Charly's Column – libcoap

Trådlös is the Swedish word for "wireless" and thus throws light on the name of the TrÂdfri smart lighting system. In the simplest case, you combine a lamp with an actuator: A remote control, a dimmer, and a motion detector are offered.

If you also want to use an Android or Apple smartphone to control the light source's brightness – and in many cases also modulate the light temperature from cold to warm white – you need a gateway, which is a small control box that connects to your Ethernet network and sends commands to the lighting systems. ZigBee Light Link [1] acts as the protocol in this case, but customers also can use compatible lamps with E14, E27, and GU10 sockets and LED panels that can be hung on the wall or built in to furniture.

That's all quite nice, but, of course, I do not want to control everything with an app. Instead, I want the lighting system to adapt dynamically to the ambient light, such as gradually becoming brighter at the onset of dusk. The Linux PC that evaluates the data from my photovoltaic system knows when it turns dark (otherwise, a simple brightness sensor will do the trick), and it is precisely this Linux PC to which I want to pass the Ikea gateway commands so that it can pass them on to the lights.

I do so with libcoap [2], a C implementation of the machine-to-machine Constrained Application Protocol (CoAP). The installation requires the commands in Listing 1. Doing this gave me the coap-client program, which I used to pass the parameters to the gateway that controls the lights.

sudo apt-get install libtool autoconf automake buid-essential
git clone --recursive https://github.com/obgm/libcoap.git
cd libcoap
git checkout dtls
git submodule update --init --recursive
./autogen.sh

Wrapped for Pleasure!

Because the syntax of the program is pretty horrific, I used a Python wrapper [3] that makes working with libcoap pure pleasure. The entry

sudo pip install tqdm
git clone https://github.com/sandyjmacdonald/ikea-smartlight
cd ikea-smartlight

installs it. I then dropped a file with the IP address of the gateway and its security code, which you will find on a label glued underneath the housing, into the current directory:

[tradfri]
hubip = 10.0.0.42
securityid = laskuhgoACBUWHG

I now have access to simple control and status commands. Figure 1 is the output from tradfri-status.py, showing that it has discovered two light sources that are combined into a group. Each lamp and the group have an ID that I can use for control purposes. For example, to switch all lights in the present group 133164 to 75 percent brightness, I just need the following command:

tradfri-groups.py -g 133164 -a brightness -v75

Figure 1: The tradfri-status.py script reports two lamps in one group and their IDs.

By December 13, the Swedish St. Lucia celebration, I will definitely have programmed a suitable candle simulation for my living room.