Tag Archive for DIY

Build Your Own Data Logger – The Software, Telling the Logger to Log

790px-Kaffeetasse_Milchkaffee_Cafe-au-Lait_CoffeeOkay, with the arduino, shield, wiring and sensor complete, we’ve got our stuff together and can get our logger to log. To do that, we need to tell the arduino what it has to do. This is done with the arduino coding language. Now, what’s that and why do we need it?

As we have seen in the last part, an arduino itself is not very intelligent. To every ordinary person you can say “Would you be so kind and fetch me a cup of coffee?” and he or she will be able to execute that task without further ado, given he or she knows where the kitchen is and all necessary tools are available. If you want the same thing from a machine, you have to speak its language (or have a translator, which is called a “compiler”) and you have to think about the task you give it in a way as if that thing doesn’t know anything about this world. Which is, in fact, true for any machine. So, to stay in the example, to code a machine you have to say:

When hearing the command “Would you be so kind and fetch me a cup of coffee?” do the following:

1. Go to the kitchen
2. If the door is locked, open it to go into the kitchen
3. Go to cupboard
4. Open door of cupboard
5. Take out 1 cup
6. Close door of cupboard
7. Put cup under the coffee machine
8. Press first button
9. Wait until fluid has filled cup
10. Take cup in an upright position
11. Bring cup to person who spoke command

Silly, isn’t it? You would go mad with an assistant asking for such precise commands. That’s why some people find it hard to code – it’s very complicated to think so basic. But, anyway, we want our logger to log, so let’s take a look at the necessary code step by step (the complete code can be copied from our Quick Start Guide):

The part that is introduced by /* and ended by */ is a comment, something that is written for humans to read, not for the arduino to understand. Think of it like spelling out certain words so the kids don’t understand. Well, we never can be sure with kids and spelling, but we can be quite sure with /* */ and arduino.

You use this comments to make sure that another human being is able to understand what you wanted to achieve with a section of code. Chances are that human being is you because after some time you won’t remember why you coded some things that way. Comments are a part of good documentation, something we collections folk like, right?

Next up we have a couple of so called “libraries” we include in our code.

We have seen what an arduino mind needs to fetch you some coffee. Well, someone has already defined all the steps beginning with “1. Go to the kitchen…” in a library, so if you want your specific arduino assistant to be able to fetch coffee for you, you just have to write “#include <coffee.h>” at the beginning of your code and whenever you write “Would you be so kind and fetch me a cup of coffee?”, your assistant will be able to do all the necessary steps to bring you a nice, hot cup of coffee. It will also include what to do if the coffee machine is turned off, the water tank is empty, there is no coffee…

Now, I have to admit that I don’t understand all those libraries that are included here, of some I only know as much as that I do need them, and I know that I need them because I saw that they were used in some example codes. I think of it like when we need a conservator – of course we have to know which specific conservator we need, but we don’t have to fully understand what she or he does. Although, of course, the better we understand what she or he does, the more effectively we can cooperate.

For our logger we have included some libraries so it:

  • understands some functions you might need from the programming language C (stdlib.h)
  • knows how to handle time, that is, knowing that there are seconds, minutes, hours, days… (Time.h)
  • knows how to read the Real Time Clock on the logger board (DS1307RTC.h)
  • knows how to communicate with I2Cs (Wire.h)
  • understands what our sensor tells it (DHT.h)
  • knows how to communicate with peripheral devices like the SD card reader (SPI.h)
  • knows how to read from and write to a SD card (SD.h).

Next up we have to define where our sensor is and what type of sensor we use. The DHT library we included is able to handle DHT11, DHT21 and DHT22, so we have to specify that we connected a DHT22 and we connected it to our pin 9. The notations behind the “//” are again comments to be read by humans, not the arduino:

Next our sensor gets a name so we can order it to do something.

To keep things simple, we called it just “dht” in small letters, but we also could have called it “Walter”, “Gretchen” or “sensor1”. It’s only important that it is named consistently and that we are careful in using upper and lower case. For an arduino “Gretchen” is something other than “gretchen”, so the program won’t run if you make a mistake here.

The next line makes sure that we can communicate with the SD-Card although we used a logger shield. In the library, pin 4 is defined for a certain action, but this is already taken by the shield, so the arduino should use pin 10 instead.

