Sorry I am asking for help again, I have a home alarm system that currently has not way of telling me if it has been set off, I am looking at linking it to my OPENHAB system, all I want it to do is sent a message if the alarm is set off, I understand I can buy extra devices to do this but I am looking for a way to adapt my current system.
I have a ESP8266 and was looking at linking it to the alarm panel, on the alarm panel there is a connection for a auto dialler and this work by have a constant 12v circuit and the current is dropped to 0v in the even of an alarm. I understand I need to drop this down to 3.3v to work with the ESP but my question is how can I use this to monitor the system could I use ESPeasy or something on the ESP and then what would I need to do in OPENHAB or is this going to be to advanced for a beginner.
This is pretty advanced for a beginner and there could be unexpected side effects. You can’t monitor and step down that 12v down to 3.5v without causing changes on that original line. You might end up messing with that original 12v enough to make the autodialer no longer work. Not a guarantee but it could be a problem.
You will probably want to search around on sites like Instructables and the like for the electronics part. Maybe post to an ESP forum. We have a lot of DIYers on this forum but they do not make up the majority of OH users.
Without knowing the model of the alarm, it is most likely that the dialler output floats, and is switched to “low” (0 volts) upon activation.
A safe way to use that would usually be to use a small twelve volt relay (not a car type!) with coil wired one side to the alarms +12 and one side to the autodialler. The contacts of the relay can then trigger a GPIO input, if you are using a Pi or similar for OH.
Example https://www.maplin.co.uk/p/12v-dc-1a-dpdt-miniature-relay-n04aw
This provides isolation, avoiding linking the alarm’s ground to your OpenHAB ground, which in some circumstances could cause trouble. You don’t want an unreliable alarm.
There are of course more glamourous ways of doing the same with opto-isolators, but I would recommend simplicity. Unless you happen to use an input with opto-isolation already, for some other purpose perhaps, or a GPIO accessory for a Pi.
example
If I could work out the step down, would I be going in the right direction with loading the ESP8266 with ESPeasy and then using MQTT on the openhab to link the two together
How you could interface binary on/off is really a choice that depends on other choices you make about the system e.g. hosting OpenHAB on a Pi gives you some GPIO capability. Or zwave devices are avail that include binary sensors. etc etc.
