Connect external siren to camera, PSU needed?

The North Face

The North Face

Getting the hang of it
Jul 28, 2019
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North of the wall
I’m considering connecting an external siren like picture attached to the Alarm Out cabling of my T5421H-AS-PV. The spec sheet says Alarm Out is rated 300 mA 12VDC. Does that mean I can connect the siren straight to those Alarm Out cables or do I need a separate PSU?

I can’t find any proper documentation except for a wiki page at DahuaWiki, but this is for DVRs. That page claims a PSU is needed, but I’m interested to know if the same goes for my camera or if it actually pushes 300 mA at 12VDC which would be enough for the siren.

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That 300ma means that the relay contacts, actually probably a transistor of some sort, is capable of switching up to 300mah. There is no power being supplied by the camera. For safety reasons, IE to make sure I wouldn't cook the camera, I'd use that to switch a real relay and not have to worry about the current limitations of the camera. That's just me trying to avoid Murphy getting involved.
 
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Yes, because I suspect that the actual "switch"inside the camera is a solid state switch which, if overloaded in any way, could easily fail causing the camera to fail along with it. Mr Murphy has a bad habit of making things like that happen even when things are designed not to let that happen.
 
Agree, I don’t want to fry the whole thing.

With the risk of proving that I’m not competent enough to hook up the entire thing, can you give me an example of the relay circuit you’re suggesting here? I’m totally fine with the answer “if you need to ask, you shouldn’t do it” though :lol:
 
A quick description will work, I hope.

A 12 volt, DPDT relay with the coil wired to the camera on one coil lead and a 12 volt power source on the other coil lead. Then the load connected to either normally open, or normally closed, contacts of the relay along with the power source for that. The power source can be used for both relay and load power. Being the cautious type, I'd also add a 1N4002 diode across the coil to suppress spikes from the coil, cathode of the diode to the lead of the coil that gets positive power.
 
North Face,
A relay is really just a mechanical switch used to energize a load. In this case it would be your siren. Relays are used when you need to control a high current load with a small current. In the case of a siren, if it is less than 300 ma, it can be switched with the internal relay. I reviewed the DahuaWiki that you referenced and this is how it shows you to wire it up. Last sentence in that section warns as Sebastiantombs points out, failure of your circuit could take out your camera or NVR.

If you are familiar with how an automotive starter works, you are familiar with a relay although, we call it a starter solenoid. You apply a small current via your key switch to the solenoid (relay) which magnetize and attracts a slug that mechanically pulls in the huge contacts that can handle the current that the starter needs. You did not need to route that huge current demand through your key switch.

Now relate that to your siren. If it requires more than 300 ma, you would power the camera relay which when called for will close and power the relay coil. The relay coil will magnetize and pull in the main contact which provides power to the siren. The higher power for the siren only goes through the external relay switch and not the camera relay.

Relays are in the couple of dollar range for what you need. Many can get complex but you just need one that has two pins for the coil and two for the load. The relay load contacts would be normally open (NO).
 
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