So far, we have just made sure that our arduino knows what it needs to know. Next up we enter the “setup”. Think of it as your new assistant walking through the door. Before you can order her/him to do anything for you, you have to show her/him around. Where is the toilet? Where is the kitchen? Where is the coffee machine… This all will happen in the curly brackets after “void setup”.

Actually, the very first thing we do is we tell our imaginary assistant how s/he should communicate with us. Our arduino will be able to tell us what it does when it is connected to a computer using a thing called “serial communication”. It will be able to send information via the USB cable which we can read in the Serial Monitor of our arduino software. The line Serial.begin(9600) is like telling our assistant that s/he should communicate with us in English.

Next up, we tell our arduino that it should use pin 7 and 8 as output. This is where our two LEDs are connected, but our arduino only knows this if we tell it so. There are two possibilities for a pin: it can be an output or an input. If we define it as an output we can send signals to it that will do something with the thing that is connected to said pin. In our case, if we send that pin a “HIGH” signal it will switch the LED on, if we send it a “LOW” signal, it will turn it off.
If we define a pin as an input our arduino will “listen” to what happens on said pin instead. If the arduino receives a signal there, it can do something according to that signal. But in this case, we only need an output for our LEDs.

Next up, we do a couple of checks to see if our SD card works properly. It prints “Initializing SD card…” to the serial monitor so we can see it.
There is again a pin, pin 10, we define as an output, because our SD-Card-Reader needs this (we know this from the example code).

Now, the arduino checks if it can read the SD-Card. If it can’t read it, it sends a message to the serial monitor saying “Card failed, or not present”.
But “in the field” we won’t have an USB cable connected to a computer, only the logger itself. So we use our red LED on pin 7 to send us the same message. If the arduino doesn’t find the SD card reader or the SD card, it switches the red LED on for 5 seconds. In the arduino language this time span is given in milliseconds. So you see that we send the LED a “HIGH” signal, then wait (delay) for 5000 milliseconds until we send it a “LOW” signal to turn it off.
This is a mission for the Q-Tip: If the red light indicates that the SD card is missing or not properly inserted, you can put your SD card in the slot and press the q-tip, which reaches to the reset button inside the case to restart your arduino and try again.

If the arduino can read the SD-Card it will send the message “card initialized.” to the serial monitor. Next it sends “DHTxx test!”. Again, we have no idea if it can read the SD card, so if this is the case, we switch the green LED on pin 8 on for 5 seconds.

With the simple statement “dht.begin();” we tell our sensor that it should start reading.

Now our setup is finished, we can now tell our assistant what s/he shall do all day long. We do this in a function that is called “loop”. This function will repeat itself forever, if we don’t code anything that make it stop (or the plug is pulled).

What we want to do repeatedly is to read how high the humidty and the temperature in our room is, right? To read from our sensor, we call “dht.readHumidity” for the humidity and “dht.readTemperature” for the temperature.
If we want to use these values repeatedly in our code we use a thing called “variables”. A variable is something like a bag. We can store a value in it and carry it around. In our case we call our variables “h” for humidity and “t” for temperature. Bags come in all sorts and sizes, so do variables. You would choose your small, black handbag for a dinner invitation and your rucksack for your day trip so you always got the bag that fits your storage needs. Our sensor readings will come in a form like 14.5 or 34.8, they come as floating point numbers. So we choose the variable type “float” for it. There are a lot of other variable types, but for now, we just learn that “float” is the right type for our sensor values.
To sum up, the following lines of code store our sensor readings in the variables “h” and “t”. Whenever we call those variables in the fllowing parts of the code, they will repeat the sensor values.

But what will happen when the sensor returns something that isn’t a valid value for humidity or temperature? The next part of our code checks exactly this and reacts accordingly.

If either the value for humidity which we stored in “h” or temperature which we stored in “t” is not a number, the arduino will inform us by writing “Failed to read from DHT sensor!” on the serial monitor. The expression for something not being a valid number is called “isnan” (for IS Not A Number. Instead of writing “or” between the conditions, we have to use wording the arduino understands, which are the two upright strokes || (there are a few of these, like “and” which is &&, “greater than” which is > or “smaller than” which is < ).

Again, with a free standing logger we won’t see a message on our computer, so we let our red LED blink frantically if the sensor values are nonsense. There might be more elegant ways to code this, but I’m just a collections manager, not an IT professional.

