Are you trying to farm mob heads in large quantity? Tired of lighting the creepers with your flint and steal? Wish you could do the whole thing from outside the blast radius? Now you can!
  You'd think a flint and steal in a dispenser would be able to automate the task of detonating the charged creepers for you, but it just lights them on fire. This makes automatically farming mob heads far more difficult, but not entirely impossible. Let me introduce the automatic charged creeper detonator!

  This contraption uses iron golems, which agro skeletons, which agro witches, which agro charged creepers, which blow up, getting you your mob head.
  The gray circuit causes the 1st skeleton to shoot at the iron golem but hit the witch, and then move out of the blast radius. The 1st skeleton must be at 7 health or lower in order for the witch to throw the harming potions necessary to detonate the creeper. The skeleton must also have the slowness effect for this to occur. This is accomplished with a second witch aiming at a second skeleton. The light blue circuit will agro the 2nd witch toward the 2nd skeleton. The 2nd skeleton must have full health so that the witch doesn't throw harming potions and heal the 1st skeleton. If that does happen, don't worry! The black circuit will damage the 1st skeleton 1 hp at a time.

  The precision crafted blast chamber, where the charged creeper meets with its soon-to-be-headless victim, was easily the hardest part to perfect. Every block has a reason to be where it is. This blast chamber will kill a creeper, skeleton, zombie, or wither skeleton at full health with 1 charged creeper blast. If the charged creeper and/or the victim are in a minecrart, it will not only collect the minecarts, but sort them into a separate chest as well.
  It also has tripwire that can be used to detect when a mob enters or explodes. The trip hook allow for further automation,(like reloading the mobs), but I haven't done any of that here. (I just put in the tripwire so you would have the option.) The soul sand allows the dropped items to be collected even if they land under the trip hooks, and the ender chest and enchantmant table ensure that the mobs and/or minecarts cannot be nudged out of the 1x1 holes where they belong.

I added command blocks to summon mobs for demonstration only. You would naturally need to hook it up to a mob farm.