NVR With Alarm Output: Wiring a Siren or Strobe to It

An NVR with alarm output has a small relay on the back panel that closes (or opens) when an event fires, such as motion on a camera or a sensor input. That relay is only a switch: it does not supply power, so the siren, strobe or light needs its own power supply, and the relay simply completes that circuit. Wire the supply's positive lead through the relay contacts to the siren, keep the load inside the relay rating printed in your recorder's manual, then link the output to the right cameras and schedule in the recorder menu.

This guide covers the wiring, the menu settings and the testing. If your camera has a built-in siren or warning light and you do not need anything louder, you may not need the recorder at all; see how to set up a camera's own siren and motion alarm first.

What the alarm output actually is

Look at the back of the recorder. On models that have one, the alarm block is usually a strip of small screw terminals or a removable green connector, often labelled something like ALARM IN, ALARM OUT, NO, NC, COM or C. Labels and layout vary by model, so check the diagram in your manual.

Three terms matter:

• COM (common): the shared terminal. One side of your siren circuit always goes here.
• NO (normally open): open when idle, closed during an alarm. Use COM and NO when you want the siren to sound only during an event. This is what most people need.
• NC (normally closed): closed when idle, open during an alarm. Rarely used for sirens; it is for devices that should be powered until an event cuts them.

Some recorders have only two terminals for the output, with no NC. In that case they are usually COM and NO, but confirm in the manual.

The relay is a "dry contact". Think of it as a light switch that the recorder flips for you. Nothing comes out of those terminals by itself. If you connect a siren directly across COM and NO with no supply, nothing happens, and this is a common reason people decide the output is broken when it is not.

Step 1: Confirm your recorder and cameras can do this

Before buying a siren, check three things:

1. The recorder has an alarm output. Not every NVR does. Many smaller home recorders have no alarm block at all. Look at the rear panel and the spec sheet for "alarm out" or "relay output".
2. The event you want can trigger it. On most recorders, motion detection and the alarm inputs can drive the output. Smarter events such as person detection or line crossing and intrusion zones may or may not be offered as triggers, depending on the recorder, the camera and the firmware.
3. Your cameras actually report events to the recorder. A camera added through a generic protocol may record fine but not pass its motion events across. If you are still adding cameras, the guide to connecting a Yoosee camera to an NVR explains what carries over and what does not.

If any of these is a "no", a camera with its own built-in siren, or a separate alarm panel, may be the simpler answer.

Step 2: Read the relay rating and pick a siren that fits

The manual or the label near the terminals gives the relay's limits: a maximum voltage and a maximum current, usually for low-voltage DC. Write them down. The siren or strobe must stay inside both numbers.

How to match them:

• Voltage: choose a siren that runs on a low-voltage DC supply your relay is rated for. Many outdoor sirens and strobes are sold in low-voltage DC versions; check the label on the one you buy.
• Current: the siren's spec sheet lists its current draw. If you plan to run a siren and a strobe together through one output, add their draws together. The total must be under the relay's maximum.
• Worked example (illustrative figures only, not a spec for any model): say your manual rated the output at 1 A at 12 V DC, and you want a 12 V siren that lists 0.3 A plus a 12 V strobe that lists 0.2 A. Together they draw 0.5 A, which is inside that example rating. If the combined draw came close to or above the rating, you would not wire them directly; you would use an interposing relay (see Step 4). Always use the real figures from your own manual and device labels.

Never switch mains voltage (household AC) through the recorder's alarm output. Those contacts are small and are not designed or insulated for it. If you want a mains floodlight or a mains-powered bell to come on, use a separate relay module rated for mains, and have a qualified electrician install it.

Step 3: Wire a siren to the alarm output

Switch off the recorder and unplug the siren's power supply before you touch any terminals. If any part of the job involves mains wiring, a ladder or a roof, stop and use a qualified electrician.

The basic circuit for a low-voltage DC siren using the normally open contacts:

1. Run the positive (+) lead from the siren's power supply to the recorder's COM terminal.
2. Run a wire from the recorder's NO terminal to the siren's positive (+) input.
3. Run the siren's negative (−) lead straight back to the power supply's negative (−).
4. Check polarity on the siren. Many sirens and strobes will not work, or may be damaged, if + and − are swapped.
5. Tighten the screw terminals firmly and tug each wire gently. A loose strand is a common cause of a siren that works on the bench and fails on the wall.

When the recorder fires an event, the relay closes, COM connects to NO, and power flows from the supply through the relay to the siren.

Practical wiring notes:

• Cable: use cable suited to the length and current. Long, thin runs drop voltage, and the siren may sound weak. If the siren is far from the recorder, it is often better to put the power supply near the siren and run only the switched pair back to the recorder.
• Outdoors: keep joints inside a weatherproof box and mount the siren where it cannot easily be reached and pulled off.
• Keep it tidy: label the wires at the recorder end so the next person knows what they are.

Step 4: When to add an interposing relay

An interposing relay (also called a relay module) is a separate, stronger relay that your recorder's output switches on, and that in turn switches the real load. Use one when:

• The siren and strobe together draw more than the recorder's relay rating.
• You want to switch mains lighting or a mains bell.
• You want one event to control several devices with different power needs.

