This is a 1 high redstone signal transmission mechanism, or to be more specific, a 1 high mechanism to activate a tripwire. This design is truly 1 high, as in, no blocks need to be underneath for things such as redstone dust. This design is 2x4x1 (not counting the length of the tripwire). The design is also very easily expandable. Although I don't think this would help compact the design, I have made sure to avoid using slime blocks, because then the blocks below and above the design have to either be air or immovable. One small downside is that fences placed under the design can in some cases block the boat from moving, and cause it not to work. Another small downside is that there is a brewing stand that emits particles that can sometimes be seen even if there are blocks covering the top of the design.

This mechanism works by using a boat to trigger the tripwire. The boat is sitting on the edge of a brewing stand, which is the shortest block that isn't restricted to being placed only on another block. It is the only block short enough to allow the boat to sit low enough to trigger the tripwire. This design also relies on the bug that boats or other entities can be inside blocks (this is something that can be set up in survival, by pushing a block inside of the boat with a piston). I am not sure if this design could be fixed if this bug is fixed, though this is an old bug so I don't know if it will be fixed soon. Another bug is used to simplify the design: the fact that when a boat is inside of a piston arm (caused by a piston pushing a boat into another block), when the piston arm retracts, it pulls the boat back. This allows the design to work with only a single piston per tripwire.