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 …

General interfacing advice using optocouplers

Zapro January 6, 2013, 1:43am 2

So that's 13V -1,25V = 11,75 / 0,03 = 391

So yes, 390 Ohms are perfect

// Per.