Next up we print our values to the serial monitor for check, in case we have a computer connected. Right now you should be able to understand what happens:

Now, we need the time that comes from our Real Time Clock. Note that to use the Real Time Clock properly, you have to set it to the correct time first, using the example provided with the RTC library. Basically here we say “look at the clock and remember anything you saw in a variable called “tm”). We will be able to call the specific day, month, hour, minute, second… that way later in the code.

What follows next is perhaps a little weird to explain and to look at. We want to store our data on the SD card later, in a form that each data point is separated by a comma. That way, we can use any old spreadsheet software, import the data in a form that is called “comma separated values” (CSV) and process it further. The thing is, our data are numbers. You remember how we defined our sensor readings to be floats? Yep.

What we need to process it further is charcaters, in other words, we need a string. To be more exact, we need a string that incorporates all the data we want to be stored when we read our sensor. We want to have something that looks like this:
“34.8, 14.5, 2017, 04, 14, 2, 45, 23,” that we can have in our spreadsheet software to process a reading of 34.8 % relative humidity, 14.5 degrees Celsius on the 14th of April 2017 at 2:45 p.m (and 23 seconds).

To achieve this, we first open up an new bag called “dataString” to store our values in. I must admit I don’t get what line 116 really does, but it has something to do with defining the size that is available for our values.

What happens next is that we put all our values that we want to store into our “bag” called “dataString”. We do this one by one, as if we open up our bag, put the tape measure in, put the gloves in, put the lipstick in… The tricky thing is that whatever we take, we first need to convert something that is a number into a string. Hmmm… perhaps like if you want to put a fluid into your bag. You first have to put it into a container. Well, perhaps not exactly so, but along these lines.

So, we put our humidity value into a container. We call this container “stringH”. The function “dtostrf” does this with our variable h, which is, as we know a float number of our relative humidity reading. Then we put our container “stringH” into our bag “dataString”:

We said we wanted to have comma separated values in the end, so what we do have to do now is to add a comma. We take our “dataString” bag and put a comma in, useing “+=” as the order to do so. Here we go:

The same goes with our temperature reading:

What does our bag now contain? Something that looks like that: “34.8, 14.5, “. You can make sure by ordering your code to let you know on the serial monitor by adding this line:

We didn’t do that here. Instead we are putting in our bag one by one the values for the day, month, year, hour, minute and second of the reading, each separated by a comma. Note: I later discovered that I wouldn’t have had to separate them all with a comma, but we will discuss this later in the series. For now, we just know it works.

Whew, that’s a lot of code. Our “bag” dataString now looks like that: “34.8, 14.5, 2017, 04, 14, 2, 45, 23,”. Next up, we want to write it to our SD card. To do that, we have to open the file the string should be written to:

If the arduino finds the file called “Mylogger.csv” on the card, it openes it, writes the content of dataString to it (at the end of all other data that is already stored there) and closes the file again. Mission accomplished!

What’s great about this is that if there is no file called “Mylogger.csv” on the SD card, the arduino will automatically create it. Only in the occasions where there is such a file, but it can’t be opened or if the SD card is missing, we will need an error coding which informs us on the serial monitor and lights up the red LED until the next loop:

Finally, we have to define how long the arduino should wait between measurements. The more often you read, the more data you get, which is a plus in detail, but also needs more storage space. In our example, we wait 5 minutes between readings, which are 300000 milliseconds. For 10 minutes, set it to 600000 milliseconds and so forth.

That’s it, that’s the whole code. There is, of course, room for improvements, for example if you need the temperature in Fahrenheit or want to calculate the dew point. But this will be another part of the series…

Read the other posts for this project:

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Build Your Own Data Logger – Arduino, the Datalogger-Shield and the Wiring

Sorry for the delay continuing this series. Actually, part of the delay had to do with developing an hands-on for explaining how gears work on a bicycle together with a colleague, using an arduino…

So, the basis of the logger is an arduino, which we presented lately as the “thing that can do things“. There are several arduino platforms depending on what you want to do. For our project we chose an arduino uno:
Photo by Clic17 via Wikipedia, CC-BY-SA 4.0Photo by Clic17 via Wikipedia, CC-BY-SA 4.0

Now, an arduino uno can do much more than just reading sensor data like we want to do, but it’s rather easy to make projects real without having to solder anything, it comes with a quite easy to understand coding language and there is a broad international community experimenting with it, so you probably find answers to your questions at the official arduino forum https://forum.arduino.cc/ or somewhere else on the web.

