40d:Repeater

A repeater is a device which creates a never-ending cyclic on/off signal. Several designs are possible, and they can be made to operate at varying rates. They (usually) use fluid or creatures to trigger pressure plates to create a cycle, continually trigger the plates, which can then also be linked to automatically and ceaselessly trigger external systems.

Fluid uses the natural flow and varying depths to trigger plates. Creatures use pathing, opening and closing doors to attract the creature to step on the plates.

As a general warning, always have a way to turn off the repeater, and/or allow your dwarves later access for repairs or modifications.

Fluid repeater

One example fluid logic repeater (described on the forum by AncientEnemy) is as follows:

≈≈≈≈≈ - river or brook
═╗≈╔═ - wall to channel out after construction
 ╠╬╣  - fortification (to filter out any creatures inhabiting your river)
 ╠F╣  - shutoff floodgate (linked to exterior lever)
 ║#║  - 1-tile drawbridge (linked to pressure plate)
 ║p┼  - pressure plate (set to 7-7 water), and access door
 ╠F╣  - floodgate (linked to pressure plate)
 ╚═╝

This repeater toggles fairly slowly, about once every 300 steps, making it suitable for operating bridges, floodgates, and upright spikes. As an added bonus, the bottom wall (immediately beyond the floodgate) can be removed and connected to a cistern whose water level will be automatically maintained at a level between 3 and 4 deep.

Goblin repeater

This is not as regular a repeater, but just as reliable in the long run.

A goblin or creature is placed (via pitting) in a loosely circular hall with one door and one drawbridge leading to a single path out. Beyond these is the bait - an exit, a restrained animal, or just your fortress in general - something the creature wants¹. One path is open, and the goblin heads that way to kill the animal. A pressure plate is stepped on, which shuts one path and opens the other² (and triggers an exterior system). The goblin reconsiders his path, and heads back around for the other hallway - where the process is repeated.

1. For instance, if using a rhesus macaque, some item of value would suffice to attract the creature.
2. Doors and drawbridges open alternately when connected to a trigger.

               A               A = bait animal (or etc.)
 ╔════════════╝  ╚═══════════╗
 ║           ┼║   ┼          ║   ┼ = access (bridge or door), currently open 
 ║  ╔═════════════════════╗  ║   ┼║= access, currently shut 
 ║  ║                     ║  ║  
 ║  ╚═════════════════════╝  ║   P = pressure plate
 ║     g ->    P             ║   g = goblin, w/ current path
 ╚═══════════════════════════╝

The only caveat is to make sure there is enough room between the pressure plate and door/bridge, to give them time to shut well before the goblin gets there. Additional mechanized doors or a cage trap may be desired if you ever wish to "turn off" the repeater without killing the goblin/creature (by safely isolating it).