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How to Design a Cheap Plant Watering Sensor (Part 6)

This is the sixth part of the meta-tutorial, where I talk about designing a cheap plant watering sensor. If you did not already read the firstsecondthirdfourth and fifth part please do it now. These parts contain a lot information which lead to this point of the tutorial.

The fifth part ended with step 24, where I talked about calculating the total bill of materials. This part will focus on preproduction of a small batch of sensors to solve some final details.

Just note, I obviously do not follow these steps in a perfect sequential way. Often I start with some tasks earlier and things are running in parallel. There are various dependencies and it would make no sense to wait with some task just to follow a strict sequence. 🙂

If you follow my blog you may already read some details about ongoing tasks. I will just briefly talk about them in this article. You will find more details in the other blog posts.

Step 25: Build an Alpha Series

Everything looks very promising, so its time to build a small batch of the final devices to see if they work as expected. This is also a test to see how a larger number of these devices can be produced and what kind of tools are needed for this task.

Order the Components

First I order the components. This is very important, because the availability of electronic components changes all the time. It is nice to have all required components, so you can order the boards with the correct footprints. If you order the boards first and are unlucky, an important component is suddenly unavailable and you have lots of boards with wrong component footprints on it.

The components for the plant sensor are really cheap, so there is no huge risk. Even it turns out a huge issue requires a component change – it will be a small loss. SMD components also do not take a lot of space, I can easily store all of them in a very small box.  Continue Reading

Successful Measurements

Using the new method of measurement, described in this post, I could successfully collect some meaningful data. This time, the read values are the exact values of the final sensor without a different kind of oscillator.

I watered the plant at day zero with quite a great amount of water. From there you can see how the frequency slowly rises, while the soil in the flower pot starts to get dry. There is a small measurement error between day two and three. Here I had a short power loss and no data was recorded which resulted in some zero records. Continue Reading

Plant Watering Sensor – Long Term Tests Take #2

I started a second take on the long term tests for the plant watering sensor. This tests are required to be sure, the measurements follow the expected cycles. After watering the sensor, the frequency should go down and while the soil is drying up, the frequency should go up the the initial value.

Logging this measurements is very important to get a good overview of the measurements and be sure if every aspect of the device works as expected. At this point, I especially test the sealing of the foot part of the sensor. If it keeps completely sealed against water, I should get very consistent and repeatable readings.


The curve on the right side shows the measurements of the last 48 hours. These small variations are normal and are most likely caused by the plant itself or because of temperature changes of the board.

If you have questions, miss some information or just have any feedback, feel free to add a comment below.

Plastic Cup Meltdown – Working with Epoxy

Currently I am working on the coating for the foot part of the plant watering sensor. Here I already tried a wide range of techniques and materials. At the moment, epoxy seems the perfect material choice – so I am trying different resins and hardeners to get the best results.

Some hardeners are very reactive and produce a very strong exothermal reaction. While I read and prepared everything meticulously for a new process as usual, I still get sometimes very bad surprises.

For the process, I dip the foot parts into the epoxy resin and hang them up for drying. To waste as few as possible epoxy resin, I used very small plastic cups for dipping.

The exact material of this plastic cups is very important I learned. You should never use something which will react violently with the mixed epoxy resin, as you can see in the next picture.


The cup literally started burning after a few minutes and I had to drop it outside on the forecourt to prevent any disaster. In the picture you can see the remains of the process: A beautiful frozen epoxy block, in the middle of the melt down. The cold air outside rapidly cooled down the process, so the burning stopped.

Mental notes for the next experiments:

  • Use only cups where you exactly know the material.
  • Keep a stop watch running with the pot time, assume 20% shorter time as specified.
  • Prepare an evacuation/meltdown plan. 😉

Plant Watering Sensor Foot Parts from Eurocircuits

I received the foot parts of the plant watering sensor from Eurocircuits. As you can see, they have the same great quality as the head parts.

Eurocircuits removes any bridges from the boards in a very clean way, so you get the boards with exactly the shape you designed. This saves a lot of work and add to the beauty of the boards.


No Success with First Long Term Measurement

The first long term measurement I made, to test the behaviour of the sensor over a longer time range was a failure. After the five days with the device introduced in this post, the readings made absolute no sense.


The sensor was not moved in the flower pot and the plant was once watered at the begin of the measurement. While it looked promising at the begin, the frequency suddenly went down again, which was very irritating. I am still investigating how this could happen.

To get closer to the real measurement of the final plant watering sensor, I started a new approach.


I soldered a header to one of the LED pads on a fully assembled plant sensor. Next I changed the device for the measurements.


Continue Reading

Eurocircuits Delivered Astonishing Perfect Panels

I ordered a single panel as a test for the production version of the plant watering sensor and I got astonishing perfect panels from Eurocircuits.


Even I only ordered just one, I got three perfect panels in gold. Thank you very much Eurocircuits! As you can see from the photos, the quality of the panels is just amazing.


This panels use the Eurocircuits registration system. This is a own set of rules for the registration holes around of the panel. Using these registration holes, I can easily align the stencil and the board on a special tool they produce. I will tell more about that in one of the next parts of the series “How to design a cheap plant watering sensor” (Read Part 1, 2, 3, 4, 5)

If you have questions, miss some information or just have any feedback, feel free to add a comment below.

Weller WT Solder Station – Great Tool with Great Issues

A little bit more than a half year ago I bough a Weller WT solder station. This solder station has a very smart design and there are many features I like.


  • The solder pencil is very small, but has exactly the right weight and grip to solder for a long time without fatigue.
  • The cable between the station and the pencil is very flexible, soft and never gets tangled magically.
  • The tip heats up to working temperature in seconds.
  • After a programmed time, the station goes into power saving mode and automatically wakes up as soon you touch the solder pencil again.
  • It has a large nice display, just showing the info you actually like to know.
  • Even digital, temperature can be quickly and precisely changed.


  • The solder pencil is very modular, you can actually have multiple heat elements with different tips and change them in the middle of the work.
  • The tips are small and relatively cheap, but from a high quality.
  • The stand is heavy and does not slide away. You can switch between brass wool and a sponge if you like.

Yet, there are two annoying issues…

Continue Reading

Soldering Boldport Club project “The Matrix”

For the Boldport Club project “The Matrix” I used hot air soldering for the first time. After some experimenting and with help from Boldport Club members, it worked really well. I could colder the 120 LEDs in less than 10 minutes to the board.

I made a video about the process. Please keep in mind, I am new with this process, so this is absolutely no instruction video how things should be done. I may be absolutely wrong. 😀 Enjoy!

Here a list of the objects and tools you can see in the video:

If you have questions, miss some information or just have any feedback, feel free to add a comment below.

Plant Watering Sensor – Long Term Logging

To gather more long-term measurements for the capacitive method I use for my plant watering sensor, I created this small logging device. As you can see, it uses one of the plant watering sensor prototypes for the measurements. Instead of using the ATtiny13A on the board, it passes the oscillator signal directly to the microprocessor of the logging device.

In front there is a very small 128×32 OLED display, where I can see the current measured frequency in kHz. On top, the current time and date is visible, and on the right there is a graph where I can see the values from the last 48 hours graphically. There is not much visible in the graph, because I took the photo just after installing the sensor.

Every minute, the current average of measurements is stored in a CSV file on a SD card. After a few weeks I should be able to analyse this file and see the results. Here I am especially interested in the cycles from watering the plant until the soil got dry again. Continue Reading

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