My Doorbell Siren Project of May 2001

OK, I was bored. I had a bunch of small 12V Universal Security Instruments "On Guard" car alarm sirens around collecting dust, and I decided to do something interesting with them. These sirens were originally designed as burglar alarms, and the noisemaking parts are identical to the Federal Model A siren (only the housing is less elaborate). The sound is similar also, a very unexciting, high-pitch, single-tone wail. Here is what one of these sirens looked like:

So, I decided it would be fun to remove the siren heads from two of them, and combine them on the same motor to make a double-headed siren like the Sterling M. I purchased a double-shaft 1/15 HP motor from Grainger, and I mounted the siren stators on it successfully. It looked like this:

...but the sound was still boring. Hey, wouldn't it be neat if it had a dual-tone minor third sound like a classic WWII air raid siren? Is it possible to modify the siren to do this? Yes it is! I removed the 6-blade rotor from one of the siren heads and cut off 5 of the 6 blades. I then remounted 4 of the removed blades 72° apart from each other using Krazy-Glue™ gel and slow-set epoxy, to create a 5-blade rotor. Now my siren has a 6-blade head and a 5-blade head... thus a minor third tone (440 and 528 Hz)! What about the stator?, you may be wondering. Well, they both still have 6 ports, but it is the number of blades on the rotor that determines the pitch. The 5 blade, 6 port mismatch at one end is surely not optimal, but on a siren this small and loosely fit together, there are no practical implications. Here is the modified rotor:

Now for the finishing touches. In the above double-head siren photo, there is a piece of galvanized sheet steel over the motor. This was a different color than the aluminum siren bells, so I decided to replace it with aluminum sheet metal for the final product. Then, I subjected each aluminum part to A LOT of polishing and buffing with Tripoli (Brown buffing compound) and white rouge. Here's the finished item:

Hear this siren in action (600K MP3)

Adam Smith 6.14.01