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Topic: Interface to existing alarm system (Read 2424 times) previous topic - next topic



I have an existing wired alarm system and wish to interface it to my Arduino.

The zone is a normally closed circuit which opens when the sensor is activated. I would like to keep the existing alarm system working but have the zone inputs going to the Arduino as well.

Not sure how I could wire this safely.

Any help would be appreciated.



A small relay (5V operated reed relay perhaps) is the safest way - I presume this is a low-voltage circuit and main-rated relay is not necessary.  A relay with normally closed contacts would be ideal.

Alternative if you have more information on the alarm circuit and voltages involved then a transistor circuit might be feasible.
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Yes its low voltage. Need to check the voltage.

I thought of a relay but I have 5 zones so would need to 5 relays.

I have attached a circuit diagram. Would this work OK ?


zone inputs going to the Arduino as well.

Alarm systems normally run a current loop through a small resistor for each zone.  Normally-closed sensors are wired in series with one another across the zone inputs.  Normally open ones are wired in parallel across the inputs.   An open circuit on a normally-closed sensor will drop the current in the circuit to 0 and the alarm condition will be detected.  A short circuit in a normally-open sensor will cause a current rise, and the alarm condition will be detected.

To snoop this, you will have to put a voltage sensor -- and it could be an A2D input or a comparator circuit -- on the zone connections of the alarm system and look at how the voltages  change with open/short conditions.  Normally there is a very wide range of currents that are accepted as "no alarm" by the system -- maybe from 1 mA through 100 mA -- a short brings the current above 100mA and an open drops it below 1mA.  Finding the corresponding magic voltage levels at the alarm system terminals will take some trial and error.

The installation manual for your alarm system unit is likely available online, and you will get better information from that.


The zone is a normally closed circuit which opens when the sensor is activated.

Are you sure about that?  A good alarm system that is correctly wired uses resistors in series with and/or in parallel with the sensor contacts.  The alarm circuitry can then detect whether the sensor is open or closed and also whether the sensor has been bypassed (shorted) or if the wires between the sensor and the alarm have been shorted or opened.  With that in mind you should see that your proposed circuit will not work.


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