In introductory physics laboratories, spring constants are traditionally measured using the static method. The dynamic method, via vertical spring-mass oscillator, that uses a stopwatch in order to measure the period of oscillations is also commonly employed. However, this time-measuring technique is prone to human errors and in this paper we present a similar setup, except for the motion timer being the B-field sensor of a smartphone, in order to measure the period of oscillations and thus the spring constant. The smartphone device as an introductory physics experimental tool is quite well established. For instance, the smartphone-based acceleration sensor has been employed in a rather quick measurement of the acceleration due to gravity and in analyzing simple pendulum phenomena. In addition, the magnetic field sensor of the smartphone device has been effectively used as well in measuring average angular velocity and the acceleration due to gravity.