NVR Alarm Out Options

There are voice drivers and they use a trigger, can repeat or time out.  The relays on a Dahua recorder have a timer as well.

Be careful of over current through the relay on the recorder. 

I've done this a couple of times. The relay output on most NVRs is rated 1 or 2 amps. 12v sirens that are over 100DB are rarely over 1 amp, so you should be fine just splitting the positive side of the power signal up into it.   I would put in an over ride cut off switch on it though.  Those 12v sirens are also really cheap, like $20

What we discovered when tripping a siren off of motion detection was that the thing was going off ALL the time and the customer got so annoyed with it he shut it off.  There was so much motion from the wind, leaves, animals in this remote area that it never stopped.

So, I'd consider looking for another trigger source other than motion. A beam break sensor would be a good option.  Dakota Alert makes some very in expensive ones.  

So I looked at the Elk 150RT. It is 6 to 14V and 1.2 amps.

I looked high and low and can not find what the specs are on the Dahua recorder. Dahua NVR4208 All I can tell is it has 2 relays out.

A relay or contact usually has 2 voltage ratings. One is what closes the magnet or opens/closes themagnet, and what the load side can handle.It seems as if I was going to use a relay, I need to know the output voltage of the NVR relay.

We do not plan on using motion. We plan on using tripwire or line crossing. A photo beam would not work due to other reasons.

The building will have door contacts, alarm etc, but the owner would like to have a system that creates a warning before anyone even damages a door, etc.

What is a good model with a siren driver?

No we have never done anything like this.

It looks like the Dahua can support 30VDC at 2A so it seems plenty to drive a siren.

We need to start wiring tomorrow. Regardless of what we do, it seems like we only need to run 18/2 to the speaker/siren?

I would recommend running the DVR/NVR output to an alarm panel that would allow arming/disarming, swinger shutdown and let it drive the siren. 

The good news is there are many intrusion alarm sirens that you can use here.

Something like this DSC model:

110db SSP15P, uses 0.5A @12VDC for about $7.

The Amseco model above is a good solution too, especially if that strobe light is a benefit here.

Like Und#4 mentions, use a relay to switch power on/off by taking the negative leg and interrupting it with the relay contacts.  Use the DVR to switch the relay contacts, but a power supply to fire the siren.

Why am I having such a brain fart on finding a relay with a 30V control? I am looking for something I can put in a small 4x4 or 6x6 enclosure.

Any suggestions?