The Vital Signs Project: Equipment
The Vital Signs Project: Equipment
The motion detector we use to record occpuancy data over time is a battery operated device
originally designed for security uses. We modify the device so it can be used with a Hobo Volt logger to track occupancy in a space. We chose
this Radio Shack model (Catalog Number 49-425) because it draws only a very small current
and we recommend these modifications be performed only on this model.
The motion detector has two operating modes: "Chime" and "Alarm." In
the "Chime" mode, the device sends a short signal to the speaker every time it
senses motion. In the "Alarm" mode, it sends a signal to the speaker for 30
seconds when it senses motion. We use the "Chime" mode. We remove the speaker
and add the small circuit shown at left in its place.
The circuit charges a capacitor with the signal that is sent when the unit detects motion.
The signal charges the capacitor to about 2.5 volts very quickly, and then the capacitor
charge dissipates completely over 15 minutes. This 15 minute period allows us to take a
voltage reading across the capacitor every 6.4 minutes (the interval corresponding to an 8
day monitoring period on the Hobo Volt), and record if a motion "hit" occurred
since the last voltage reading.
The chart on the left shows the voltage as measured by the Hobo Volt as a function of time
after a motion "hit" at time=0. The voltage drops in logarithmically, and the
voltages at any particular time is a function of the time since the last "hit."
Motion Detector Throw Spectifications
Detection Angle: 60°
Detection Range: 30 ft.
Detection: Passive Infrared
The
Vital Signs Curriculum Materials Project