I'm making a SAW-map with (of course) traps where someone has to die. But I can't come up with something that'll make doors/pistondoors open when someone dies! Is there a way with command-blocks or can I do it without them?
you could either have a scoreboard object that counts deaths or use spawnpoints.
The scoreboard is easy to use if you have more than one trap that you want them to die in, you can keep them separate just by making the objective names different. To use the scoreboard, type the command /scoreboard objectives add *name* deathCount. Then have another command block that says /testfor @p[score_*name*_min=1] and is constantly being powered by a clock. This will output a signal through a comparator whenever a person has died. You can take that output and do whatever you want with it, and add another command block with /scoreboard players reset @p. This will clear their deaths, making the detector ready again.
The spawnpoints use less lag, and are a bit simpler to use. Essentially, when the player starts the map, a command block with /spawnpoint @p x y z will set their spawnpoint to whatever coordinates you want. Then, whenever they die, they will spawn here. You can put a pressure plate here that will turn on whenever they spawn. Take this wire wherever you want, and you can then teleport them back to the map.
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I like to do redstone builds on my channel, and I can probably help you with your in game troubles.