Shallow water depth alarm

Claim 1

IN A DEPTH ALARM DEVICE HAVING TRANSDUCER MEANS FOR GENERATING AN ECHO RANGING PULSE SIGNAL, THE COMBINATION COMPRISING (A) A FIRST MEANS FOR DERIVING A VARIABLE WIDTH GATING SIGNAL IN RESPONSE TO THE LAGGING EDGE OF SAID ECHO RANGING PULSE SIGNAL INCLUDING DIFFERENTIATOR MEANS FOR DIFFERENTIATING SAID ECHO RANGING PULSE SIGNAL AND PRODUCING FIRST AND SECOND SPACED APART SIGNAL SPIKES AND A MULTIVIBRATOR CONNECTED TO SAID DIFFERENTIATOR MEANS FOR PRODUCING SAID VARIABLE WIDTH GATING SIGNAL IN RESPONSE TO SAID SECOND SIGNAL SPIKE, (B) ALARM MEANS, (C) COINCIDENCE MEANS COUPLED TO SAID FIRST MEANS FOR COUPLING AN ECHO SIGNAL DERIVED FROM SAID PULSE SIGNAL, UPON TIME COINCIDENCE BETWEEN SAID GATING SIGNAL AND SAID ECHO SIGNAL, TO SAID ALARM MEANS TO ACTIVATE SAID ALARM MEANS WHEN THE TIME LAG BETWEEN SAID PULSE SIGNAL AND SAID ECHO SIGNAL IS LESS THAN A PRESET VALUE, (D) MEANS CONNECTED BETWEEN SAID ALARM AND COINCIDENCE MEANS FOR INTEGRATING SUCCESSIVE SIGNALS AND PREVENTING ACTIVATION OF SAID ALARM MEANS UNTIL THE RATE OF RECEPTION OF SAID ECHO SIGNALS EXCEEDS A PREDETERMINED VALUE, AND (E) CALIBRATION CIRCUIT MEANS CONNECTED TO SAID MULTIVIBRATOR FOR TESTING THE RESPONSE OF SAID DEPTH ALARM DEVICE INCLUDING DIFFERENTIATOR MEANS RESPONSIVE TO SAID GATING SIGNAL FOR PRODUCING THIRD AND FOURTH SIGNAL SPIKES SPACED APART BY AN AMOUNT INDICATIVE OF THE MAXIMUM DEPTH AT WHICH SAID ALARM IS ACTUATED.