A general characteristic of yantras is that they are small in size.
With the exception of yantras placed below temple statues at the time of consecration and yantras installed permanently for worship in mathas or temples, and a few other cases, yantras are generally mobile, whereas mandalas are not.
Texts may prescribe that the lines of the yantra be traced with a specific colour, for example, with turmeric or blood, but the space inside is never filled with colours….images are generally not found in yantras.
Brunner defines Yantra as a linear representation on a specific surface, such as birch-bark… yantras almost inevitably have letters, seed (beeja) syllables or mantras inscribed in them….yantras have inscribed mantras.
...yantras, like some mandalas are used for worship in desire-oriented (kamya rites), but their main purpose is magical.