Dry Contact or Voltage Input Alarm Siren? How to Choose for Your PLC, Relay, or Sensors

1. Start With the Controller Output, Not the Supply Voltage

When checking a PLC, sensor, or controller manual for this type of application, go straight to the Output Specification, Digital Output, Alarm Output, or the output wiring diagram.

Common terms include:

• Dry contact
• Volt-free contact
• Potential-free contact
• Relay output
• Voltage output
• PNP / NPN
• Transistor output
• NO / NC / COM

You do not need to turn this into a controls-engineering exercise. First determine whether the output changes a contact state or actively provides a powered signal.

If the Output Is a Dry Contact

A dry contact may also be described as a volt-free contact or potential-free contact.

For selection purposes, think of it as a switch controlled by another device. The controller changes the contact state; the contact itself is not being used as a powered trigger output.

If the manual clearly says dry contact, volt-free, or potential-free, a dry-contact siren input is the logical place to start.

If the manual only says Relay Output, check one step further. Look at the output description or wiring diagram and see whether the relay contacts are identified as dry, volt-free, or potential-free.

NO and NC answer a different question. NO means normally open, and NC means normally closed. Those labels describe the normal contact state; they do not by themselves tell you whether the output is dry or powered.

If the Controller Actively Outputs a Voltage

Some PLCs, controllers, and sensors provide a powered electrical signal when an alarm condition occurs.

Depending on the manufacturer, the manual may call this a:

• Voltage output
• Active signal
• Powered output
• Wet contact

For siren selection, the useful information is: what appears at the output when it becomes active, and what voltage that signal uses.

For example:

Alarm output: 24V DC when active

Here, 24V describes the actual alarm output. That is useful when matching a voltage-input siren.

Now compare it with:

Power supply: 24V DC

That only describes how the controller itself is powered. It does not prove that the controller also sends a 24V alarm signal.

Why “My PLC Is 24V” Is Not Enough

Keep three specifications separate:

Controller power
How the PLC, sensor, or controller itself is powered.

Controller output
What the controller provides when the alarm condition occurs.

Siren power
What power supply the siren itself requires.

A PLC powered by 24V DC may use a relay output, a transistor output, or another output arrangement. The siren can also have its own AC or DC supply while using a separate signal input for activation.

That is why “My PLC is 24V” does not automatically mean “I need a siren with a 24V trigger input.”

If your main question is the siren's own power connection, see Do Alarm Sirens Come With a Plug, Switch, or Power Adapter?

If you are checking 12V and 24V supply compatibility, see Can I Connect a 12V Alarm Siren to a 24V Power Supply?

This guide is focused on the other side of the system: what signal tells the siren to activate.

2. Match the Controller Output to the Siren Input

Once the Output Specification is in front of you, most applications narrow down quickly.

If the device clearly provides a dry, volt-free, or potential-free contact, start with a dry-contact input.

If the device actively provides a powered signal when triggered, confirm the actual signal voltage and match it to the appropriate voltage-input configuration.

“My PLC Has a Relay Output.”

Check whether the manual goes on to describe that output as dry contact, volt-free, or potential-free.

If it does, the dry-contact direction is fairly clear. If the manual only says “Relay Output,” check the terminal description or output wiring diagram before ordering.

“My Controller Outputs 24V When the Alarm Is Active.”

Confirm that 24V refers to the actual output signal, not the controller's own supply voltage.

If the specification clearly states that the output becomes 24V DC when active, match the siren to the corresponding powered-input configuration.

“My Sensor Says PNP or NPN.”

PNP and NPN are common electronic sensor output types. They are not another name for a dry contact.

They can be used as trigger sources when the siren input is configured for the corresponding signal. Check the sensor output specification and voltage level before choosing the siren input.

“I Don't Know What My Device Outputs.”

Do not choose an input version from statements such as “The PLC is 24V” or “The sensor is 12V.”

Look for any one of these instead:

• Output Specification
• Digital Output page
• Output Wiring Diagram
• PLC / sensor / controller model number

Before ordering, the useful information can usually be reduced to four questions:

1. What device will trigger the siren?
2. Does it provide a dry contact or a powered output?
3. If it provides a powered output, what is the actual signal voltage?
4. What power supply is available for the siren itself?

3. Choose the Input Type First, Then Choose the Siren

Once the trigger input is confirmed, the next question is no longer just: “Will it work with my controller?”

Now you can compare the physical format, adjustment method, and other features that make one siren a better fit for the installation than another.

SLA-01HK: Traditional Horn + Strobe Design

The SLA-01HK uses a traditional horn-and-strobe layout. It has separate power and external signal leads, an on-unit volume adjustment knob, and USB support for replacing the stored alarm voice.

For the dry-contact configuration, the control logic is direct:

Signal contact closed → Alarm on
Signal contact open → Alarm off

If the controller provides a powered signal instead, confirm the actual signal and voltage so the corresponding input configuration can be selected.

This format is a practical option when the installation suits a conventional industrial horn + warning-light design and local volume adjustment is useful.

SLA-B02K: Box-Type Design With Remote Adjustment

The SLA-B02K is also controlled by an external signal, but it uses a different box-style housing.

For the dry-contact configuration:

Signal contact closed → Alarm on
Signal contact open → Alarm off

Its adjustment method is different from the SLA-01HK. The SLA-B02K supports infrared remote adjustment for volume and alarm tone, as well as USB voice replacement.

The difference between these two models is not that one works with a PLC and the other does not. The selection order is: match the trigger input first, then choose the siren format and control style.

If you want to compare more signal-triggered models before choosing, browse the Dry Contact & Voltage Input Alarm Siren collection .

If the controller output type, powered-signal voltage where applicable, and siren power supply are already clear, you can move directly to the matching product configuration.

If they are not clear yet, the most useful information to provide is:

• PLC / controller / sensor model number
• Screenshot of the Output Specification
• Output Wiring Diagram

Those details are more useful for checking compatibility than a statement such as “My PLC is 24V.”

4. Frequently Asked Questions