Use an SW-520D ball-type tilt switch in a stationary bench model to explore head-angle alerts, never as a driving safety device.
Demonstrate relative methane-response trends with MQ-4 and local indicators; it must not replace a certified household gas alarm.
Build a low-level alert only after choosing a sensor suited to the tank and calibrating actual empty and full points.
Detect a nearby object with HC-SR04 and drive a local indicator as a tabletop proximity demonstration, not a certified security system.