The recorder's contacts then switch only the module's coil (powered from your low-voltage supply), which draws far less than a siren or lamp, so the small contacts on the recorder are protected. The module's own contacts carry the heavy load. Choose a module whose coil voltage matches your supply and whose contacts are rated well above the load. Many ready-made modules include a suppression diode across the coil; if you are using a bare relay, ask an installer whether one is needed. For anything on mains, have an electrician fit it in a proper enclosure.

Step 5: Link the output to events in the recorder menu

Wiring is half the job. The recorder still needs to be told when to close the relay. Menu names differ between recorders and firmware versions, but the path is usually similar:

1. Open the recorder's main menu, then Event, Alarm or Motion settings.
2. Select the camera channel you want to trigger the siren.
3. Turn on motion detection for that channel, and set the detection zone and sensitivity. The same principles as on the camera apply; our guide to motion detection zones and sensitivity explains how to stop leaves and headlights from triggering it.
4. In the linkage or "actions" section, tick Alarm Out (sometimes shown as "Alarm Output 1" or a relay number).
5. Set the alarm duration or "latch time", which is how long the relay stays closed after an event. Start short. A siren that wails for a long time after a cat walks past will annoy neighbours quickly.
6. Set a schedule: for example, only overnight, or only when the premises are closed.
7. Save, and if the recorder asks, apply the same settings to other channels you want linked.

Some recorders also have a separate Alarm Out page with a mode setting (for example "auto", "manual on" and "off"). Make sure it is on auto, or whatever your manual calls the mode that follows events.

Step 6: Test it properly

1. With the schedule active, walk into the detection zone of a linked camera.
2. The siren should start within a moment or two and stop after the duration you set.
3. If the recorder has a manual trigger or "alarm test" button, use it to check the wiring separately from the detection settings.
4. Test at night too. Infrared lighting, insects and shadows behave differently in the dark.
5. Tell neighbours before you test a loud outdoor siren, and keep tests short.

Reading the symptoms

Nothing happens at all. Check the power supply first: is it plugged in, and does the siren work when you briefly connect it straight to the supply? If yes, the problem is the relay path or the settings. If the manual trigger works but motion does not, the wiring is fine and the issue is in the event linkage or schedule.

The siren sounds all the time. You have probably wired to NC instead of NO, or the output mode is set to "manual on". Swap the wire to NO, or change the mode back to automatic.

You hear the relay click, but the siren is silent. The relay is working. Look at polarity, the supply, and loose terminals at the siren end.

The siren is weak or crackly. Voltage is dropping along the cable or the supply is too small. Use thicker cable, a shorter run, or move the supply closer to the siren.

It triggers on rain, cars or animals. That is a detection problem, not a wiring one. Narrow the zone, lower sensitivity, use person detection where offered, or switch to a line-crossing rule if your setup supports it.

It worked, then stopped after a firmware update. Re-open the event and linkage menus. Settings can reset, and option names may move between versions.

The recorder reboots when the siren fires. Something is drawing too much current or putting electrical noise back into the recorder. Disconnect the siren straight away, then check that it runs from its own supply (not from a power port on the recorder) and that the load is within the relay rating.

When to get help

Ask for help if:

• The job needs mains wiring, a new power point, or work at height.
• You want the siren to tie into an existing intruder alarm panel. That is a bigger design question; read how CCTV and a burglar alarm work together before deciding which system should control the siren.
• The recorder's manual does not list a relay rating and you cannot confirm it from the seller.
• You live somewhere with rules on outdoor sirens, such as limits on how long they may sound. Check local rules before mounting one outside.

A qualified electrician or security installer can confirm the rating, fit an interposing relay where needed, and make the outdoor wiring weatherproof.

Install or resell cameras?

For installers and shops, the alarm output is a useful extra to explain, because customers often do not realise their recorder has one. During the survey, check the rear panel for an alarm block and note the relay rating before quoting a siren. Bring a low-voltage siren and a matching supply as standard kit, and use an interposing relay as soon as a customer asks for mains lighting. At handover, show the customer the alarm duration and schedule settings so they can quiet it themselves, and leave the wiring labelled. Dealer and partner details for the Yoosee range are at business.yoosee.com.

Frequently asked questions

Can I power the siren from a DC output on the back of the recorder?

Some recorders have a small DC output terminal intended for sensors. Check its rating in the manual before using it for anything else; it is often too small for a siren, and overloading it can upset the recorder itself. A separate supply sized for the siren is the safer choice, with the alarm output only switching it.

What is the difference between alarm input and alarm output on an NVR?

The alarm input receives a signal from something else, such as a door contact or a motion sensor, and tells the recorder an event happened. The alarm output is the relay the recorder switches in response to an event. You can link the two, so a door contact opening makes the recorder record and sound a siren.

Can a wireless camera trigger the NVR alarm output?

Often yes, if the wireless camera reports its motion events to the recorder and the recorder lets you choose that channel as a trigger. Whether events carry over depends on the camera, the recorder and how the camera was added. Test it with a walk-past before relying on it.

Can I connect more than one siren to a single alarm output?

You can, as long as the combined current of all devices stays under the relay rating in the manual. If the total is close to or above the limit, use an interposing relay module so the recorder's contacts only drive a small coil.

Why does my siren keep sounding after the person has left?

The alarm duration or latch time in the recorder is set too long, or new motion keeps restarting it. Shorten the duration, and tighten the detection zone so passing traffic or swaying plants do not keep re-triggering the event.