Arrow Traps fire an arrow when you step in front of them (let's say 6 block range?) or on top of them, damaging the player for 4 damage if it hits them. The typical arrow "thunk" sound should be played as if the arrow was being fired from a dispenser with a small bit of smoke or something from the device.

The Arrow Trap remains "on" as long as you stand in front of it, firing every 4 seconds.