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Reactron collector: Kitchen scale

An inexpensive kitchen scale becomes a connected device

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Sometimes you want to monitor the weight of something remotely and wirelessly. But, you may also want or need to implement your own wireless protocol.

Many cheap electronic kitchen scales have an LCD display, which may be easily hacked to read the results. By just reading the LCD, you do not have to interface with the Wheatstone bridge sensor itself and provide amplification and scaling. For around $10, the scale already has that all built in, no need to replace it or build your own.

Adding a small Arduino clone (Moteino) and a HopeRF transceiver, it is possible to keep the scale awake so it does not reset itself, and then take and transmit the readings over wireless to any requesting system.

Hacking the LCD on a device is a really nice, non-invasive way to get a device’s information. Once you’ve done one LCD, all other LCDs are easy to do, with some adjustments. I use this technique a lot. I like it because it does not interfere with the device’s own designed behavior or circuitry, so that it can continue to function properly, while still allowing for data output that can then be used to turn a non-connected device into a connected one. Mine connects to my wireless Reactron network.

With the kitchen scale, you can get very precise readings (1 gram resolution) over a range up to 5 kg. This exact same hack can be applied to jewelers scales, which will have a tighter resolution (0.1 g or even 0.01g - advertised, but maybe not real) and will have a smaller range, typically < 1kg. A shipping scale may have resolutions in the 5 to 10 gram range, and a max capacity between 15 and 50 kg. Bathroom scales may have 20 gram precision and > 100 kg range. You can use this one hack to read them all, and choose the scale according to your application. In my case, I do a lot in the kitchen scale range, so this project will focus on that.

This scale originally took two AA batteries, but I put in a 3.7V LiPo so I could recharge it. I am keeping it awake a lot, and overriding its one minute idle timeout, so that it stays on and gives me continuous readings. Consequently I wanted a rechargeable setup, otherwise it would have resulted in a large number of spent batteries. The Arduino clone and the Hope RF radio both run at 3.3V, so that’s all an easy match.

I have three of these units currently operating. They are used for two purposes, but I have plans for more.

The first purpose was to measure the amount of water distillate precipitating from a Graham condenser. The distillate enters a beaker that has a limited capacity. I wanted the system to shut itself off before any overflow condition occurred. Ultimately I decided that this was important enough to add a second scale to weigh not only the beaker with the distilled water, but the first scale as well. That is, one scale on top of the other. Since I know the weight of the scale, it is just a constant, an offset. If either scale fails, a reading is available from the other. Just a little redundancy for something important.

The second purpose was to detect a cup for my coffeemaker, and determine if it was already filled or not. This prevents my coffeemaker from initiating an action resulting in overflow.

A further purpose, to be implemented in the future, is to detect the presence of cargo on material movers I am building, to increase the success rate of generalized product handoff from material processor (such as distiller or coffeemaker) to mover, and from mover to destination. It is important that my coffee gets to me - the machines must know when they have it securely, and detect emergency conditions like a spill.


I will put details of the build and the LCD hack itself in the project logs.

  • Before and after

    Kenji Larsen06/16/2014 at 03:08 0 comments

    The internals of the scale before modification:

    and after:

    And here is a stack of scales with the same conversion (these are the ones that have good vias to solder to):

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Syrum wrote 08/19/2014 at 00:02 point
I'd love to hear more details about this project since I'd like to do something bet similar.

  Are you sure? yes | no

Kenji Larsen wrote 08/19/2014 at 02:59 point
I will be adding a lot more detail to this and my other projects in the upcoming weeks, but for this particular one, I did an earlier writeup with more detailed information at

https://lowpowerlab.com/forum/index.php/topic,188.0.html

The hack uses the wonderfully engineered Moteino of LowPowerLab, and that level of interface is described there. Later, I added the Reactron structure (in software) and changed some of the RF details to allow this scale to communicate across all the nodes of my network. However, if you only need the info about the interface to the scale circuit board, that part may not be relevant. There may be enough detail in that post for you as is. I will clean it all up and post it here with the additional info at a later time.

By the way, the Moteino is now on Projects - check it out, I recommend it highly for hacks like these. http://hackaday.io/project/2197
There may be other aspects you may find useful.

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