This works as an easy on, easy off version. The timer can be primed, started and stopped from a single lever and the only external requirement is water if the timer gets reset.
The bridge and both floodgates in the Z-1 tunnel (with the pressure plate) are linked to the on/off switch. The 'safety cutoff' floodgate is optional depending on how OCD you are with
accident prevention fun.
If my math is correct this layout can be used with these specific widths and number of floodgates:
w = tunnel width or area (not counting floodgates)
fg = number of floodgates necessary
p = pressure plate setting
w: 4 fg: 2 p: 5 (shown in the image above)
w: 6 fg: 2 p: 7
w: 10 fg: 3 p: 5
w: 12 fg: 3 p: 7
w: 6 fg: 4 p: 4
w: 7 fg: 4 p: 6
w: 16 fg: 4 p: 5
w: 18 fg: 4 p: 7
I also accidentally made a sort of metronome that cycled very regularly, ~215 steps open, ~160 closed. I'm positive that it's on the wiki somewhere but when I saw the design I was too noobish to understand wtf. My layout went something like this:
FBBPX
F - Fortification
B - 2x1 bridge (I think it can open either way)
P - Pressure plate (7/7)
X - Floodgate
Or something like that, I saved the design off but didn't pay a lot of attention at the time.
Actually, I took the time to look for it and it's under 'repeater' in the 40d section of the wiki. This is a much safer version:
≈≈≈≈≈ - 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)
╚═╝
My 1x1 drawbridges don't seem like they open/close which is why I've been using 2x1 bridges.