Portable Motor-Driven Siren

Why Build a Siren?

I recently made a VERY loud portable motor-driven siren because:

1. I had some spare RC parts, I’m a big fan of repurposing functional bits rather than throwing them out.
2. I just got my first new 3D printer, a Prusa
3. I wanted to make something cool.
4. And finally I saw a picture of an air raid siren, I thought, "I can make that "

The result? A ridiculously loud and fun build with surprising practical applications. While this is far from a "refined" design, it’s proof of concept—and with a little know-how in electronics, some soldering skills, access to a basic 3D printer, and a few RC components, you can make one too.

The overall shape of my design was influenced by a picture of a massive Cold War-era siren, which I later discovered to be an ACA (Alerting Communications of America) Allertor 125. Its bold, retro aesthetic and functional design inspired me to create this scaled-down, portable version for the project.

RC Parts

1. 1x Brushless DC Motor, I used a 2830 BLDC out-runner motor, rated at 1000KV from Banggood
2. 1x Electronic Speed Controller, I used a Hobbywing Skywalker 2-4S 50A UBEC Brushless ESC With 5V/5A BEC from Banggood
3. 1x Li-Po Battery, I had used a Turnigy 3 cell 2200mAh 20-30C Li-Po

Electronic Parts

1. 2x 555 timers
2. 2x 100n capacitors
3. 2x 10n capacitor
4. 1x 330k resistor
5. 1x 1k5 resistor
6. 1x 10k resistor
7. 1x 10k potentiometer
8. 1x push button switch (I used a 15A rating)

3D printed parts

1. Base green
2. Cover red
3. Guard black
4. Holder blue
5. Horn black
6. Rotor yellow
7. Stator yellow
8. ElectBox green
9. ElecKnob pink

I have no commercial affiliation with any suppliers. If I include a link to a supplier, it’s simply where I sourced my parts. I’m based in Australia, I try to shop locally whenever possible.

The siren works by forcing air through spinning parts to produce a loud, wailing sound.

At its core is a spinning component called the rotor (green), which has openings, or ports, around its edge. The rotor is driven by a motor that spins it very fast. Surrounding the rotor is the stator (yellow), a stationary part with matching ports. Together, the rotor and stator form the heart of the siren.

When the rotor spins, it draws air in and pushes it out through the ports at high speed. As the rotor's ports align and then block the stator's ports repeatedly, bursts of air pressure are created. These bursts generate sound waves.

The ports play a critical role in determining the frequencies that the siren can produce. On this design, the stator has three groups of ports, allowing for a richer beat frequency and a more distinct sound. The rotor has 5 ports.

The pitch of the sound is controlled by the speed of the rotor—the faster it spins, the higher the pitch. To create the classic "wailing" effect, the motor's speed is varied, causing the sound to rise and fall.

Surrounding the rotor and stator is a cover and horn, which amplify the sound and make it more directional.

In this design, the rotor is spun using a standard RC motor, like those found in drones or RC planes. This makes the siren compact yet incredibly powerful—perfect for this portable build.

In short, a mechanical siren works like a super-powered whistle, chopping air into powerful, oscillating sound waves!

The motor is connected to the ESC (Electronic Speed Controller), which is powered by the battery—this follows a very standard RC model configuration. Additionally, the ESC provides 5V power to other circuitry.

Initially, I tested the setup with an RC transmitter and receiver. However, this design uses a 555 timer-based circuit to generate the required PWM pulses (1–2 ms every 20 ms). This is essentially the same type of PWM signal you’d get from an RC receiver.

Once the ESC initialises, the motor's power/PWM can be controlled using the potentiometer located at the top of the siren.

On my ESC, I have to start with the minimum PWM setting (effectively the throttle-down position in RC terms). I consider this an excellent safety feature, ensuring the motor doesn’t start unexpectedly.

If your ESC supports it, you can also program its settings using the potentiometer as a throttle control. However, for my build, the out-of-the-box settings worked perfectly.

I found maximum loudness is achieved at about 50% PWM and only the frequency increases after that point, although it does appear to sound louder.

For my tests, I used a 50A Skywalker ESC paired with a 3S Li-Po battery pack rated at 20–30C. At 100% PWM the peak audio frequency: 912 Hz (measured using an audio spectrum analyser on my PC).

So this means the rotor speed is equal to 912 Hz × 5 ports = 182.4 rotor revolutions per second × 60 seconds = 10,944 RPM . This result aligns with expectations for a 11.1V power supply and a 1000KV motor., i.e 11,100 RPM.

For this project, I used a Prusa MK3S+ printer and some no-name PLA filament I had lying around. Since I was finishing off the ends of various rolls, the final siren ended up being quite colourful! If I were to print another one, I’d opt for a stronger, tougher filament (like PETG, ABS, or Nylon) for the stator and rotor, as they experience the most stress during operation.

You’ll need to enable supports when slicing the cover to ensure the internal shape prints correctly.

Files for Printing

I’ve included the following files so you can print and build your own:

1. STL files for slicing in your preferred software
2. G-code for direct use, optimised for a Prusa MK3S+ (just in case you’re using the same or compatible printer)

For those interested in modifying or remixing the design, I’ve also uploaded tried uploading a FreeCAD design file. Unfortunately, I couldn’t upload the FreeCAD file here, but you can find it by searching for me on Printables.com.

