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?
  • 1
    \$\begingroup\$ Someone is getting confused between NO and NC - Normally Open means open when there is no magnet. That's better, since the circuit is closed (current can flow) when the magnet is near, and cutting the wire will trigger the alarm. \$\endgroup\$ – Reversed Engineer May 27 '15 at 16:01
  • \$\begingroup\$ Dave, that is exactly the question. Both @AdamDavis and IgnacioVazquesAbrams gave great explanations as to why. \$\endgroup\$ – Phil May 27 '15 at 16:57

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.

  • 9
    \$\begingroup\$ In the "olden days", before all alarms became wireless, this also allowed several switches to be wired in series in a large loop and brought back to the alarm panel. The panel would have the ability to supervise several such loops. I used to install alarm systems some 50 years ago in college. One of the trickiest jobs I had was to put gold foil on the inside of a jeweler's window (which would break the circuit if the window was broken). \$\endgroup\$ – tcrosley May 27 '15 at 14:24
  • \$\begingroup\$ @tcrosley any reason why it was gold? Where I come from (eastern europe), foil strips around the outside of windows used to be a common sight, but they were certainly not gold. Looked like aluminium. \$\endgroup\$ – Roman Starkov Jun 5 '15 at 13:15
  • \$\begingroup\$ Probably just the jeweler's preference. I think the proper name is gold leaf. Surprisingly, it's not terribly expensive, due to it being so incredibly thin. \$\endgroup\$ – tcrosley Jun 5 '15 at 13:51

I think that a clarification is in order. When a switch, relay, transistor, etc. is NO or NC, it is in this state/condition, when the device is not activated. What this means, is that a NO switch (for example) will be closed when it is activated (a magnet is in proximity; door closed). With this kind of switch, you make a "closed loop," which will be "sensed" if the loop is broken (the door opened; wire cut). This type of application, is the reason why the switch has to be NO, when not activated!


Normally Open magnetic switches can be put them in parallel rather than series when you want one wire pair to monitor several openings.

Unfortunately if the wire is cut, it doesn't alarm, but this is taken care of by having each switch in parallel with a large resistor. If the wire is cut, the resistance changes upward, and you can tell that the wire was cut, rather than a door opening (which reduces the total resistance to zero) or closed (which has a maximum resistance based on the number of openings in the string).

It's a different topology, and is more useful when you are mixing sensor types. For instance, a floor pressure switch is normally open, and closes when someone steps on it. If you wanted a door and a floor switch to be on the same circuit, it's easier and cheaper to use an NO switch at the door, than to find a NC floor pressure pad.

Most alarm systems today have enough zones, though, that mixing various types in one zone isn't needed.

  • \$\begingroup\$ If an alarm device watches for changes in resistance, I would think one could easily combine normally-open and normally-closed devices on the same loop. A cut wire wouldn't be distinguishable from a tripped switch, but if the system should alarm in either case I don't see that being a problem. \$\endgroup\$ – supercat May 27 '15 at 15:32
  • \$\begingroup\$ Thanks @AdamDavis, I appreciate all the additional input and explanations. Although I think it would be best if I award the "answer" status to the other post, which is seems more focused on the specific subject, despite both answers making sense. \$\endgroup\$ – Phil May 27 '15 at 15:40
  • \$\begingroup\$ @supercat If the alarm is intelligent, yes, but the alarms of old just watched switch opens and closes, and only the really nice alarms watched the loop current, and even that had no intelligence - they merely had a comparator that could be set to a trip point. They wouldn't have had the intelligence to know that the resistance had changed, so much as the ability to know that it was moderate, but not open. So there were only three states: short (door open), moderate resistance (door closed), and high resistance or open (wire cut). You still wouldn't mix and match NC/NO on such a circuit. \$\endgroup\$ – Adam Davis May 27 '15 at 16:14
  • \$\begingroup\$ If one had an alarm which would trip if the resistance went above 100K or below 10K, and one ran a two-conductor cable to all the switches and stuck a 33K resistor at the end of it, one could wire NO switches across the cable and NC switches in series; opening an NC switch or cutting the wire would increase the resistance from 33K to near-infinite; closing an NO switch would drop it from 33K to near-zero. \$\endgroup\$ – supercat May 27 '15 at 16:23

It is called Fault Condition. Both conditions: Open door and Power Supply failure are covered.

  • \$\begingroup\$ excellent. would you like to elaborate though? \$\endgroup\$ – Phil May 28 '15 at 14:17

The reason NO switches are used is simply that the MB circuits are cheaper to produce, monitor, service and rectify faults because they are simple circuits without spurious parts such as resistors to go wrong than with NC switches.

  • 1
    \$\begingroup\$ What's "MB"? Why is the circuitry so different for NO switches and NC switches? How does it make it cheaper to rectify faults? \$\endgroup\$ – Finbarr Jan 3 '18 at 10:55
  • \$\begingroup\$ Why would "spurious parts such as resistors" be needed in one configuration and not in the other? \$\endgroup\$ – Transistor Jan 3 '18 at 12:08

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