Droid mine

The Droid mine was a type of battle droid used by the Confederacy of Independent Systems.

Overview
These were a form of sentry droids used by the Confederacy during the time of the Clone Wars. They were a type of battle droid that were placed underneath the ground in an inert state where their antennae were only visible on the surface as well as the top of their heads. If an object brushed by the antennae then the activation signal resulted in the droid awakeining whereupon it dug itself out of the ground to target nearby living objects.

Once activated, the droid mines began its attempt at eliminating its threat. The secretive nature of their deployment made them a dangerous threat to Clone troopers who stumbled into large minefields of these droids. What was even more dangerous in such circumstances was when clone troopers destroyed the droids who's shrapnel debris served to only activate more hidden droid mines.

During the Battle of Christophsis, a number of droid mines were deployed near the shield generator where they were accidently activated by Ahsoka Tano.

Source

 * Star Wars: The Clone Wars (film)
 * Star Wars: The Clone Wars (novel)