Hello Vikash and Blade2012,
I'm new to the forum. You had a few questions in your posts, and I'm not sure if you got an answer.
- supply external power for motor, buzzer, etc: The simplest way is to use a relay (solid state or electro-mechanical). The "coil" of the relay needs to match the output voltage of the controller board (Arduino outputs). The relay "contacts" need to be rated for the voltage and current of whatever you are trying to turn off/on. You may need to use a reverse biased diode in parallel with the coil to keep it from over-voltage condition when you try to shut off the coil due to the inductance of the coil. You should not have this issue if you use solid state relays.
- Alarm an severed wiring. This is usually called "fail-safe" wiring. The simplest way is to use normally closed wiring. The sensor passes the current (the switch is closed) during normal times, and stops passing the current (the switch opens) when there is an alarm. However, if you are stuck with sensors that can only operate the other way (passes current when there is an alarm condition), then there are still ways to do this. A resistor can be put in parallel with the sensor at the sensor location. In this way, during normal (non-alarm) times, the circuit is completed through the resistor and you get a lower current in the circuit or lower voltage on the return wire. In an alarm state, the switch shorts across the resistor to allow a higher current in the curcuit or the full voltage on the return wire. On advantage in using this sort of circuit is that you actually get 3 states: alarm, not alarm, and broken circuit. Whereas the normally-closed switch only gives 2 states since the broken circuit and alarm give the same signal.
Now the next question is how to inject these signals into an Arduino controller, I suppose. I have not started to play with these controllers yet, so I don't know the specs on the inputs. But I'm sure the above will trigger enough ideas to get you started. If you have to, you can always use an op-amp comparator circuit to drive inputs by sizing the resistor correctly. So all your EE courses start to come together.