The story of Gideon can be found in Judges, Chapters 6 through 8.  The part in which he defeats the Midianites is found in Chapter 7.  Gideon used trumpets and torches hidden in clay jars to defeat the Midianites.  But really, what he used was God's instructions.

Gideon defeated the Midianites by doing exactly what God told him to do.  The last of the things that God told him to do was to take his 300 warriors and equip them each with a trumpet and a torch hidden in a clay jar.  At his signal, they were to break their jars, blow their trumpets, and give a battle cry.  When all of this was done, 

the Lord caused the men throughout the camp to turn on each other with their swords.