Alarm systems can and are used by the most observant Jews on Shabbat. Some systems such as the Israeli made PIMA alarms
have built in Shabbat modes.
The two main issues that observant Jews have to decide upon are how to arm and disarm the system within the framework
of 'halacha' and whether the blinking of sensors as you walk past them is problematic.
Fortunately solutions to both of these issues can be built into all the major alarm control units that we use.
There are many articles dealing with using alarms on Shabbat and a quick Google search will find most of them.