Arduino is fantastic as an intermediary between your computer and a raw electronic circuit. Using the serial interface, you can retrieve information from sensors attached to your Arduino. (You can also send information via the serial interface to actuate circuits and devices (LEDs, relays, servos, etc.) connected to your Arduino.) Once you have the data in your computer, you can do all sorts of things with it – analyze it, display it, or share it on the internet, for instance.

In this post, I will be reading and displaying analog data from a pair of LDRs connected to an Arduino. Here is the schematic:

Here is how you hook it up to the Arduino:

The Arduino sketch is very simple – it just reads the values from analog pins A0 and A1 (in the range [0, 1023]) and prints it to the serial port. Here is the code:

The serial port sends values in the format:

512 300
513 280
400 200
...


On the computer side, I need to read these values, and plot them as a function of time. I am using Python and the Matplotlib library for this. I wanted to display this as a scrolling graph that moves to the right as data keeps coming in. For that, I am using the Python deque class to keep and update a fixed number of data points for each time frame.

You can see the full implementation here:

And here is what the plot looks like. It scrolls to the right as data keeps coming in.

Here is the schematic:

In the above circuit, the reference voltage at the non-inverting terminal of the op-amp is VCC/2. When it’s dark, the LDR has a high resistance (over 20K), and the voltage at the inverting terminal (pin 3) is going to be less than VCC/2. Hence, the output of the op-amp will go to high when it is dark. When sufficient light falls on the LDR, its resistance falls, and the voltage at the inverting terminal (pin 2) exceeds VCC/2. At this point, the op-amp output goes low. We can control the threshold at which it goes from low to high by adjusting the potentiometer R1.

Here is what the circuit looks like on a breadboard. The supply is 5V regulated.

In this case, I am using the LM358 – a very popular general-purpose Op-Amp IC. This works fine for our purpose, but do note that there are dedicated comparator ICs with better switching characteristics for critical applications.

In my next post, I will describe how to hook this up to an ATmega168 and wake it up from power-save mode.

The code needed to transmit serial data is very simple, and the datasheet has most of what you need:

Here is the schematic of a simple setup that will let you send debug data (strings) from an ATmega168 to your computer. If you are completely new to AVR programming, I recommend that you read Hackaday’s tutorial on the subject. Note how the TX/RX lines are flipped when you connect it from the ATmega168 to the FTDI adapter.

A few things to remember in order to for this to work correctly (the Makefile in the GitHub link below takes care of all this):

• For getting a 9600 baud rate, the chip needs to run at 8 MHz, and for this, you need to unset the  CKDIV8 fuse.
• For full sprintf formatting support, some additional flags are needed in the linker.

Here is what the output looks like on CoolTerm, a serial monitor that I use on my Mac:

Having a “printf” function is very handy for debugging your projects – so choose a chip that will let you support this functionality without too much pain.

Downloads

Here is all the code used in this project – do pay attention to the flags in the Makefile.

https://github.com/electronut/atmega168-serial-hello

Here is the schematic for the supply. You need to adjust the 1k pot while checking Vout with a multimeter or oscilloscope, till you get the exact voltage you are looking for.

Nothing fancy, but I try to practice a little pen & ink drawing when I can.

For this project, I wanted to try and make a simple night lamp – something that you can put under your cot and use it simply by picking it up. Useful in case you get up in the middle of the night and need a quick light. So I used a tilt switch and a 555 IC to design the circuit.

I used SMD components for this project for compactness. If you haven’t done any SMD soldering before, don’t be intimidated by it. You can solder many types of SMD packages (SOT23, TQFP, 1206, etc) using just a soldering iron with a small chisel tip, and a pair of tweezers. If you add a hot air rework station to your tools, you can even solder tiny QFN packages which have all leads under the device.

Here is a very informative and entertaining cartoon guide to SMD soldering.

Here is a video of the project:

You can find all relevant files at our GitHub link below:

https://github.com/electronut/nocte-lux

I also made a video with my old iPhone 3GS and uploaded it to youtube.

What blew my mind was the popularity of the post – in a few weeks, over 15000 people had seen the video, and the project was picked up by Hackaday and LifeHacker.

This has been a great inspiration for me to share the projects I am working on.