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[LordBaal]

Can somebody please point out or guide me to a clear way, step by step if necesary, of making an automatic reservoir? I found some (contradicting) guides. Put one pressure plate, put two, one at the bottom other atop. Set them to 0-7, 5-7, 7-7, 0-1.... 

Then the wiki says the pressure plate only sends open signals, does that mean it can only open the floodgates but not close them?  This certainly have happened to me before. I was under the impression that with a single pressure plate at the bottom, set on reset and going trough a gearbox to send the signal to close the flood gate once the 7-7 trigger was filled would do, but it didn't. What I'm doing wrong?

The sad part is that years ago I managed to do one but now can't do it for the life of me. Very annoying.

[origamiscienceguy]

pressure plates are either sending a signal or not. When they send a signal, they do what you want (open a floodgate) when the signal stops, the flood gate closes. (Not the other way around unfortunantly)

[smeeprocket]

couldn't you technically reverse that by hooking up a lever to the floodgate, pull the lever so that the floodgate is open and then set up your pressure plates?

[origamiscienceguy]

I believe so, haven't tested it though.

[Larix]

No, the "open" and "close" signals have fixed effects for most buildings (only exception are gear assemblies): an "open" signal opens a door/floodgate or raises a bridge, a "close" signal closes a door/lowers a bridge. You can't revert these effects (you can get around that restriction with extra machinery, though).

Verbatim quote from the wiki article "pressure plate":

(Reusable) Pressure plates send an open signal the instant their criteria are met, and a close signal 99 ticks after their triggering criteria are no longer met.

I.e. they send both types of signals. With water-triggered plates, choosing the correct triggering depth governs the behaviour of the plate. If you e.g. want an automatism that lets water into your reservoir when the level there gets too low, you could use a plate responding to 0-4 water: when water level is 5 or higher, the plate is inactive and the door or floodgate remains shut; when water level falls enough, the plate activates and opens the door, letting in water until the level gets high enough to switch it off again.

Plates have a delay before switching off, so this might be insufficiently safe if you're drawing water from a river, which could push quite a bit of extra water into the system during the delay, possibly leading to flooding.

[omega_dwarf]

You can also have it be passive...dig your vertical shaft down to the level you want the top of the reservoir at, use the diagonal flow pressure-killing semi-exploit to get the water into the top level of your reservoir, and then let it fall to the bottom of your reservoir and fill it. When I make cisterns, that's always how I do it. Mechanisms are tedious.

Only disadvantage is that you can never service your cistern if your shutoff mechanism breaks, unless you drain it faster than it's filling - into the caverns or something. If your embark doesn't freeze and you use a bridge, you shouldn't have to worry about that, though - unless something fun breaks your lever.

[smeeprocket]

tbh, I just use a levered floodgate and seal off the cistern while filling it.

[LordBaal]

Soo... If I make it 3 levels deep, and let's suppose I want it to be filled just the first two levels, if I put a pressure plate on the second level floor set to trigger by water from 0 to 6 then in theory it should keep open the floodgate until it reaches a pressure of 7 (filled) and then close the floodgate until it gets lower again?

[smakemupagus]

In principle you don't even need a floodgate -- You could just use a diagonally connected tunnel to de-pressurize the incoming water on the highest level you want the cistern to fill.  Floodgates are nice for a backup plan, if you ever want to be able to dry out the cistern for "maintenance*".

*dredging out drowners from the bottom of the well

[LordBaal]

Well, I already did it. Three levels deep, gravity filled from the uppermost level with a flood gate in place, on the bottom there's a pillar and on top of it (that would be the second floor) a pressure plate set to trigger from 0-6. After that I connected the floodgate to the pressure plate but nothing happens. So I connect a lever to the flood gate and open it manually hopping the pressure plate will work.

Water rush in, it completely fill both levels bellow and once the water reach 7 the pressure plate activates and voila! The flood gate is closed, leaving me with a perfect cistern.

Now I'm digging trying to find a cavern to make a emergency flush system and also see if the pressure plate open the floodgate too.