A.C. bridge intrusion alarm system

An alternating current alarm system for the protection against unauthorized intrusion having an alarm, comprising:

• two adjacent bridge arms, each arm having at least one fixed resistor constituting a sensing resistor, and at least one variable resistor in series with the fixed resistor, constituting a balancing resistor, the combination constituting a rectangular connected electrical bridge circuit;
• normally open switch means shunted across at least one of said fixed resistors, said switch means being open in a safe condition of the system and adapted to be closed during an unsafe condition thereof;
• an alternating circuit transformer having a secondary winding connected to opposite points of said bridge arms of said bridge circuit, said secondary having a center tap winding connected to a common ground of the system, and
• at least one diode connected to the intersection of each variable and fixed resistor in each bridge arm,
• a pair of detection latch means each connected to the output of said diodes for detecting a change in the resistance value of said bridge arms and for producing a current in response to said change which actuates said alarm.