Wireless Sensor Network, Part 1

With all components finally having arrived from China, I built the first couple of wireless sensors a few months ago. They are using the cheap nRF24L01 transceiver modules for data transmission based on the RF24Network network layer. I’m using a Raspberry Pi with the transceiver module directly connected to its GPIO pins as the root node. The sensor nodes are controlled by an ATmega328 microcrontroller flashed with the Arduino firmware for convenience.

Each sensor node currently features a DHT11 humidity sensor, a DS18B20 digital thermometer, a BMP180 barometric pressures sensor and it’s powered by three AA cells. The controller spends most of its time in power-saving sleep mode, waking up to send a data packet about every minute. Each packet contains the readings from the three sensors as well as the battery voltage as determined against the bandgap reference.

The setup is far from final. For one, I’m still not sure how to store the data. Currently, it is just logged into a text file with an occasional and experimental import into OpenTSDB. I haven’t really come to trust OpenTSDB since I haven’t found a suitable and convincing way to backup the HBase data, yet.  Also, I might still replace the Raspberry Pi root node with an Arduino based MQTT relay.

First though, the power consumption of the sensor nodes needs improving. When I started out I was hoping for each node to run for close to a year on a single set of batteries. As you can see in the graph above, with the current design a set of three NiMH batteries with low self discharge lasts for no more than 6 weeks. What comes to mind in order to improve this, is to switch off the sensors and the transceiver in between samples. That is probably what I’ll try next. Please let me know if you’ve got any other ideas!



Minimal Arduino using an ATmega88

In preparation of the quest to build my own wireless sensor network I spent some time in getting a minimal Arduino up and running based on the ATmega8 chip. It was not before I finally succeeded that I realized it would be of no use in this case, because the ATmega8 lacks the ability to trigger an interrupt handler through its watchdog timer. So, I wouldn’t be able to use its power down sleep mode.

ATmega88 using internal clock

I then found a couple of ATmega88 chips in my stockpile, which aren’t subject to this limitation. Fortunately, setting this one up as a minimal Arduino turned out to be much the same. I mostly followed an existing guide, but added the Optiboot bootloader to the mix.

I first needed to compile the bootloader for the use with the internal osciallator at 8Mhz. So I added the following lines to arduino-1.0.5-r2/hardware/arduino/bootloaders/optiboot/Makefile:

atmega88_8: TARGET = atmega88
atmega88_8: MCU_TARGET = atmega88
atmega88_8: CFLAGS += '-DLED_START_FLASHES=3' '-DBAUD_RATE=19200'
atmega88_8: AVR_FREQ = 8000000L
atmega88_8: LDSECTIONS  = -Wl,--section-start=.text=0x1e00 -Wl,--section-start=.version=0x1ffe
atmega88_8: $(PROGRAM)_atmega88_8.hex
atmega88_8: $(PROGRAM)_atmega88_8.lst

I limited the baud rate to 19200 as greater values turned out to be less reliable in my setup. For this to work, I had to explicitly disable SOFT_UART in optiboot.c:

/* Switch in soft UART for hard baud rates */
#if (F_CPU/BAUD_RATE) > 280 // > 57600 for 16MHz
#ifndef SOFT_UART
//#define SOFT_UART

With that you can build the bootloader by running make atmega88_8 or omake.bat atmega88_8 on Windows in that same directory.

Next, I added the new Arduino type to arduino-1.0.5-r2/hardware/arduino/boards.txt. It should have been possible to add the board definition to a boards.txt file below the sketchbook directory but that did not work for me and the current version of my Arduino IDE.

atmega88_8.name=ATmega88 Optiboot (8MHz internal OSC)

Finally, with the ISP connected, I installed the new bootloader using the Aduino IDE and can now upload sketched through the serial connection.

There seems to be one annoying issue left though. When the compiled sketch size is getting close to the limit of 7680 bytes the upload will fail with an error message. If anyone can shed some light on what I might have missed here, please leave me a comment.

OpenWRT and Scripting Languages, Part 1

The RGB wall light project is currently on hold due to a massive lack of time. Still, I’m taking little steps of preparation for the second phase of the project, the software intended to control the RGB matrix. I could use the gcc cross compiler for the job, but using a scripting language to develop directly on the device promises to be far more comfortable. I’d like to narrow this down to languages readily available as packages for OpenWRT which leaves me with Erlang, Lua, Ruby, Perl, and Python. Since I feel like trying something new I’m going to rule out Perl for now.

Make Room

The installation of the bleeding edge OpenWRT version called “Attitude Adjustment” on the TL-MR3220 leaves about 1MB of space for additional packages to be installed – which isn’t even close to being enough for this purpose. So, the first step is to extend that space and the currently recommended way to do this is to move the overlay mount to an external USB device: Continue reading

Fixing Broken PSU of Asus WL-500gP Router

I’ve been using the Asus WL-500g Premium under OpenWRT for years. I always keep a second cold-spare device in stock to minimize our household’s downtime in case the router breaks. Last year, it was actually the router’s power supply unit that ceased to function. After I replaced it with the PSU from the spare device everything was back to normal. The same thing happened again this week, so I decided to document how I fixed the wall-wart back then.

