Portable motor powered siren

I had some spare RC parts and a new 3D printer and needed to make something.  I saw a pic of this great air raid siren, and thought I can do that…

It is designed to fit a 2830 BLDC out-runner motor, mine is rated at 1000KV (Banggood, ID: 1549208) and I used some of the included mounting hardware.  I used a 50A ESC, because that's what I had lying around from an old helicopter,  but 20-30A rating would probably do.

Only two parts are required to be printed at a minimum - the rotor and stator.   Next parts are duct/cover and directional horn if wanted.  And finally a stand, holder for battery and ESC and an area for electronics and control knob.

The design was motivated by this massive 1950s Cold War siren.  

Maximum loudness is achieved at about 50% PWM and only the frequency increases after that point, although it does appear to sound louder.  The frequency and relative sound pressure levels have been plotted against PWM input.  For these tests I used a 50A Skywalker ESC with a 3S  20-30C Li-Po.  At 100% PWM with my setup, peak audio frequency is 912Hz / 5 ports =  182.4 revs per second x 60 seconds = 10,944 RPM.  About 11,000 RPM which you would expect from 11.1V and a 1000KV motor.

The stator can be seen below in yellow, and also houses the motor.  The rotor, also yellow, fits inside stator with about 0.5mm of side clearance.

The red duct part is designed to direct the sound from the stator's 5 side ports into the black horn for directional effect, but I haven't tested the effectiveness of this.   

The blue tube (holder) contains battery and ESC.  For my setup, it is a tight fit, but a smaller battery and ESC would also work.  Specifically, mine are: Turnigy 3 cell 2200mAh 20-30C Li-Po battery with a Hobbywing Skywalker 2-4S 50A UBEC Brushless ESC With 5V/5A BEC (Banggood ID: 1041723) and it just fits.  The ESC also provides the power for the electronics.

The electronics is housed in the green tube above the holder, which is press fitted onto the holder for easy access to assemble and wire up.  The pink knob on top is attached to a 10k potentiometer which controls the speed. 

The green stand at the bottom presses tightly into the holder but is removable to provide access to battery charging connector.

There is a black push button on back but you will need to drill the hole to suit your switch.  My push button is rated at 15A.  Hold to power up, and keep it held -  I designed it this way for safety. 

Four M3x25 hex screws are required to attach motor body to stator, and optionally to the blue holder.  I've used hole tolerances that facilitate self tapping.  I used the included (with the Banggood part) mounting hardware to attach the rotor to motor shaft as well.  Make sure you do it up real tight as it can become detached during de-acceleration, which is super dangerous if you are not using the duct cover!

I used hot glue to attach the red duct to the stator, and also for the front safety grill.  Just a few dabs on the back of the stator does the trick and is easy to remove if needed.

These are the critical two components.    

Everything else is optional.

Motor body mounts inside stator body and is fixed with original mounting hardware that came with the Banggood motor.

The 4 holes are required on the outside of the blue tube, otherwise very hard to tighten with hex keys.  

WARNING: The blades are sharp and fast and would likely cause injury if touched whilst in operation.  I would strongly suggest using the guard mesh.

CAUTION:  Use hearing protection during testing, you'll need it.  Also use eye protection if running without a cover or speed testing for the first time.  

I initially tested with an RC transmitter / receiver.  Now I am using a 555 based circuit to generate the required pulses (1-2 ms every 20ms).  Once the ESC initialises, just turn it up and make noise.  On my ESC I have to start with the minimum setting (effectively the throttle channel on an RC model), which I believe is a great safety feature.  BTW you can also program your ESC using the knob as if it was a throttle control, if your device supports this.

The thing was designed using FreeCAD.  Original source is attached, however I'm still learning so some of the elements are a bit dodgy.  It's in lots of colours in cheap no-name PLA because that's what I have lying around.  

If there is enough interest I'll make improvements - maybe.  Hope you enjoy.

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