The following is the text from the Empty Room, Tricks, and Traps document. The additional information is added to increase agency.
This week our options are less about how to trigger traps, and more about our tools for creating interesting challenges for players. We're looking at light and timing.
Light Detection: This trigger only functions in always lit or never lit areas. It works by using materials that react to the interruption or exposure to light. The source of light can be natural (sunlight/moonlight), magical (light, continual light), or man made (incandescent, torchlight, lasers). The trigger can be set to go off after the first interruption or detection of light, or it can be set to be triggered after a certain number of counted interruptions or time exposed to light.This simplistic description of light detection neglects the various ways it can be used.
This is an effective trick or puzzle when the sensor responds only to a certain color of light.
The default stance of the trick, is of course that once torch, lantern, or sun-rod light is shone on the trigger, the trap is set, but it is also a useful stance for constructing a puzzle.
Often the sensor will be visible as a colored opaque crystal sphere. This should be breakable of course but doing so should ruin the mechanism.
The trigger most certainly does not have to be binary. It can require a certain degree or configuration of light, or perhaps be part of a multi-part puzzle where the light that triggers the light detector also gives a clue to the next step in the process while allowing the puzzle to succeed (like opening a door or portal where something must be thrown).
Timed: This trigger just regularly springs the trap. Note that the trap may be set to use a timed trigger after the initial trigger is sprung.Timed triggers are great obvious traps. This can be gouts of flame or swinging blades. They are rarely a danger themselves, but can make other situations more interesting. Having timed teleporters or walls or blades or other things can create interesting puzzles and situations.