I had looked around for off-the-shelf devices that could serve this purpose, but they were all deficient in one area or another, be it battery life, recording format or storage capacity.
My solution is based on the Android smartphone platform, but the choice of device would depend upon budget and the precise application requirements.
Here are some general system specifications:
- fully weatherproof
- portable unit weighs 900g
- continuous recording for up to 24 hours - or timed recording
- periodic unit status and image updates over the mobile phone network
- web-based interface for viewing and control of unit
- uses the h264 video codec
- USB data interface
Star N9770 (£100) and HTC One V (£170)The cheap phone I initially used is based on the Dual Core Arm Cortex A9. The camera module is not of high quality, and though fine during daylight hours, during periods of low-light, the frame rate is reduced to 8fps. The HTC phone costs a little more, but offers significantly better low-light video quality, and faster shutter speeds. Outside the hours of daylight, the HTC phone provides acceptable quality footage, whereas the Star N9770 does not.
Software DevelopmentMy preferred programming language is Python, and where possible I would always choose this over Java, but for the smartphone software I decided that using Java (Android's standard development language) would give me easy access to the core libraries required for this type of application.
It was relatively easy to implement this functionality, so the verbosity of Java didn't cause me too much frustration, and there were only a couple of challenges that required some deeper consideration.
- Since the application needs to run for an extended period, I needed to allow the recording process to continue whilst the screen was switched off.
- Because I wanted regular updates to be sent to my web server for viewing/control, I needed to establish how to create snapshot images and upload them over the mobile phone network without impeding the video recording too severely.
Hardware DevelopmentIn order to protect the device from the elements, I decided to use a polycarbonate IP66 rated (weatherproof) enclosure, and mount the phone and other parts inside in such a way that they can be fairly easily replaced. This is especially important when the enclosures themselves constitute a significant part of the unit cost, and when smartphone devices and their cameras are improving all the time.
To ensure there was sufficient power for a typical survey, I added a USB battery portable pack that provided an extra 12000ma without adding too much to the weight or volume of the unit.
A few options for mounting the enclosure were considered, but I settled on a mechanism that allows the enclosure to be clamped on a lighting column using two light telescopic poles. This allows the unit to be mounted at up to 4.5m which means views are largely free from vehicular obstructions.
This spring clamp allows quick installation (around 30 seconds) on lighting columns up to about 90mm in diameter.
I subsequently also added a basic shield in the form of a small section of plastic pipe to the enclosure to protect the sensor area from rain and sun glare.