To work with an arduino you will connect the sensor and other devices you need to the ports of the arduino. You got digital ports (0 to 13), analog ports (A0 to A5) and some ports for connecting to power (marked 3.3 V, 5 V and GND). We take a close look on what we connect how later on.

First, we need another useful device, a data logger shield. So, what is a shield you ask? Well, as your arduino comes out of the box, it is a generalist. It can do anything, from showing the bike speed to logging your climate data. You connect it to all the parts you need for your specific project. Depending on the project, this can be a lot of wires, resistors, capacitors, switches… So, for some specific tasks you need for certain projects some fine people have developed boards that have everything you need on them and which you can connect to the arduino by simply “clicking” it on. So, the shield sits on the arduino like a backpack, is ready to perform certain tasks and you still have a reasonable amount of ports for your specific task, although some ports are now used for the connection.

And what is a data logger shield? Well, look here:

Taken from Adafruit website https://www.adafruit.com/product/1141

Taken from Adafruit website https://www.adafruit.com/product/1141


A data logger shield has two very useful devices: a SD card reader to log your data for further use and a real time clock (RTC). We need a real time clock because an arduino is incredibly dumb. When we tell it what time it is, it just remembers it as long as it got power. When we disconnect it from the power source it immediately assumes that it’s 0 o’ clock. The RTC has a small battery on board that safeguards the arduino remembers the time even when disconnected.

When we clicked the data logger shield to the arduino we can start wiring our logger. We can do that by soldering the wires directly to the shield, which comes with a convenient hole matrix for this task, or we can use a little breadboard and breadboard cables to do the wiring like in our example. For long-term use, I’d prefer soldering because it is less vulnerable to bad contacts. You can see the exact wiring in this scheme:

logger

The data pin of the sensor is connected to digital port 9, the green LED over a 100 Ohm resistor to digital port 8, the red LED over a 200 Ohm resistor to digital port 7. The “buckled (short) legs” of the LEDs, which are the minus pole, and the ground pin of the DHT22 are connected to the GND port. The power pin of the DHT22 and the data pin over a 10 K Ohm resistor are connected to the 5 V power port.

For our logger, we mounted the breadboard with the sensor and the two LEDs on the outside of the casing, while arduino and shield are inside of the casing. You can see the breadboard wires being connected from the inside to the outside:

DatLog_2

Finally, we need something that allows us to push the reset button in case we want to restart the arduino. In our case we used a q-tip with a broad plastic peg on one end.

Next up, we will take a look at the coding for our logger.

Angela Kipp

Read the other posts for this project:

This post is also available in Russian translated by Helena Tomashevskaya.

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Build Your Own Data Logger – The Sensor, Heart of the Logger

The first thing that can do things that actually DOES things for the TECHNOSEUM: A data logger that records the climate in a certain area of our museum.

The “Q-Tip-Logger”, a simple logger that writes temperature and humidity to a SD card

Quite to my surprise the last post about building a data logger caused quite a discussion. As I mentioned, this is not concurring with professional solutions. It’s an alternative for people who like to build things or who are searching for ideas for projects when cooperating with schools and STEM classes. For someone who isn’t keen on learning something new, maybe a bit unusual and building something, this project is not interesting. For someone who has a sufficient collections care budget to buy professional loggers and send them in for calibration regularly it might be interesting anyway to compare the professional loggers with the DIY versions. For someone with a collections management budget next to zero who sees a chance to find sponsors for buying the parts if he or she initializes a cooperation project with a school group, it opens up the possibility to have an alternative to the cheap loggers from the hardware store and spark interest for collections care in young people at the same time.

The critical questions – which I really appreciated – made me switch the order of this guide: instead of starting with the main parts, the arduino and the data logger shield, I start with the very heart of the logger: the sensor.

The sensor is the part that decides how good or bad the logger is suited for its purpose. No matter how careful you are building your project, if the sensor is bad, the results will not be satisfying. On the other hand, good sensors have their price. Like often it’s up to you to weigh the pro’s and con’s and decide how good your sensor has to be for your purpose. What helped me a lot were the tests conducted by Robert Smith with the most used sensors in the hobbyist sector, the DHT11, DHT22 and SHT71: http://www.kandrsmith.org/RJS/Misc/Hygrometers/calib_dht22_dht11_sht71.html. This comparison, together with his previous analysis of six DHT22 sensors made me use the DHT22/AM2302 which measures temperature and relative humidity for my projects.

