Kitube

okay, so i want to build some sort of mechanism which is able to be activated by an object to start a stop watch and be stopped by the same object to measure the time it takes to travel a chosen distance. (i want to be able to calculate the force of for example a bb or dart). i thought about taking a simple stopwatch and controlling the start/stop button with an electrical current (since all the button does is close a circuit) so i could make a switch (for example some aluminium foil) which when i shoot a bb gun for example at it flips the aluminium foil to connect two wires which are connected to a power source and the stop watch, closing the circuit and starting the watch. and after 5 meters or so there's a second switch which the bb would hit stopping the watch. ive got a problem with this idea. for the first switch to work it needs to be open and the second switch closed so when the first closes the watch starts, however as soon as the bb closed the switch it needs to open again (since the button push is only a short connection of the circuit), thats all good. but when it gets to the second switch for it to work it needs to be open and the first close to send another electric signal. there are two ways i can think of that might work, 1. the stop watch could start with all switch closed and hopefully this doesnt cause it to start and that the opening and closing of the switch cuases it to start/stop. or 2. have 2 separate stop watches and the bb activates the first when it is shot and the second at the end and that i could calculate the time difference between them, but since the difference would be very small it would be hard to stop them at the same time and get the correct time. so, anyone got any ideas?