Hi all. I am interfacing my alarm system with an Arduino.
Here is the write up from the alarm manual.
OUTPUTS (J3): These outputs are programmed as active low output. They are held at 13V and fall to 0V when active, it can source or sink 10mA. These outputs would normally be connected directly to the input channels of wire in type communicators. Alternatively each output can be used to drive a relay (coil resistance > 1200 Ohms) connected between the output terminal and the 13V supply terminal. The relay will energise when the output port operates. It is recommended that a back EMF protection diode is used in parallel with the relay coil.
I have some MOCD207M optocouplers handy, so I was going to connect the diodes to the 13V and the outputs so when the outputs went low, current would flow through the diodes and activate them.
Looking at the optocouplers datasheet http://www.fairchildsemi.com/ds/MO/MOCD207M.pdf a forward voltage of around 1.4v @ 30mA is suggested. Should I use a 390R resistor to drop from 13v? The alarm manual says the outputs can source or sink 10mA so would 30mA flowing into them overload them? Or should I try and limit the current to 10mA and hope it is enough for the optocouplers - I can't see the minimum current in the MOCD207M datasheet?