An alarm bell went off.

警钟响了。

Wire the thermometers up to trigger off an alarm bell if the temperature drops.

把温度计的电线连接上，当温度下降时，就会触发警报。

In case of fire, ring the alarm bell.

如遇火警，即按警铃。

It should have set off an alarm bell, but it didn't.

他的画本来应该为我敲响警钟，但是我却没有意识到。

Sound the alarm bell in case a fire breaks out.

万一发生火灾你就按响警铃。