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 first, second, third, fourth 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.
This is the third part of the meta-tutorial, where I talk about designing a cheap plant watering sensor. If you did not already read the first and second part, please do it now. These parts contain a lot information which lead to this point of the tutorial.
The second part ended with step 14, designing a first prototype PCB. So let us start with the next steps in this journey. This article will be the smooth transition from prototyping to the initial planing for a final design.
Step 15: Assemble and Check the Prototype
After receiving the prototype PCBs from OSH Park, I assemble one completely, including the cable and with one of the sensor plate prototypes as foot part.
Set the Fuses of the Microcontroller
The microcontroller ATtiny13A requires programming using SPI before it can be soldered to the board. There are special bits in the memory, called “fuses”, which control very basic settings of the chip. One of this fuse controls if the chip can be programmed and debugged via the debugWire protocol. This protocol just uses one single wire to program and debug the chip, bus has to be enabled first.
So I put the microcontroller into the programming adapter and connect everything via the Atmel ICE to the computer.
This is the second part of the meta-tutorial, where I talk about designing a cheap plant watering sensor. If you did not already read the first part, please do it now. It contains a lot information about constraints and decisions made which lead to this point.
The first part ended with step 11, building a working prototype with the selected key components. So let us start with the next steps in this journey.
Step 12: Analyse and Measure the Prototype
Never forget why you actually built a prototype. It is your tool to verify all assumptions you made in the design phase. To do this you need the right measuring instruments.
The Power Usage
I start measuring the current of the circuit. This will show if my assumptions about the battery life will be true. For this test I use a multimeter which has a good resolution measuring in the µA range. The multimeter I use is the Testo 760-3 which is not a very well known brand. Multimeters are usually really poor at measuring low currents on low voltages, so let us see if this will work.
I also use a Fluke 114, but this one has no current measurement. It is sometimes very handy to have two multimeters, one to measure the voltage and a second one to measure the current.
For the first test I program the MCU to do all the tests in a loop and connect the power directly to the second part of the circuit. Now the power is always on and I can measure the current used by the MCU while doing the measurement.
In this article I will talk about how I designed a cheap plant watering sensor. My goal is some kind of meta tutorial, where you can see the steps involved from the initial idea to the final sensor. If you ever planed to create a own device, I hope this article give you some inspiration to start your own project soon.
Why a Plant Watering Sensor?
I have a couple of plants in flowerpots and this plants not only like some light, they also need water from time to time. Watering this plants is something I often forget, with sad results. There are ready made solutions for this, but I have some objections with all of them. To be clear: There are really smart products out there – it is absolutely nothing wrong with them. It is just as I like to build my own fan controller, I like to build my own plant watering sensor in my very own fashion.
Here a list of already existing projects and devices I own or checked out:
After deciding to create a own plant watering sensor, I spend some days to think about my expectations and goals for this sensor.
For my personal case, I like to put a small sensor in each flowerpot. There will be five and more pots, therefore that number of sensors are required. A single sensor should be really cheap, so I can distribute as many of them as I like. Battery life should be at least one year, better two years. I collected all this thoughts into the following list:
Cheap: Ideally less than €5 including the PCB.
Visual Signal: A flashing LED, simple to notice but easy to ignore.
Simple: Easy and simple to build with few components.
Beautiful: It should ideally look like a decoration.
Long Battery Life: The battery should last at least 1 year or longer.
Small: The sensor should be almost invisible from far away.
Reliable: The measurement should be reliable and the sensor must not corrode or degrade over years.
Safe: The sensor must be safe for the plant and environment.
Low Battery Indicator: The sensor should detect if the battery is at end of life and signal this.