More light!

Charly's Column – Hue and Rasp Pi

Article from Issue 218/2019
Author(s):

Since his Trådfri Smarthome article over a year ago, sys admin columnist Charly has been receiving messages from readers with two questions: "Can you do that with the Philips' Hue system?" and "Can this also be done with a normal brightness sensor?" Yes and yes!

In a previous issue [1], I described how I control a smart light in my living room with Ikea's Trådfri system and Linux. The darker it gets, the more I turn up the lights. Around the same time, Martin Loschwitz then explained how he used a Raspberry Pi and a transmitter module to control Hue and other Zigbee devices [2].

Now I'm going to do it again, without the Zigbee module, but I expect to use a Hue bridge, which Martin's approach doesn't need, and because not everyone has a roof-top photovoltaic system, this time I focused on a simple brightness sensor.

Getting Started

First, I discovered the IP address of my Hue Bridge: 10.0.0.8. Then I pressed the button on the bridge and entered the following at the command line:

curl --data "{\"devicetype\": \"huelr\"}" http://10.0.0.8/api

The huelr string (for "Hue living room") is freely selectable. I got some output like:

[{"success":{"username":"T3VGtGWmFUgJwHufxkudY1bizvqoZMhCSqu1ySwm"}}]

This unpronounceable string is an authentication string, which I have to specify with every call in the future. A (confusing) list of all connected devices is returned by the command:

curl --request GET 10.0.0.8/api/T3VGtGWmFUgJwHufxkudY1bizvqoZMhCSqu1ySwm

If you only need information about lights, you can append /lights to the command or /lights/1, /lights/2, and so on for each lamp. The following command switches lamp 1 to medium brightness (value 125):

curl --request PUT --data "{\"bri\":125}" 10.0.0.8/api/T3VGtGWmFUgJwHufxkudY1bizvqoZMhCSqu1ySwm/lights/1/state

For the brightness sensor, I used a $2.00 module named BH1750 (online, e.g., [3]). I connected this to the I2C bus of a Raspberry Pi (Figure 1). I used the Zero W model with WiFi – it has to communicate with the bridge.

Figure 1: The case contains a Raspberry Pi Zero, with a seven-segment display peeping out. In the upper right corner of the picture, you can see the brightness sensor clamped in place.

Cat's Eyes

Raspbian comes with almost everything you need, with only a few packages to add:

sudo apt install build-essential wiringpi i2c-tools python-smbus

The program in Listing 1 reads the sensor in the program I named lux:

cc lux.c -lwiringPi
mv a.out lux && chmod 755 lux

Listing 1

lux.c

01 #include <wiringPiI2C.h>
02 #include <stdio.h>
03 int main (void) {
04   int handle = wiringPiI2CSetup(0x23);
05   wiringPiI2CWrite(handle,0x10);
06   sleep(1);
07   int word=wiringPiI2CReadReg16(handle,0x00);
08   int lux=((word & 0xff00)>>8) | ((word & 0x00ff)<<8);
09   printf("%d \n",lux);
10   return 0;
11 }

If you run lux, you will see the current measured value for the photo sensor in lux units. Depending on this, you can now control your smart lighting.

Infos

  1. "Charly's Column: libcoap" by Charly Kühnast, Linux Pro Magazine, issue 202, September 2017, pg. 49: http://www.linuxpromagazine.com/Issues/2017/202/Enlightened-libcoap/(language)/eng-US
  2. "Controlling Zigbee Devices with the Raspberry Pi" by Martin Loschwitz, Linux-Magazin, September 2017, pg. 76: http://www.linux-magazin.de/ausgaben/2017/09/raspi-spricht-zigbee/ [in German]
  3. Debo BH1750: https://www.ebay.com/i/292393778070?chn=ps

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