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[sum1won]

I'm currently trying to make a hatch that closes whenever someone is standing under it.  This is for workplace safety, a very important and not silly consideration.  (Currently, my mines are set up so that ore/rock is loaded into a minecart and dumped into the appropriate shaft).

The wiki suggests that pressure plates can only open via a trigger, not close (or, more accurately, they close in the absence of a trigger).

I would prefer to make it so that whenever someone is standing there it sends a close command, and an open command n ticks after nobody is standing there.  Can I?

[Thisfox]

Could you build the hatch, reverse it using a lever, so that it was open instead of closed, then attach the pressure plate? Or would that not work?

[bloop_bleep]

That would not work. When you flip a lever, it doesn't tell the linked object to "toggle", it tells it to either "open" or "close" depending on the new state of a lever. This is why if you link a hatch to a lever, flip it, making the hatch open, and then link a floodgate and pull it again, the floodgate will stay closed and the hatch will close: The first time you pull the lever, it sends an "open" signal to the hatch, and then the next time you flip it, it sends a "close" signal to both the hatch and the floodgate, even though the floodgate is already closed.

[PatrikLundell]

There are ways to make an inverter (I've never done it myself). I believe water and mine carts (in conjunction with pressure plates) can be used for this.

An alternative to solve the problem is to have the mine cart being pushed down the chute when suitably full or regularly, land on a ramp and roll to a track stop where it dumps the contents, and then return up using a spiraling impulse ramp track. Statues are said to be usable to block foot traffic on the track while letting carts through (haven't tried that myself yet: I've used pressure plates and doors).

[Gigaz]

Even if you build an inverter and close the hatch upon plate activation, you would likely still not get what you want. The pressure plate signal arrives at its destination 100 ticks later, whatever you do. So when the hatch cover is closed, the dwarf is long gone. And when he is on the plate, the hatch is open either way.
What you want can still be done, but you need to release the all stones in regular intervals and design a system so that during stone release, no dwarf is in the danger zone.
The easier solution is probably to transport the stones down in minecarts.

[taptap]

Tons of ways to do inverters, e.g. you can have a minecart circulating over a pressure plate once you get an ON signal to linked furniture the circulation is stopped (cut power from roller, open a door/hatch that lets the cart circulate on different track), normal delays will make it useless, however. And even with special customized short delay inverters (same design but multiple pressure plates that never reset while cart circulates, but when circulation stops the first OFF signal does the work) you will have limited use. There is always delay and when you only trigger the inverter when you are already in danger it will be too late as a rule.

I believe a better design is to work the other way round. I.e. enforce that drops (or only the last drop on the exposed tile) only happen when nothing is below.