Collin; ya, that's a good question that I need to ask the actual instructor that will be working with the students. The way I have it now, with a single 555IC would, I believe act in the second way you descibed. But, after staring at your question for a while, I'm inclined to believe the first way would simulate the use of the extinguisher more closely(push, release, push again resulting in a total of 15 seconds or more, not requiring a continuous 15 second push) but, the preference of my customer may require a continuous push. I'll repost with the results of my visit with the customer and an image of my initial design in about 24hrs.