Air gun bullet detector / sencor. I'll take pictures of air gun bullets and have therefore tested a lot Ir interupt, photo gates, laser triggers, piezo triggers and sound sensors. None of them are fast enough and stable enough for a modern air rifle. Then I came across this link which shows an old-fashioned transistor controlled interupt. It was a success !!
(I have no SCR, but it works just fine with a BC337 transistor instead of SCR).