Legend

1. Base green
2. Cover red
3. Guard black
4. Holder blue
5. Horn black
6. Rotor yellow
7. Stator yellow
8. Electronics Enclosure green
9. Knob pink

You’ll need to solder the circuit according to the circuit diagram. Feel free to use whatever soldering technique you’re most comfortable with—there are no high-current connections on this board, so it’s quite forgiving. The key requirement is that the circuit fits neatly into the electronics housing (the green component at the top) and includes a connection to the potentiometer for control.

The battery, motor, and ESC should be connected using the provided high-current wiring, following the standard practices for RC setups.

Be extremely careful not to short the battery during this step, as Li-Po batteries can be dangerous.

Before installing the electronics into the plastic components, it’s important to test the circuit and ensure everything is working correctly.

Clamp the motor securely during testing—without proper restraint, it could spin up and shoot off the desk! This is especially important with high-RPM motors like the one used in this project.

Don't try full speed.

The back of the motor is securely attached to the mounting bracket using the supplied hardware (a bracket and four hex bolts). For my setup (a Banggood motor), these parts worked perfectly.

The bracket is then attached to the stationary stator body using self-tapping screws. There are pilot holes to help with alignment—be careful not to over tighten these screws, as they will be removed in a later step.

The rotor is fixed to the motor shaft using the provided mounting hardware, which is typically used for attaching propellers. Ensure the shaft bolt is tightened securely—this is critical to prevent the rotor from coming loose during operation.

As an added precaution, I applied CA (super glue) to the bolt to make sure it stays firmly in place.

Carefully align the rotor and stator. There’s only 0.5 mm of clearance between them, so proper alignment is crucial. The rotor must spin freely without touching or scraping the stator. Take your time to get this step right.

DO NOT Power Up Yet!

As shown in the image above, there are four holes on the inside of the holder. Bolts or screws pass through these holes, securing the motor bracket to the stator. For my build, I used hex bolts and CA glue for added precision, but self-tapping screws would likely work as well. The connection must be tight and secure.

After assembly, ensure you can still spin the rotor and stator freely by hand without any obstruction or scraping. This step is crucial to avoid damage during operation.

To make assembly easier, I added four holes to the outside of the holder, allowing easier access for tightening the bolts using hex keys (Allan keys) - but these are optional.

Finally, at this stage, drill one hole large enough for your push-button switch to fit through.

At this stage, you can perform the first real test of your siren.

In an earlier step, you should have built and tested all the electrical components. Now, connect these to the motor for the first time.

Clamp the holder very tightly to your workspace, but be careful not to crush or damage it.

Push the button to power on the circuit.

Once the ESC initialises (mine plays a tune through the motor), slowly turn up the potentiometer.

Pay close attention for any scraping or unexpected movement. If anything seems off, release the push button immediately to shut it down.

Safety Precautions

1. Hearing Protection is Essential: This test will be extremely loud, so always use proper hearing protection.

1. Eye Protection is Essential: Especially if running the test without the cover or conducting speed tests for the first time. Bits can shatter, break off, and cause injury—I’ve experienced this firsthand, and it’s not worth the risk!

1. WARNING: Rotor Blades are Sharp and Dangerous: The rotor blades spin at very high speeds. Avoid any contact while the siren is in operation, as the blades can cause serious injury.

1. Do Not Push to Full Power Yet: At this stage, DO NOT attempt full power.

The top part of the holder houses the electronics board, with the potentiometer installed above and attached to the knob.

The battery is positioned at the bottom of the holder. Ensure it sits securely but remains accessible for charging.

All the parts should push together easily without the need for glue. If you decide to use glue, avoid applying it between the holder and stand, as this will prevent access to the battery for charging or maintenance.

I used hot glue to attach the cover to the stator. Hot glue provides enough working time to carefully align the components before it fully sets. Once I achieved proper alignment, I applied additional glue to ensure a secure bond.

If you ever need to open the housing, hot glue can be easily cut away, making disassembly more convenient.

The horn (black) simply presses onto the cover.

The guard (yellow) is secured to the cover using hot glue. Check to ensure there is no contact with the spinning parts.

Your completed siren should look similar to the pictures provided—but hopefully in better colours than my "use-up-the-rolls" approach! Feel free to get creative with your colour choices to make your build truly unique.

1. Safety Precautions:
2. Always wear hearing protection. The siren is extremely loud and can cause hearing damage, especially at close range.
3. Hold away from your body. Move bystanders to a safe distance.
4. Start-Up:
5. Turn the potentiometer to its minimum setting.
6. Push and hold the button—the siren operates only while the button is held down.
7. Once the ESC initialises (it may play a tune), slowly increase the speed by turning the potentiometer.
8. Sound Effects:
9. Adjust the potentiometer to create a wailing-type sound.
10. Alternatively, you can pulse the button momentarily to produce a different type of sound effect.
11. Operational Limits:
12. Do not hold the button for more than 10–15 seconds at a time. The parts can heat up, and any deformation (especially with PLA components) could be critical. Using stronger materials, like PETG or ABS, can significantly improve durability.

Check local regulations

Since sharing this project on YouTube, I’ve learned that sirens like this may be illegal in some countries or jurisdictions. It’s best to verify your local laws before use.

Check out the video to hear what mine sounds like.

Portable Motor-Driven Siren © 2025 by Mark Makies is licensed under CC BY-SA 4.0

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