravity stays centered on the bottom part of the scale. Without some sort of connection between the top and bottom plywood circles, the top will inevitably slide over time, messing up all the center of gravity calculations. On the other hand, if this connection between the top and bottom has much vertical friction, it will take some of the load of the scale, throwing off the weight calculation.
In my previous post I soldered the weight scale parts to a proto-board. In this post, I design and 3D-print the part that keeps the Load Sensors from slipping.
The Load Sensor is an oddly-shaped thing that has a few tricky constraints: the T-shaped part in the middle must be free to bend downward (my wooden mounts take care of that), and I don’t want it to slide out of place horizontally or tilt off of its position when I’m putting the top plywood piece on the scale.
In my previous post I described how to use long break-away headers, and started soldering the circuit together. In this post I finish transferring the scale circuit from the breadboard to a protoboard, and do a quick test mount of the circuit on the plywood scale base.
A reminder: I found that the Load Cell Amplifier was (by design) so sensitive to changes in resistance that just touching the resistors on my solderless breadboard caused large changes in the Amplifier output. So I wanted to solder all the parts down.
It’s a good time to recap: This project is a scale that will sit underneath my dog Pippa’s bed, so that I can measure her weight automatically, at night while she sleeps. The project-in-progress is Open Source, at my CurieBLEWeightMonitor Github repository. I occasionally tweet about it (among other things) as @bneedhamia.
In my previous post I covered how to choose matching resistors for the Load Sensor to convert the Load Sensor into a Load Cell that can be wired into Sparkfun’s Load Cell Amplifier. In this post, I nearly finish building the breadboarded circuit and start transferring it to a soldered protoboard.
In my previous post, I worked through the calculations of weight and center of gravity when using four Load Cell Amplifiers instead of one. In this post, I build the circuit for the first of the four Load Sensor / Load Cell Amplifier combinations I’ll be using.
The Sparkfun Load Cell Amplifier is designed to connect a Load Cell to an Arduino. A Load Cell contains a full (4 resistor) Wheatstone Bridge, but a Load Sensor contains only half of a Wheatstone bridge. To connect a Load Sensor to a Load Cell Amplifier, I need to add two resistors: R3 and R4 in the following diagram. The dotted box represents the Load Sensor. The triangle in the middle of the diagram represents the Load Cell Amplifier. As the weight on the Load Sensor increases, R1 decreases, which causes the voltage V1 to increase, causing the digitized amplifier output to increase.