Elk - Arduino integration

[42etus]

I'm wanting to have an Arduino output control a zone on my Elk M1. The Arduino doesn't have access to the Elk main board, only to an input expander board in a out building. I could have the Arduino control a relay and have the Elk zone connected to the relay contacts, but there must be a more elegant way to accomplish this. Any ideas would be appreciated.

 

[RAL]

I think that using a relay is going to be the most straight forward way of interfacing them.  The Elk zone inputs drive a voltage on the zone loop, so you can't connect an Arduino output directly to the Elk's input.  So that means you need a relay or some other form of isolation between them.   You might be able to do it with an opto-isolator, but I've never tried connecting one to an Elk input. 

 

[jpmargis]

A transistor will work to add an open collector output to an Arduino.  The open collector can then be connected to the Elk input.