I've been building a number of redstone circuits lately, and to keep them protected I've been enclosing each entire circuit in obsidian. However, recently I noticed that a zombie managed to spawn in a dark 1x1x2 void within the redstone circuit. I fixed this by placing ceiling slabs to make these voids 1x1x1.5, too small for spawning.
Unfortunately, I came across a part of my design where I have a redstone torch in the top half of a 1x1x2 void, and a redstone wire on the bottom half. I can't fill the void in any way, and I'm afraid monsters can still spawn there. The only solution I can think of is to find a way to add another block of space to the 1x1x2 void where I can place a coal torch, keeping the light level in the void high enough to avoid spawning.
Are there any other tricks or design components that help keep redstone circuitry from becoming a breeding ground for baddies?