PIR motion sensors for time-lapse photography

Using a household security sensor to trigger time-lapse cameras “when stuff is happening”

How do you efficiently capture something about as big as a truck being sporadically hand assembled over a period of one year?  This is a question I recently faced when setting up time-lapse cameras on a major instrumentation project at the Australian Astronomical Observatory (AAO).

A typical time-lapse approach is to take photos at fixed intervals for the duration of whatever process you’re trying to capture.  The AAO’s assembly schedule for HERMES, a high resolution astronomical spectrograph, didn’t lend itself to this approach for three reasons: i) nothing was going to happen on evenings or weekends; ii) there would be many ‘dead’ periods when work was being done outside of the main assembly area and therefore out of shot; and iii) it would go on for a year, which meant camera shutter life might have become a limiting factor.  What I needed was for photos to be taken only when work was being done, and to do so at short intervals so that these events translated into a reasonable number of frames in the final video.

I decided to use a Passive Infra-Red (PIR) motion sensor to govern when the cameras were triggered.  These sensors are used all the time for outdoor security lights, and they’re super cheap (about $20 from a DIY store).  By using a PIR sensor, the cameras would only fire when someone was in the room, i.e. “when stuff was happening”.