Games : How to make command block stop outputting signal?

I want to create a command block with the command

/clear @a[r=5,m=0] 570

(Yes, it is an ID from a mod, but it does not matter anything). I want to detect when this command has succeeded using comparator facing from the command block.

So, everything works good, but the comparator after that is always outputting. How to make it stop outputtng, so the command success can be detected several times?

I tried to hook up the command block with /blockdata ~ ~ ~2 {SuccessCount:0} where the coordinates point at the first command block, but this didnt work.

I'm running Minecraft 1.7.10.