Der DHT22, auch unter der Bezeichnung AM2302 zu finden.

The DHT22, also named AM2302.

Under real testing conditions it doesn’t reach the +/- 2% accuracy in measuring relative humidity mentioned in the data sheet on page 3 (which would have been a real surprise as only really high-prized loggers reach that accuracy in real life) but they bring good enough value for the money invested in my opinion. It can be found in electronic stores, in the adafruit store or on ebay and is easy to integrate into a project.

Sensors are generally not built for eternity. That’s why you have to send in the professional loggers for calibration. Fortunately, there is a method for testing loggers for accuracy by using saturated salt solutions described by Samantha Alderson and Rachael Perkins on the website of Connecting to Collections Care: http://www.connectingtocollections.org/wp-content/uploads/2012/03/Calibration-using-saturated-salt-solutions.pdf
That’s what you should do with your homebrew loggers, too. You can even built a little testing device so you only have to expose the sensors to the test, not the whole logger.

Before you let your logger log the first time you should check it against a reliable professional device. In two occasions I found linear differences between my own logger and the other device (for example one was always 1% below the reference device), which I could correct in the software. Then it’s important like I already mentioned to check your sensor regularly. If the sensor is not reliable anymore you should replace it. That’s where the price of about 9 USD comes in handy.

So much for the sensor. The next part will be about the arduino and the logger shield.

Angela Kipp

Read the other posts for this project:

This post is also available in Russian translated by Helena Tomashevskaya.

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Build Your Own Data Logger – Quick Start Guide

The first thing that can do things that actually DOES things for the TECHNOSEUM: A data logger that records the climate in a certain area of our museum.

The “Q-Tip-Logger”, a simple data logger that logs temperature and humidity to a SD card.

Like I promised before, here I show you how to build a data logger with the arduino. I want to do this step-by-step so you can follow the development and considerations that led to “our” logger. Many considerations had to do with the necessities of everyday museum work and so they might be of interest even if you don’t plan to build your own logger but want to hand the project over to your STEM group or your museum’s DIY club. For the impatient reader who has already did some building or raises a technology nerd, here comes the parts list, wiring scheme and code and some considerations for a good head start.

First of all: of course there are commercial products for this task and I don’t want to concur with those. But often I see that colleagues can’t afford the professinal solutions and therefore buy cheaper loggers that are not suitable for the task. Here, building it yourself can be an interesting alternative. Because it’s you who decides on which components to use you are the one who decides on the quality. And because you built it yourself you can maintain and repair it yourself.

Another remark: Wiring and code worked well in our setup but I can’t take responsibility for any damage that is done because you built your logger according to this guide.

What the device does:
This logger logs the current temperature in degrees Celsius and the relative humidity in percentage, then writes this data along with the date and the time as comma separated values to the file MyLogger.csv. This file can be imported into a spreadsheet software for further processing.

Needed Components:

  • 1 Arduino Uno
  • 1 Logging Recorder Shield with SD-Card reader and clock
  • 1 Temperature and humidity sensor (used here DHT 22 / AM 2302)
  • 1 Minibreadboard
  • 1 LED red
  • 1 LED green
  • 1 resistor 200 ohm
  • 1 resistor 100 ohm
  • 1 resistor 10 kiloohm
  • Breadboard cables
  • 1 SD-Card (because only small data is stored this is quite deliberate, I used a 2 GB card)
  • 1 power adapter (230 V AC to 9 V DC)
  • Case according to personal preferences

Wiring scheme:

logger

Arduino Code:

The code is based mainly on examples provided with the libraries used. It might be that I have drawn certain solutions from the web and haven’t noted the source. Should someone discover his/her code used here, please drop me a line so I can give propper credit.

The Casing:
Unleash your creativity here. I used a cardboard box that was roughly the right size and cut openings for the cables and the SD Card into it with a cutter. The only really important thing is that you can operate the reset button on the shield. You can spare it in the casing, we used a q-tip with a broad plastic peg on one end so the reset button is pressed reliably.