Note: This is potentially dangerous so don’t try this at home! You’ve been warned.

Actually, the hardest part was to open the PSU. Both parts of the case are glued together, so separating them will inevitably scar the case. After the case was open, the worn out capacitor that broke the unit was clearly visible. I just de-soldered the capacitor, replaced it with a new one, and closed the case using two strips of adhesive tape. Done.

Wifi Enabled RGB Matrix Wall Light, Part 1

Inspired by the so called Lampduino and several other related projects I decided to build my own version of a RGB matrix wall light some time ago. During the last few weeks I finally managed to start out on this project.


The matrix of 64 individual cells and the LED back-plane are made from 4mm thin plywood that I spray-painted with a silvery varnish. The outer frame is made from a stronger 14mm birch multiplex board, yielding a very solid construction. I also bought some thin, white PVC board intended as the material for the front cover, but I still have to figure out how to cut that nicely.

Lacking the ambition to create my own LED driver board I settled for the “Rainbowduino”, an Arduino compatible board developed and sold by Seeed Studio. In addition to that, I wanted the matrix to be controlled and programmed over the air. One of the cheapest, easiest, and most versatile ways to accomplish this was to modify an OpenWRT based router for this purpose.

Continue reading

Serial Communication Between an Arduino and the TL-MR3220 Router

Last time, I installed OpenWRT on the TL-MR3220 router and added a connector for it’s serial interface. Today, I wanted to try and connect this interface to an Arduino to make the two communicate. The router’s interface operates at 3.3V while the Arduino’s UART operates at 5V, so we need to convert between those levels. The cheapest way to do this that I could find is described here. It involves using a 74LS04 hex inverter chip to convert the 3.3V signal up to 5V and two resistors for a simple voltage divider to convert the 5V signal down to 3.3V.

Continue reading

OpenWRT and the Serial Console of the TL-MR3220 Router

The TP-LINK router TL-MR3220 is a very cheap (around €22 in Germany) yet feature-rich wireless 802.11n router that supports the open source OpenWRT firmware. It offers a USB port, allowing a bunch of additional peripheral hardware to be connected. Installing the matching OpenWRT nightly build was easy enough, the original firmware offers to do so using the standard web interface.

I was thinking about connecting one of them to an Arduino in a project to come. The integrated serial interface seems to be the obvious way to achieve this. The router’s board readily exposes the interface, so I added a 4-pin header to do a few tests. The pin-out of the interface is documented in the OpenWRT wiki. Using a standard FTDI adapter I then got access to the router’s serial console. From here it should not be to hard to use the same interface to connect to an Arduino instead.

Seeeduino Stalker: Writing to the SD Card

After I managed to upload the first sketches to the Seeeduino Stalker last week, I was eager to try the special features offered by this special kind of Arduino platform. I decided to try writing to an SD card first and followed the example included in the Stalker’s documentation. Which turned out to not work at all.

A couple of hours later and looking for errors in all the wrong places, I finally managed to find out how to use the FileLogger library:

  1. The SD card needs to be FAT16 formatted.
  2. The file that the the Arduino is writing to must exist, the library does not create files.
  3. The file must not be empty, it must contain at least one byte of data.

If these conditions are met, the following tiny sketch will work just fine and append data to the file:

#include "FileLogger.h"

void setup(void)
 byte buffer[] = "Hello World!";
 FileLogger::append("data.txt", buffer, 12);

void loop(void) {}

Update 10/8/12: Please note that this example will work with version 1.0 of the Stalker, only. For newer versions of the board you should try sadfatlib.

Connecting to the Seeeduino Stalker

A few weeks ago, Seeed Studio were celebrating their anniversary with a bunch of limited offers. Among other things, I acquired a pair of Seeeduino Stalkers v1.0 with an Atmega 168. Unfortunately, the serial interface on those boards is not labeled. So it took me a while to figure out how to connect the FTDI breakout board. With a small adapter board, programming the Stalkers with the Arduino IDE now works like a charm. (Select board type “Duemilanove w/ 168”.)

Unboxing the DSO Nano

On Friday, I received a late Christmas present to myself: after about 4 weeks of travel, my DSO Nano finally arrived! A long time, but that’s alright, it didn’t take much longer than I had expected. Also, the current weather conditions are not really fit to speed up delivery times.

The oscilloscope and the few other parts I had ordered were well packaged and thus arrived in excellent condition. Seeed even replaced the spare probes I had ordered with the adapter I would have liked instead, just as they promised. Unfortunately, my time is somewhat limited right now, so I can’t get to thoroughly trying out my new toy right away. All I managed yesterday, was to install the battery and to turn on the DSO. So far, everything seems to be working fine.

Oh, I really like the white box that the DSO is packaged in. Very professional!