What they do
Downer detectors are devices you can find scattered across the village of Maidenholm and the Parade District. They can be found inside houses and along the streets. If the player is not on joy and they walk through a downer detector, it will set off an alarm which will alert the wellies.
What to do about them
Most downer detectors will be connected to a trap controls box by a wire that you can follow to find the control box. When you find the trap controls box, you can either disable it with a clever trap tool or reverse the polarity with a polarity device.
If the player uses a polarity device on the trap control box, it'll reverse what the downer detector does. Instead it'll set off an alarm when somebody on joy walks by, and will be fine when somebody not on joy walks by.
A few downer detectors like the one in the top image (top right) will also be connected to spankers. Reversing the polarity on a downer detector will also reverse the polarity on the spanker meaning it will shock NPCs on joy and the player if they are also on joy, but will not care if they are not on joy.