Improvements:
By looking close you realize that this project was an emergency response, designed to be quickly available. So, there is room for a couple of improvements.
Instead of the mini breadboard it is logical to solder the components to the logger shield wihich has already a built in prototype board. The only thing to think about is that the sensor has to be outside of the casing because the outside climate should be monitored.
Then, the logger remains comparably numb. To know about your climate, you have to read the SD card or plug in your laptop ans see what the serial monitor tells you. A LCD shield would be a comfortable solution to this issue – and you could skip the LEDs and print the error messages on that LCD monitor.
Last but not least a LAN or WiFi shield could send the data to the web.

Have fun experimenting!
Angela Kipp

Read the other posts for this project:

This post is also available in Russian translated by Helena Tomashevskaya.

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A thing that can do things – Taking a look at the Arduino from the perspective of a collections specialist

An Arduino with a LAN shield - a thing that can do MANY things

An Arduino with a LAN shield – a thing that can do MANY things

Being the collections manager for a Science and Technology Museum has a whole range of downsides. For example, you never, ever get big industrial storage halls so tight that dust and pests aren’t an issue, people think you are crazy when you insist on various security systems and archival packaging for “old junk” and one of your duties is to explain that, no, you don’t sell spare parts for vintage cars or old radios. But sometimes it has its advantages, for example that you are much closer to all those techy, nerdy things that happen “out there” (Because, yes, there IS a world outside of the museum, I know people who have been there).
One day a colleague showed me a little blue thing that was blinking frantically.

“What’s that?” I asked.
“An Arduino”, he replied.
“What’s an Arduino?”
“It’s an amazing little thing! It’s a thing that can do things. It can do everything!”

Basically, when he showed it to me all it could do was blink a little red LED. But as I was digging deeper I discovered that there is a whole community of makers out there who do amazing projects with that little thing. You can really do everything, from reading sensor data to steering an electric motor and everything in between. When I saw that someone had realized a game of Tetris inside of a pumpkin (https://www.youtube.com/watch?v=8PCp5xk-9Qo) I was sold. I needed such a thing.

Actually, if you look sharp at the picture on the post for our second Registrar Trek Birthday 2015 you get the idea when I first made contact with this thing...

Actually, if you look sharp at the picture on the post for our second Registrar Trek Birthday 2015 you get the idea when I first made contact with this thing…

Granted, the moments when a registrar or collections manager is in urgent need of a tetris in a pumpkin are rather rare. But for a whole range of tasks it would be useful and – compared to a computer game in a pumpkin – not too hard to put into action.

How about an alarm when it’s much too bright in a gallery and someone should close those curtains? Or a data logger that writes temperature and humidity in your off-site storage to a SD card? If there is LAN or even WiFi available, it’s starting to get really interesting as you can monitor the climate via the internet and even get alerts on twitter or in your mail when someone turns on the light or a climate value exceeds a certain level.

The big advantage of the arduino is that you can do those projects yourself and at a reasonable price. It requires that you familiarize with the topic, but, compared to former times, you don’t have to be an expert in electronics to do it. The few necessary components are available via the internet and thanks to a large, world-wide community that is committed to the spirit of Open Source you find a solution or even complete code to nearly every problem which you can adapt to your own situation with a little thinking and a few changes.

The first thing that can do things that actually DOES things for the TECHNOSEUM: A data logger that records the climate in a certain area of our museum.

The first thing that can do things that actually DOES things for the TECHNOSEUM: A data logger that records the climate in a certain area of our museum.

Recently I have experimented a lot with this “thing that can do things”, so I plan to use this blog to present some of my projects that have to do with museum work every now and then for you to have a look and maybe try yourself. Our readers who are not so much into technology will hopefully forgive me. One or the other might even feel inspired to have a closer look into the world of microcontrollers…

For a start I suggest the starter kits who contain not only an arduino but a whole bunch of useful accessories like resistors, sensors and LEDs so you can experiment right there and then. There’s nothing more frustrating than missing one little component when you start learning and who has a fully equipped electronics workshop at home? Usually those kits come with some instructions for simple experiments (if they don’t, you can find plenty of them on youtube) that I highly recommend to conduct. Along with a general understanding of the topic you explore what can be done, may it be a tetris in a pumpkin or a data logger.

And there is one thing I can promise: when you manage that a little red LED is blinking exactly how you planned it for the first time, you get a feeling like you just discovered new territory…

This post is also available in Russian translated by Helena Tomashevskaya.

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