I made a creeper farm in my survival world with the normal idea of trapdoors, carpet, a cat, etc. I know that the hopper system for picking up the drops will work, but no creepers will spawn in!
This is the tutorial I used if it helps any:
Originally I built it just like he had it (which was relatively low to the ground), and nothing would spawn. So I raised it to be 128 blocks off the ground, because of the 128 spawn radius (which was really scary to build in survival lol).I added torches on top of the farm so they wouldn't spawn there, and made a little room about 50 blocks under it to camp, and got nothing. After this I tried camping 50 blocks above the farm, and still got nothing. I built an antenna that went 20 blocks above the farm & 15 blocks out, still nothing.
To get the obvious questions out of the way, I am not on peaceful while I am camping. I am also the only player on my world. I am on windows 10 edition, I am not sure if that impacts anything. Also, it is definitely dark enough for mobs to spawn.