Invention - Sleep Inducing Powder Turret

You create an invention which functions as an automatic turret. The turret shoots puff of powder at the first creature that gets within 15 feet of range after it is activated by a voice command. 

This powder sends creatures into a magical slumber. Roll 7d8; the total is how many hit points of creatures this puff of powder can affect. Creatures within 20 feet of the point it shot at are affected in ascending order of their current hit points (ignoring unconscious creatures).

Starting with the creature that has the lowest current hit points, each creature affected by this powder falls unconscious until the effect ends, the sleeper takes damage, or someone uses an action to shake or slap the sleeper awake. Subtract each creature's hit points from the total before moving on to the creature with the next lowest hit points. A creature's hit points must be equal to or less than the remaining total for that creature to be affected.

Undead and creatures immune to being charmed aren't affected by this powder.

The invention goes inactive and can't be triggered again after the first shot while also going inactive after the inventor finishes a long rest. 

At Higher Levels. When you create this invention using components of 2nd level or higher, roll an additional 2d8 for each component level above 1st.