Acoustic Trigger Circuit. The circuit utilizes a very sensitive carbon button (13) tuned to 250 cps., having a current consumption of 1.5 milliamperes (N.B. the battery life of the AE 1 is about six hours, and the AE 101 about 17 hours continous running.) The voltage output is stepped up and the steady d-c component removed by transformer (14). The transformer output is rectified by (15) and operates a sensitive relay (16). A large condenser (17), in parallel with the coil of relay (16), prevents operation by noise of very short duration such as distant countermining. It also prevent repeat operation by self noise of the mine.

The Rotary Converter. The rotary converter (18) consumes two amperes at 12 volts from the main battery, and supplies 200 volts d.c. to the echo sounder transmitter, and to the H.T. line of the receiver amplifier.

The converter is switched on when the audio frequency sound from a ship actuates the carbon microphone (13) which operates relay (16) as described above. Relay (16) operates relay (7), which connects the converter armature and field winding across the main battery (6).

The converter also drives three camshafts as described below:

The High-Speed Camshaft. This gives 12.5 impulses per second to five sets of con-tacts in the echo sounder transmitter and receiver circuits as described below.

The Low-Speed Camshaft. This is geared down to rotate once in 100 seconds. It de-termines the mine for which the echo sounder remains switched on after it has been started by the audio frequency acoustic trigger. It operates six sets of contacts.

It closes contact (19), in parallel with the contact of relay (7), and holds it closed until the end of the 100-second period. In this way it ensures that the converter remains switched on for the whole of this period, although releays (16) and (7) are switched off.

At the same time as the cam closes contact (19) it also closes contacts (20), (21), and (22) which connect the three filament batteries (23), (24), and (25) to their respec-tive tubes in the echo-sounder receiver circuits.

It was found that the noise from the converter was sufficient to operate the sensi-tive carbon microphone, and hence the converter switched itself in again at the end of every 100-second period. To prevent this, the low-speed camshaft has been made to operate another contact (26), which short circuits the condenser (17) which is in para-lell with the relay (16) of the acoustic-trigger circuit. The condenser is short circuited one second after the start of the 100-second period and unshorted again one second before the end of it. The sensitive relay (16) can not operate until after the converter has stopped at the end of the 100-second period, however, as a result of the delay period, however, as a result of the delay introduced by the parallel condenser (17).

The low-speed camshaft also operates contact (3) in series with the detonator. The contact is closed five seconds after the start of the 100-second period, and opened one second before the end of the period. In this way the mine is protected from spurious fires due to volatge surges in the amplifiers either while the rotary converter is running up to speed when it is first switched on, or while it is slowing down after being switched off.

The Lowest-Speed Camshaft. This camshaft is geared down 5:1 to the low speed camshaft. It operates a circuit whose purpose is to prevent the mine batteries from being run down by an audio frequency accustic sweep, and at the same time to make sweeping more difficult by making the mine passive under certain conditions. Every fifth live period the lowest-speed camshaft operates contact (27), which connects the coil of relay (28) to battery (29). Relay (28) then opens its contact (30) and so breaks the cir-cuit of the carbon microphone, thus making the mine passive. Contact (30) also con-nects relay coil (28) across battery (29), so that the relay remains locked in unitl battery (29) runs down. This takes from about a half hour to an hour. When the battery volatge has fallen sufficiently, the relay drops out and the mine again becomes active.

Battery (29) is normally in a run-down condition because it is shunted by resistance (31). It is charged up, however, during live cycles other than the fifth, because in each live cycle the resistance is disconnected by contact (27). Thus the mine will be rendered passive only on the fifth live cycle as described above, if the previous four cycles have occured in fairly rapid succession.