A blog about random projects

Brief service announcement on TP4056 based Lithium Ion charging modules from AliExpress: You need to configure them to your target batteries capacity. This is done by changing resistor R3 on them accordingly. Check out https://www.best-microcontroller-projects.com/tp4056.html for details.

If you don’t, this can lead to thermal runaway (aka setting stuff on fire) caused by overcharging the cell.

Water filters are essential devices that help remove impurities from water, ensuring clean and safe drinking water. Over time, these filters will become saturated with contaminants, leading to reduced efficiency. However, through the process of regeneration, water filters can be revitalized and reused, extending their lifespan and minimizing waste. In this post, I will explore how to regenerate simple cation resin cartridges using dishwasher salt, a cost-effective and readily available solution.

Understanding Dishwasher Salt

Dishwasher salt, also known as water softener salt, is a specific type of salt designed for use in dishwashers equipped with water softening systems. Unlike table salt, dishwasher salt is primarily composed of pure sodium chloride without any additives like iodine or anti-caking agents. Its main purpose is to regenerate water softeners by removing calcium and magnesium ions, which cause water hardness. This same principle of ion exchange makes it a suitable option for regenerating cation resin cartridges used in water filters.

Cation Ion Exchange and How It Works

Cation resin cartridges are used in water filters to remove positively charged ions from water, such as calcium (Ca2+), magnesium (Mg2+), and certain heavy metals. The resin within these cartridges is composed of small polymer beads, which are negatively charged and referred to as “cation exchange resin.” These negatively charged beads attract and trap positively charged ions as water passes through the filter.

As the cation resin beads capture these ions over time, they become saturated and lose their ability to efficiently filter water. This is where the process of regeneration comes into play. By using dishwasher salt, which is rich in sodium ions (Na+), we can trigger an ion exchange process to recharge the cation resin.

Regenerating Cation Resin Cartridges with Dishwasher Salt

Here’s a step-by-step guide on how to regenerate water filters with cation resin cartridges using dishwasher salt:

Step 1: Prepare the salt solution Take dishwasher salt and dissolve it in water to create a concentrated salt solution. The correct salt-to-water ratio is not that important. As a rule of thumb, make sure the cartridge is fully covered with a layer of salt.

Step 2: Fill the pitcher with tap water – add more salt if necessary. Just make sure it’s always covered with salt. The water should always be fully saturated with salt.

Step 3: Allow sufficient time Let the cartridge soak in the salt solution for at least 20 minutes.

Step 4: Give the Cartridge another run with salt solution. You can use the same water as used before.

Step 5: Rinse the cartridge After the soaking period, clean the cartridge from the salt solution and rinse it thoroughly with clean water. Rinse 3 pitchers or so. This step helps eliminate any excess salt and ensures the filter is ready for use.

Conclusion

Water filters (may) play a crucial role in maintaining water quality necessary to brew good coffee and tea. By understanding how to regenerate cation resin cartridges using dishwasher salt, you can prolong the life of your water filter and reduce the impact on the environment through reduced waste. It also helps you saving costs – OEM cartridges are not cheap! Regular regeneration helps your water filter perform optimally, providing you with clean and safe drinking water for an extended period. I guess this process can be repeated 3 to 5 times per cartridge before the resin degrades to an unusable level.

I’ve read many articles on how to build IoT projects with Arduino – most of them covered the basics of micro controllers and how to use them in a more or less useful scenario in your home (“Build a self watering plant with an Arduino”). I’ve always missed the HOW part. That’s why I want to summarize my own experiences and mistakes made. I want to give you some tips on how to avoid problems building very quick prototypes to try a certain use case. See this article as a collection of thoughts and not as the definite truth.

In my understanding, Rapid Prototyping is the process of building a working “system” of some sort in relatively little time with the goal of experiencing how this system feels in a certain scenario and learn from this experience. The outcome of this process is not a product but knowledge about usability, haptics and possible problems in operation and manufacturing.

With that said, let me summarize some of my thoughts:

Put it in a case.

A case protects your electronic prototypes from external influences like water, sun and people/cats touching it.

Firmware

If it should be a quick prototype, try to avoid writing code as much as possible! I know, it’s fun – but try to avoid happy engineering. It’s sometimes quicker to use already existing software. For many automation tasks you can use esphome. ESPHome creates a custom firmware on demand, provides Over-the-air firmware updates, an API to access sensor data, simple preprocessing/filtering of data, and a rules engine to trigger events. It is possible to build robust, functional prototype firmware within minutes with this approach. All you have to do is writing a yaml file with your component configuration.

Use 3D printing – and Laser Cutting if you can

Getting started with 3D printing requires some training, but it’s worth it. The learning curve is steep and the results will benefit your project immediately. Operating a 3D printer is only half the story.

In many cases you want to design special parts for your project. You won’t find those 3D Models on Thingiverse; you’ll have to design it yourself. I find the Software Fusion360 the most useful.

Learning to design a simple, structural part in Fusion360 is relatively easy. Start with a Sketch and use the history function to modify dimensions / contours in the timeline.

The finished CAD Model goes straight into a slicer software. I like to use Cura for it’s simplicity. The slicer is creating machine code – instructions for the printers controller to turn on fans, heat beds, nozzles and move motors etc… (BTW: Great hacking potential – take a look what else you can do with g-code)

From there on, hit print and wait….

I hope this gave you some inspiration.

Have you been in the situation where you start an app and want to intercept network traffic because you are curious what messages are exchanged between the client and the API? So you spin up mitmproxy, charlesproxy, burpsuit or whatever and you see.. nothing?? Hmm, that might be because certificate pinning implemented in the client. Luckily there are some methods to bypass that. One method, that sometimes works for Android apps is to patch the app.

get it

My preferred way to get an APK from an Android device (for reverse engineering purposes etc) is this helpful bash script. No root required, adb installed, phone in developer mode.

appname=targetapp;
i=$(adb shell pm list packages | awk -F':' '{print $2}' | grep $appname);
adb pull "$(adb shell pm path $i | awk -F':' '{print $2}')";
mv base.apk $i.apk 2&> /dev/null;

decompile it

This steps generates smali code. Smali is an assembler for Dalvik Virtual Machine bytecode; The assembled dex (Dalvik executable) bytecode can be decompiled into smali code. That’s what we are doing now.

apktool d targetapp.apk

There is an interesting thread on smali on the xda-developer forum.

EDIT 1: For those who are interested, checkout the Dalvik bytecode reference: https://source.android.com/devices/tech/dalvik/dalvik-bytecode

patch it

Ok, now that we have the smali code, we can start browsing where the app checks the x.509 certificate. “checkClientTrusted” and “checkServerTrusted” are really good candidates. We patch those two functions to return before the actual check executes by adding “return-void” (line 453 and 467)

 

compile it

apktool b targetapp -o modfied_targetapp.apk

sign it

First, we generate a key and then we use jarsigner to sign the apk

keytool -genkey -v -keystore my-release-key.keystore -alias somealias -keyalg RSA -keysize 2048 -validity 10000

jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore my-release-key.keystore modified_targetapp.apk somealias

install it

adb install modified_targetapp.apk

 

If the app doesn’t crash ;-), you should be able to analyze the network traffic now. There are other methods available too. Take a look at FRIDA