A very nice alarm circuit can be constructed using one thyristor  (SCR) a buzzer and other few external  components  . This alarm circuit can work in parallel loop or closed loop configuration . As you can see in this electronic schematic circuits the device is very simple . For the parallel loop configuration is used four switches connected in parallel and are used for monitoring . When one of the four switch is closed SCR1 triggers , which sound the alarm .

The function of the closed loop circuit alarm is same as parallel loop alarm , but in this case the alarm is activated when one of the switch ( that is connected in series at this alarm type ) is open .

This alarm circuits ( parallel loop and closed loop ) need to be powered using a power supply that will provide and output DC voltage between 6 and 12 volts .