Looks like an older issue. I just used wood from a new tree (made sure I emptied my inventory of any older wood). I started playing in 1.5.2, so I don't think it's the wood slab issue. I had toggled to peaceful recently. I have also made daylight sensors in 1.6 before as well. I am using Optifine B4 for 1.6.2, but I have disabled that to see if it changed anything. I can however create it in creative mode via a crafting table.