If you change a block above the sticky piston a little, there is a permanent redstone signal.

Which is then connected to a door, for example. The whole thing works with a block update detector (BUD). So if you put a block over the Piston Earth, the signal changes. When it becomes a grass block, the signal changes again because the block (specifically the ID of the block) has changed. The secret part: only you know which block to change (as seen from the surface of the earth). The best way to do this is to remember the position of that block (or write down the coordinates) and then, whenever you want to open the door, take down that earth block. Either you close the door again manually from the inside with a lever (but then the hole in the ground remains, which could cause a stir in multiplayer), or you build a grass block again (it must be be a grass block, an earth block would later change to grass and thus a 3rd time the signal: The door would be open again) into the hole, the ID of the block ( air becomes grass = ID change)changes and the door is closed again.

Instructions:

Build the BUD as shown in the picture (ATTENTION! There must be a hole under the block which is under the piston [i.e. between the two blocks on which the amplifiers are located], otherwise the piston cannot move it!) The earth above it would have to be built up like this afterwards:

Then you connect a door (or similar) to the redstone, which flows back into the flask via an amplifier, and invert it again depending on how you use it.