The circuit is very simple and using only two transistors and few other components. The circuit can be operated from 6V to 12V DC. The operating voltage of the relay should be same according to the supply voltage.
The figure below shows a project of a very sensitive low current triggered relay circuit or low current relay driver circuit which can triggered a relay switch with only 5uA of current. The project can be used with wide variety of electronic circuits and projects, which has a very low current output.