Does that gadget just open/close a switch? If so you hardly need those transistors. Or does it output a voltage?
If it closes a switch you just need to set pin 2 to INPUT_PULLUP, and then detect a FALLING interrupt (that would be when the switch is closed). In other words, wire the switch from pin 2 to ground, and when the switch closes pin 2 will become low briefly. Don't use a LOW interrupt, that will tend to hang up your program.
Check with a meter if there is a voltage output, or just a switch closure.