Room Climate Monitor

Connecting a WebCam

Since I haven’t connected a keyboard and mouse to my Pi, so that only one USB port was occupied with a wireless adapter, I decided to connect a Logitech C510 HD webcam which I had lying around. I tried to to capture a live-stream in order to place it onto this page, but it turned out, that this requires much more DSL bandwidth than I currently have. Thus, I decided to grab single frames each 5 minutes with fswebcam. The images are stored as JPG files in a subdirectory on the SD card, are labeled with a timestamp and converted into thumbnails using ImageMagick’s convert program. The WebCam is pointing onto the street in front of my house. Update: Meanwhile I migrated to pygame/opencv to grab single frames, because fswebcam frequently caused the webcam to fail.

Remember that different laws may apply in your country for publishing webcam pictures of public places.

Measuring light conditions

After taking some test shots with my Logitech C510 HD webcam, I soon realized that it required some exposure adjustments according to current light conditions. Most webcams are optimized for indoor use. As a consequence, pictures taken through the window are often overexposed in bright sunlight and underexposed during night. Therefore, I wanted to make my RasPi light-sensitive. Photoresistors (light-dependent resistors, LDR) provide simple means to achieve this. However, the RasPi lacks analog input pins. On the Adafruit Learning System one can find a tutorial how to do analog measurements without using an analog to digital converter (ADC), but this method is based on measuring the length of time to (dis)charge a capacitor, which is dependent on system load and therefore not that accurate. With an ADS1015 analog to digital converter, one can add up to 4 single input channels. Wiring is achieved via I2C, so that GPIO pins (SCL, SDA) can be shared with other I2C devices. For analog read-out, the LDR is placed into a voltage-divider circuit (as R1 or R2), with Vout connected to the A0 input pin of ADS1015.

Voltage divider. The LDR (e.g. A1060 #183563 – 62) can be either connected as R1 or R2. In addition, a 10kΩ resistor is required. Image source: Wikipedia

Instructions to convert voltage readings to light-intensity (LUX) can be found on this page.