Doorknob

Star of David doorknob in an uninhabited house in Valparaiso, Chile

Picture is courtesy of "d.composed" who published it on Flickr

Star of David as a Jewish Symbol on a Turkish Flag

The Star of David was used as a Jewish Symbol in the 16th century CE. It was drawn on the war flag of Chief Hizir Hayreddin (Barbarossa) along with the symbols of Christianity and Islam.
Chief Hizir Hayreddin (Barbarossa) was the Great Admiral of the navy of Suleiman the Magnificent from 1534 until his death on 1546. Now the flag is at the Naval Museum of Istanbul.

Picture is copied from Wikipedia

Tetragrammaton

Christian hexagram with Tetragrammaton Dove and Cross

on a blackprint issued for celebrating 400 years to the Synod Zilina

Sent to me by Chastven Miloje (c) From Ashdod Israel

Tetragrammaton magen david at Templo de la Congregación Israelita Buenos Aires Synagogue is courtesy of Robert Pollack who published it on Flicker and it shows a Tetragrammaton surrounded by a Star of David surrounded by a circle.
Tetragrammaton in Greek means "The Four Letters"; it stands for the name of God which is comprised of four Hebrew letters (Yod, He, Vav and He).
Robert Pollack wrote to me:

It is a strong symbol, and one that suggests unity and interconnectedness. It always reminds me of an elaborate knot.