Tactile sensing can enable robots to perform complex, contact-rich tasks. Magnetic sensors offer accurate three-axis force measurements while using affordable materials. Calibrating such a sensor involves either manual data collection, or automated procedures with precise mounting of the sensor relative to an actuator. We present an open-source magnetic tactile sensor with an automatic, in situ, gripper-agnostic calibration method, after which the sensor is immediately ready for use. Our goal is to lower the barrier to entry for tactile sensing, fostering collaboration in robotics. Design files and readout code can be found at https://github.com/LowiekVDS/Open-source-Magnetic-Tactile-Sensor}{https://github.com/LowiekVDS/Open-source-Magnetic-Tactile-Sensor.