I received this in the latest news email from the University of Idaho here in Moscow where I live.   

Lawrence Johnston, a UI physics professor emeritus, traveled to Washington, D.C. this week to recall the 1945 detonation of the first nuclear weapon. The July 14 symposium marked the 60th anniversary of Trinity, the first manmade nuclear explosion. Johnston witnessed the successful early morning test July 16, 1945, and the later use of nuclear bombs at Hiroshima and Nagasaki, Japan. He is believed to be the only person to witness all three. He lives in Moscow with his wife, Millie.

Johnson, if I recall correctly, invented and received a patent on the detonator for one of the bombs.  He also, again if I recall correctly, built a lot of the instrumentation used to record the effects in the planes that dropped the bombs.  And since they hadn’t asked him to teach anyone else how to run the instrumentation when it came time to drop “the big ones” on Japan he was the only person that knew how to operate the equipment.  Whoops!  He’s was required to go on both missions.