Siren Control System WB600
Police Station Equipment
Mounted on the front of the "Operating Unit WB401A" was a small panel, a "Control Unit WB600A", consisting of interlocking push buttons, each containing a small lamp and a key-switch. To activate the sirens, the key-switch must be operated with a key, which was often kept in a break glass unit. Pressing either the top right button for the Attack Warning or top left button for the All Clear would activate the remote sirens. In between the two siren buttons was a third Stop button.
In the apparatus room, duplicate "Unit Signalling WB600A" generated the tone signals used to modulate the WB400 carrier. Each "Unit Signalling WB600A" had a "Control Unit WB600A" mounted on it, the sirens could be controlled from this point too. Two audio tones were employed, a 1500Hz G-signal and a 2160Hz S-signal. To prime the siren circuit, the G-signal was pulsed 0.4 Seconds on and 0.4 Seconds off for 10 seconds. This was followed by a pulsed 4 Seconds on and 4 Seconds off S-signal, for the Attack Warning, or 60 Seconds of S-signal for the All Clear.
An earlier topic in the sequence described the police station equipment with an interactive picture too. If you haven't visited the page, a link is included in the cross reference.
Siren Point
At the siren point, a "Receiver Carrier WB600" looking like a grey plastic box approximately 18 inches wide, by 9 inches high and 9 inches deep, was connected to a telephone line via a "Filter Unit WB600A" The receiver was wired into the Home Office"s mains circuits controlling the siren.
The circuitry contained a 72kHz carrier receiver, the audio output of which could be monitored on the back panel. This audio was amplified and filtered to extract the 1500Hz and 2160Hz tones. All this was done with Germanium Junction transistors, however the G-signal pulses were detected by a pendulum relay. This device swung back and forward in response to the G-signal pulses until after a few seconds the pendulum travelled far enough to make an electrical contact. This gave the necessary immunity to prevent the receiver being trigger by the warning message speech signals.
Approximately 6 seconds of pulsed G-signal primed the S-signal circuitry. The subsequent S-signal operated a small relay, which further operated the siren 3-phase Mains contactor. The siren could also be controlled locally from the autowailer, a small box on the Home Office equipment, having buttons to sound the "Attack" and "Raiders Passed" warnings and a Stop button.
None of the WB400 or WB600 equipment circuitry contained any EMP protection. After a thunderstorm it was in my experience, often necessary to replace faulty WB600 receivers, especially those in water towers and other high points. After changing the receiver, it was good fun to run the siren for a couple of seconds.
As the WB600 control signals modulated the WB400 carrier, these would have been heard by the warning points on their "Receiver Carrier WB400". Their instructions did make mention of the remote control signal. I have made a reconstruction of the remote control signal's audio tones and siren sounding the attack warning. This is a stereo MP3 recording, with the control tones on the left channel and the siren on the right channel. Thank goodness this was never heard for real.
A Typical Air Raid Siren
This nice detailed picture of a siren, was sent to me by Patrick Bean. If you look carefully, two cables come up the pole. One cable connects directly into the siren body. This is the 3-Phase supply to the motor. The other goes to the 4 way junction box under the grey thermostat box. At the 3 & 9 O'clock position, wires come out to the fan units and go into another junction box where two 'Pyro' type of copper clad wires come out. These are the heaters to prevent the fans icing up in the British weather.