S.H.E.L.F. - Dual load cell and Board - Pi Chef Design Challenge - Blog post #8

Hi guys, this will be a short update blog, I'll be showing the small board i soldered up for the amplifiers and other stuff that may be needed in the project as well as the way I am using 2 load cells per amplifier. The plan for the full shelf is to use 4 load cells in total, 2 per shelf, as mentioned in the previous blogs one part will be aimed towards lighter stuff like spices and such, while the other one will be for the bigger and bulkier stuff. I can always go with 4 hx711 amplifiers, but that would mean that there will be 4 less GPIO pins available (this may not be that big of a deal in this project, but I want to keep everything to a minimum). The ones I am using are dual channel (as most of those I saw online are, I don't know if all are or if there are single channel ones, I didn't research that), this is how I connected them to the amplifier:

• E+ and the red wires from both load cells
• E- and the black wires from both load cells
• A-  green wire from one of the load cells
• A+ white wire from the same load cell as A-
  
• B- green wire from the other load cell
• B+ white wire from the same load cell

This of course meant that some changes needed to be made, the library I used was tatobari's hx711 library, about which I talked about in the previous blog (S.H.E.L.F. - The one item shelf - Pi Chef Design Challenge - Blog post #7 ), I made minimal changes to the code to get readings from the other load cell when they are connected like this. Here is how (this is an addition to the last blogs code):

hx2 = HX711(5, 6)
hx2.set_gain(32)

The rest of the code is exactly the same as in the previous blog, just we have to double every command, and also calibrate the second load cell in the same way we did with the first one. For the second part of this update I'll be talking about the small board i soldered up, it is half finished for now, it is missing the underside connections and the power supply jack. With this board I wanted to have a clean way to connect the sensors with the amplifiers as well as have direct power output from the power supply directly rather than from the Raspberry and a power switch. Some of the other thing I did is I left a lot of the pins empty as well as easy to connect to other things to be possible to easily upgrade, the hx711 are easily removable, if I ever need them in a hurry for another project, and there are two USB ports which can be used for Raspberry/Arduino. Here are the pictures of the components as well as of the (half) finished product:

Since the deadline is getting ever so close, I think it's a good time to do a kind of a recap of what has been done for now, and what is left to be done until the end of the project. Let's start with the plan of the project itself, my goal was (and still is) to make a simple, smart, low cost device which would using load cells, a Raspberry and a camera as main components monitor the levels of ingredients on the shelf, while it sends all of that data online, so it can be read using a phone, tablet, computer or anything else. The basic idea is scanning the label on top of the object when it is placed on the shelf and updating the system then, these are the things that have been done for now.

1. A base plan and a 3D model of the shelf
2. Getting to know OpenCV using the Raspberry and PiCamera module
3. Making of the real life model, which will be used in the prototypes and possibly in the final stage of the project
4. Getting the sensors to work with the amplifiers and the Raspberry
5. Designing the labels
6. Making and testing the label detection program
7. Adding real world elements like a buzzer and lights
8. Getting the label detection and the load cell to work together in the one item shelf

Now for what is left, there are some steps that are a mere days work away, but also some which will need a little bit more time, but nevertheless they will be fun to do.

1. Uploading data online
2. Making the first prototype
3. Troubleshooting
4. Making the full shelf
5. User end (the way the user views the data)
6. Final product

Of course in every of these points there are more things hidden but there are the main things that are left, for the next blog I will hopefully have the first prototype, a multi item shelf which stores and uploads data. Thanks for reading the whole blog, hope you liked it!

Milos