Right now, I have a monitor in place for stove usage. This monitor will detect if the stove is "on" but unattended. This is a non-invasive monitor (i.e. no modifications made to the stove). The monitor utilizes a motion sensor (for the whole kitchen) and a USB temperature probe (TEMPerl).
The probe is mounted on the back wall (behind the stove) up near the vent. It is not in the direct path of heat, but it doesn't need to be.
I am taking rolling/moving averages of sampled temperature (in 30 second intervals) and any "sudden" spike in temperature (say from 73.3F average to 75F -- one or two degrees) will make the monitor believe the stove is on. While the stove is "on", if there is no motion in the kitchen for N minutes (e.g. 30 minutes) then an alert is emailed. If there is motion, the timer is reset. If the sampled temperature drops (a degree or two) below the now high average, then the stove is cooling down (turned off?).
There is a lot of tuning to do, but the smoothing caused by the rolling/moving average helps significant rises and declines to stand out while normal warming/cooling of the kitchen (due to the sun or thermostat) is averaged out. It may be worth looking into Bayesian classifiers to see if it can better tune out normal warming/cooling trends.
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