Coasters are dumb and stupid, so let's make this one a smart one. The idea is to put a microprocessor inside a table coaster with a connected load-cell, granting our table cork the ability to weigh the amount of liquid of your choice.
This project requires you to solder, 3D print, cut open a kitchen scale and program.
Step 1: Find Parts
First things first, gather the required materials.
This is what we found
- 2x IKEA Table Coasters
- 3 LED's
- Adafruit Bluefruit LE (Arduino with Bluetooth)
- A Load Cell Amplifier (also Adafruit)
- 3x 1 kohm resistors (depending on the resistance in the load-cells)
- Salvage load-cell from Kitchen scale
Step 2: Wiring
Find yourself a prototyping breadboard, follow the load-cell instructions and connect the load cell to the Arduino.
To wire the load-cell to the amplifier you need to build a Wheatstone Bridge and replace one of the resistors with a load cell. The resistors resistance needs to be close to the strength of the load cell itself, so you need to measure the load cell. You will find that three cables are coming out of the load cell, this is an excellent opportunity to find which two we should use. Between two of them, you will find twice the amount of resistance than the other combinations. Choose one of the combinations that produce the lesser resistance (which does not matter as we are only using one load cell; you will notice later that your numbers may turn out negative, but that can easily be solved in code).
Then hook up the Wheatstone Bridge to your amplifier following the instructions from its manual.
Step 3: Code and Testing
Time to start coding. Find and install the drivers for the amplifier and your Bluetooth module. We decided to build a drinking game; you do whatever floats your boat.
Make sure you get nice readouts, you may have to implement some averaging of the values to hide stray measurements. We choose to present the results of the readouts on the app and with three colored LEDs.
You may have to return to this step ones everything is put together to insert the conversion into weight units if that is required for your app.
Step 4: 3D Print Skeleton
The load cell requires pressure in a specific way, design and 3D print a plastic skeleton in you favorite 3d modeling software.
Step 5: Fit Inside Coaster
Make sure you get proper readouts before gluing everything together.
Step 6: Calibration
If your app requires proper units, you will have to find known weights or use an already calibrated scale. Note that the amplifier should be linear but may behave differently under different load. This exercise is also good to get an idea of the accuracy of your setup.
A good way to visualize your values is taking deciliters of water and plot it on the scale as a deciliter of water is always 100 gram.
Step 7: Finished
Well, now you are done! Good luck, and thank you for taking an interest in making coasters smarter, future humans will thank you for helping us make the table coasters of the world smarter.