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How to Automate the Turn-On Time of a Deco Light

I own a decorative light for the winter which I controlled using a simple time switch in the past. The time of dusk is continually shifting, so I had to adjust the turn-on time multiple times each year.

This year, I planned to automate the turn-on time. I tried to find a light controlled switch, which automatically enables the decoration on dusk and disable it on dawn. I saw no suitable one.

I ended adding an own switching circuit to the decoration. This article describes how to build an own controller as I did and highlights a few problems you may encounter.

The Issues with Off-the-Shelf Solutions

My requirements for the off-the-shelf solutions were:

  • Setting to adjust the light level.
  • Suppression/identify the light from the decoration.
  • Turn off at a given time or dawn.
  • It shall work at a temperature of -10ºC.

Sadly, I found no simple device, which matched all my requirements. I also noticed, most of these devices had horrible ratings and reviews – mainly because they lacked the described features.

Adjusting the light level seems a straight forward requirement. The brightness, where you like to turn the decoration on is a matter of personal taste. Most devices I found do not let you adjust this.

If the decoration is turned on, it will naturally get brighter outside. If the switch does not compensate for this fact, the light will start to blink. In this case, you had to place the sensor somewhere carefully it is not affected by the light of the decoration.

Most devices just let you select a duration, how long the light stays on. This is the cheapest way to implement a device like this. It makes not much sense though. Because the time of the dusk is moving, the time when the light turns off shifts as well. Useful is either turning the light off at dawn, or at a fixed time.

Then, there I found one device which had all the previous features, but it was rated to indoor use only. The minimum working temperature was 5ºC.

Initial Situation

The initial situation is shown in the following illustration:

A timer connects to the mains plug. The mains adapter of the decoration connects to this timer and converts the 240V AC power to 24V AC. This AC voltage is somehow a standard way to power old, filament lamp based decorations.

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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

How to Design a Cheap Plant Watering Sensor (Part 5)

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

The fourth part ended with step 20, where I did usability tests and stability tests using the preliminary firmware. This article will focus on designing the final board for the project.

Step 21: Design the Final Board

Designing a good board is like one of these puzzles with quadratic tiles, where you try to lay down a 3✕3 set where all edges match. Often a small change result in many follow up changes, so you have to rip-up a lot of routes and design them in a new way.

My goals for the board were:

  • Everything, except the two LEDs, should go to the top side of the board.
  • Reduce the amount of vias to the absolute minimum.
  • Create a ground pour, especially around the oscillator part, to reduce noise.
  • Move the button as far as possible from the oscillator to minimise the influence if the user presses the button.
  • Make it as small as possible.

I worked with small iterations, checking the design after each iteration and checked the design against my goals. To keep track of the changes, I versioned each larger iteration. This way I could go back at a later stage for comparison or if a change did not turn out well.

The Tools

I worked with Autodesk Eagle to create the board. This tool is in the current state far from perfect, but it is cheap and has all required features for the task. For me personally, these are the features I need to design a board:

  • Smart routing editor which is linked to the schema.
  • Quick and easy way to create vias and see the required connections.
  • Good library support for symbols and packages.
  • Design rule checks.
  • Quick board preview to check label placement and design.

I described some issues of Eagle in this post:

12 Time Wasting Issues in Autodesk EAGLE

As you can see, these are not very advanced features and are supported by almost all good board editors. I never use the auto router, because I do not have time pressure or have to do repetitive tasks. Continue Reading

How to Design a Cheap Plant Watering Sensor (Part 3)

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.

lucky-resistor-1 Continue Reading

How to Design a Cheap Plant Watering Sensor (Part 2)

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.

lucky-resistor-1

Just running, doing the measurement of the oscillator, the second part of the circuit uses at maximum ~1.2mA. This measurement phase should be as short as possible. Later we will analyse the timing. Continue Reading

How to Design a Cheap Plant Watering Sensor (Part 1)

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:

Step 1: Define the Expectations and Goals

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.

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How to use the Scripts in the Cat Protector Project

It seems the cat protector project documentation has a small gap. There is just a hint that you have to use some scripts to prepare the SD card to play the audio files, but no details about this.

To use the scripts you need some knowledge how to use a command line interface on your operating system. The scripts are very simple to use on Linux and Mac OS X, but on Windows it is very tricky. I strongly advice you to use either Linux or Mac OS X. If you are working on Windows, just use a free virtual machine application and install a Linux (I suggest Ubuntu) or a Linux live CD where you don’t have to install anything.

Why? …. Why … that … complicated

The used microcontroller is not very fast, if the software on the microcontroller also has to deal with a complicated filesystem, no sequential blocks and other obstacles, changes are small to produce sound output in a good quality. Therefore I prepare the samples in a very simple format and instead of a complicated file system, I just store the blocks with the audio data sequentially onto the SD card.

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