Some generic, low-cost "door alarm" magnetic switch components are made of using "Normally Open (N/O)" type reed switches, ie, when the magnet is in proximity, the current stops flowing.
If I were building such a system, I think I would choose a Normally Closed type so that the current would flow when the magnet is pulled away, eg, when the door is open, and I could sense it and ring the alarm.
My question:
- Why are generic, low-cost magnetic switch components go for Normally Open reed switches instead of the opposite?
Best Answer
N/C magnetic switches are unfortunately nearly useless for security. Since their "unalarmed" state is for them to be open, if the cable going to them is cut then the alarm will not trigger, and of course not be triggerable anymore.
With a N/O magnetic switch the circuit is closed in the unalarmed state, and cutting the cable will trigger the alarm.