Mobil Directo / Allertor Yelp Signal - Explained

[jerrylovessirens]

There seems to be quite a few people making dubious claims about the fabled Mobil Directo/Allertor "Yelp" signal. So I decided to go straight to the source...

[The Yelp signal was accomplished by a] Louis Allis motor that had 9 leads. It used three contactors that switched the motor from 2 pole 3500 RPM to 600 RPM 12 pole. That made the yelp sound. During the yelp the motor became a generator and put power back on the line. When Louis Allis went out of business we bought special made motors from Doerr Electric in Cedarburg, Wi. I designed a copper core motor that could take the heat made during three phase reversal. We used a two contactor magnetic starter with a timer that reversed the motor for 2 seconds to make the yelp then waited 5 seconds before switching to forward again. - Jim Biersach 12/2022

[siren fan]

Any videos exist of this function?

[jerrylovessirens]

No, not that I'm aware of. I know there are quite a few synths, some done better than others

[FSThunderboltfan1000]

Although it is not a Mobile Directo, Here is the audio of the Allertor Yelp signal
https://youtu.be/ijO0BrOq_z4?t=860

[jerrylovessirens]

Thanks Tom, totally forgot about that audio clip Justin had gotten from Mr. Biersach. Thanks for posting