12VDC Trigger

Does anyone know how to trigger a signal to be sent to a SmartThings hub generated by a 12vdc alarm output? I can change it to a contact closure with a 12vdc relay as another option. Tried to use the Aeotec Dry Contact Sensor but seems unreliable, not supported.

I’ve seen fancy integrations, but any one of some pretty simple hacks could work…

  • hook an electromagnet to the 12v DC (ummm… coil one of the wires around a nail… ummm… Not sure if you need resistance to avoid a hot short circuit or whatever…) and use that to trigger a basic cheap magnetic contact sensor. I’m only half goofing… This would absolutely work, I just forget my Electronics 101 regarding the “load” part of this circuit.

  • use the 12v to flip a DC relay, whose contacts are soldered to a cheap ZigBee or Z-Wave panic button or the dry contacts available in certain Monoprice Contact or Tilt Sensors… (or just short across the sensor reed).

What’s wrong with using a dry contact sensor?

Or be fancy and use an MCU…


I use a 12vdc relay to control a water sensor - just cut the plug off the water sensor and connect to the dry contacts on the relay, then power the relay from the alarm panel either n/o or n/c just configure the water sensor reaction to whatever you want wet or dry
I have a number of these running various ‘dumb’ switches & relays


That will work perfectly. A little surprised that someone isn’t making a low voltage logic device like this yet. Thanks


Me too… i would buy 20 of them if they were a good price :slight_smile:

Btw I use this to notify me if my house alarm goes off (and if the alarm stops too) The only thing extra I did was ‘tweak’ the dth to show ‘alarm active’ & ‘alarm inactive’ instead of wet & dry


Hi Cobra - what water sensor do you buy that has a plug? The one’s available in the UK (http://www.argos.co.uk/product/7497805) don’t have a plug to cut off. I suppose I could solder wires to the connectors at the back of this but was hoping to find an easier solution.

UPDATE: I found the answer to my own question by looking at your historic posts. (Foxx Project Z-Wave water sensors obtained from Amazon.co.uk). These are cheaper than the SmartSense ones and have wires to make it easy. Thanks for all your valuable posts!

Maplins had them much cheaper than amazon if they still have some


Just realised that they are even cheaper so going to buy some more myself

The door contacts are on special offer too

One thing to note…
I have found that if they are normally ‘Dry’ then go ‘Wet’ when an alarm is triggered then they are ok.
Constantly holding them at ‘Wet’ runs the battery out in just a couple of days.
For some of mine I ran them from the same PSU with a little voltage regulator from China.

Like I did for this project: