The June 1942 issue of Service magazine carried this ad for an air raid alarm to be added to any radio.  It noted that the first sign of an air raid would often be local radio stations leaving the air, lest they serve as a beacon for incoming bombers.  If you were listening to the radio during the day, you would know immediately.  But at night, with the radio off, you would be caught unaware.

With this home alert, you would leave the radio on standby, and if the station you were tuned to left the air, this would be the sign of a possible air raid.  The ad noted that during air raid alarms in Los Angeles, radios equipped with this device sounded the alarm from six to ten minutes before the sirens sounded.

The unit sold for $5, and could be installed by a local service shop.