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Originally Posted by Carl
There are two obvious possibilities. If Mineoro's claims of ionic detection are correct, then the act of pressing and releasing the brake must somehow produce ions. Since the brake includes a switch for activating the brake light, we could suppose that the opening and closing of the switch might produce an electrical arc, which could generated ions. This would suggest that moving the FG80 farther from the switch (at the top of the brake pedal, under the dash) would make detection more difficult. Not so. In fact, I found that I could place the FG80 close to the rear brake light, outside the car, and just as easily get a response to the braking.
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If the brake test shows the FG80 is sensing transient EM fields, then a simple experiment may further demonstrate this: Consider that in a 12vdc circuit switching power to a light bulb with a resistance filament, there are usually several spikes when the power is first applied to the filament. These spikes are caused by the minute arcing as the switch contacts close. As voltage is suddenly applied to the light filament, a surge of current can be expected for a small fraction of a second, then as the filament heats up, its resistance increases and the current drops and levels off to a constant amount. Releasing the power usually does not cause this large rush of current. Now some filaments are wound into a coil shape rather than a strand of wire. These bulbs with a coil shaped filament could be expected to send out more intense transient EM fields when the power is applied. Furthermore, I would suspect the field would be directional, due to the coil shape where the transient current flows.
A simple test would be to switch on and off a DC bulb that has a coil shaped filament. Then test the FG80 for beeping at different angles to the axis of the filament coil. I would also take note of the orientation of the coils inside the FG80 to maximize this effect. If I am correct, we might find that the FG80 also has directional properties in sensing a DC light turning on.