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Old 06-04-2016, 07:03 AM
Fridgemagnet Fridgemagnet is offline
Join Date: Feb 2005
Location: Hampshire, UK
Posts: 627
It sounds like Dunmurry's duck is being triggered by the big electromagnetic spike produced when the fluorescent light is switched off. Even if the duck's optical sensor is masked off with Blu Tack the EM noise is finding its way into its electronic gubbins.

Fluorescent strip lights have a large inductive ballast in the circuit. When the light is switched on this inductance will limit the current ramp-up so you won't get much of an EM spike then. With the fluorescent running there will be a current through the ballast inductor that gives rise to a magnetic field, and when this current is removed the magnetic field collapses and all that magnetic energy has to go somewhere, so it gets transformed into a voltage spike. This voltage spike is often large enough to arc over the open switch contacts, so you might see a little flash under the switch upon turn off.

Duck may also be susceptible to the fridge/freezer thermostat turning off due to the large inductance of the compressor motor. To prevent spurious quacking you could either fit surge suppressors to the fluorescent light, or keep the duck in a metal box. Steel is good as being ferromagnetic it will also attenuate magnetic fields as well as electric fields; mumetal is even better, but possibly too exotic and expensive for a randomly-triggered duck workaround.