How do you smoke weed in a room with a smoke alarm without setting it off?

You would simply need to keep the smoke away from the sensor, either by isolating it physically or by directing it away. Some sensors also do not detect very small amounts of smoke, so it could be sufficient to open a window and dilute the smoke with fresh air. However, some ionizing sensors can be set for extremely high sensitivity and will detect combustion byproducts almost instantly.