This is the blow dart trigger for my HS trig. The block of wood has a 1/4", 20 tpi Tee nut attached to the bottom.

There are holes drilled into the block at 90 degree angles The larger hole holds a cheap clear plastic drinking straw. The straw is placed all the way through the block of wood and there is a little tape on the straw to hold it in place.

The sensor has an IR LED on one side, and a photo sensor on the other. The photo-sensor's output is wired to an input on the Arduinio controller. When a foreign object breaks the beam of light the signal on the Aduinio switches from a logic 1 (on) to a logic 0 (off.) The Arduino recognizes this "falling edge" to mean that a dart has been launched.

The Arduino then triggers the camera shutter and also turns off all the LED room lights