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I'd like to add a bit of redundancy to a security system. I use reed switches for doors. How can i safely connect the same reed switch to two separate arduino boards? (the boards are not connected between them in any way, they use separate adapters).

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  • \$\begingroup\$ That's not redundancy. \$\endgroup\$ Oct 1, 2012 at 9:05
  • \$\begingroup\$ I'm using the arduino just to read various sensors. The data processing is done on a linux machine (i have two of these). I'd like to connect all the sensors to two boards, to have two identical sensor-reading systems. I dont mind not having duplicate sensors since i have plenty of various types of sensors in each location (motion, ir sensor, video, etc) \$\endgroup\$
    – vlad b.
    Oct 1, 2012 at 9:23

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... How can i safely connect the same reed switch to two separate Arduino boards?
(the boards are not connected between them in any way, they use separate adapters).

If they are not electrically connected and if you wish to maintain the isolation you could use an optoisolator or similar. The reed switch could provide current via the LED in the opto isolator or voltage switched by the reed switch could apply voltage to a resistor connected to the LED. A reed could drive 2 x LEDs in two optoisolators this way this way if it had its own power supply or using the power supply from one Arduino.

Placing the LED from an optoisolator in series with the reed switch adds a reliability hazrad - if the LED fails then the input fails on both Arduinos. Placing a few diodes or a zener across the LED such that Vdiode_string is slightly higher than V_LED_on will protect against LED failure as long as there is enough drive voltage.

There are other isolating means available but an opt-oisolator is about as easy and cheap as any.

If you do not wish to maintain isolation then by connecting the two systems' grounds together you could share a sensor. But, isolated systems seem with having in the context. One mode of attacking a security system is to use a TASER or electric cattle prod or similar. This tends to be poorly tolerated [tm] in most cases. If you wish half your system to live long enough to report the other half's death when under TASER attack you will need to rate the cross coupling isolators accordingly. One method I've heard of is to use a dummy video camera which is presented as first line target. It's health is monitored by the system proper. It's role is to act as a honey-pot TASER-attack sensor. [I won't tell you what country I heard about this from :-) ].

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  • \$\begingroup\$ Didn't think of the taser issue. The two systems will be checked by a third system that can also send out gsm / email notifications in case of failure. I have a couple of linux-based systems that check each other and can do a "vote" to restart an unresponsive system. \$\endgroup\$
    – vlad b.
    Oct 1, 2012 at 11:21
  • \$\begingroup\$ I'm now thinking of doing most of the sensors as a separate circuit (+5v - wire - reed - wire - led - -5v) and connect 2 light sensors, one for each arduino, and tie the led and the sensors in electrical tape or put them in a small box with no light. That way i can "read" the led easily, and in case of a taser-related event i will only have to replace the 5v power source and the reed switches + leds. the arduino's and the linux systems would be on a different electrical sources. \$\endgroup\$
    – vlad b.
    Oct 1, 2012 at 14:19

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