Bury one of these puppies and watch the dead bodies fly, This bomb is 100% stable, and it WILL NOT BLOW UP until a player is within it's range, guaranteed. When you get close to it, cave spiders spawn inside, and jostle each other around until one bumps the side of the fence, and then lights the TNT. Other, more simple proximity detectors have a good chance of blowing up before a player reaches it due to natural spawning of monsters, but this works off the fact that spawners can create spiders mid-air, while the half-slab and fence flooring creates a spawn free zone. This means that there is a 0% chance of it blowing up unless you modify it in such a way that it leaves it open for monsters to climb in. Placing a redston lamp above the spawner can allow you to turn the bomb on and off, Given that it is a player detector, it does not HAVE to blow up, you could wrap redstone wiring around the fences for another use (or you could always add a bigger payload of TNT :3.)