How to reset a command block's status?

Question

I'd like to use a comparator to act when e.g. a testforblock command block attached to a button triggers. But the comparator changes its state only when the button is pressed after the test result changes, while I'd like to have a short pulse if nothing changed, i.e.

previous command block state | new command block state | desired output
           on                |           off           |      off
           off               |           on            |      on
           on                |           on            |      briefly off, then on
           off               |           off           |      briefly on, then off

How can this be achieved?

Answer 1

Try this contraption:

It will work exactly as you described.

Answer 2

Instead of making a machine, just set up another command block coming out of the comparator with this command:

/blockdata <Insert Coords> {SuccessCount:0}

Answer 3

In my case it is actually sufficient that an on-signal is reset after a short while, which can be simply achieved by splitting the input signal and additionally feeding it to the comparator's side.