Automotive burglar alarm with gas siphoning detector

A security alarm apparatus for warning of attempted siphoning of gasoline in an automotive vehicle having a battery, an electrically energizeable sonic alarm, and a first variable resistor whose resistance is varied by a gas tank float, said apparatus comprising:

• a window comparator means having first and second input terminals and an output terminal, a resistive voltage divider means including said first resistor and a second manually adjustable resistor;
• means for connecting said voltage divider means across said battery;
• said voltage divider means having output signal means coupled without inversion to the first terminal of said window comparator means to form a first signal at said first input terminal, an inverting amplifier means fed by said output signal means of said voltage divider means and feeding the second input terminal of said window comparator means to form a second signal at said second input terminal;
• said second resistor being variable to initially set said alarm apparatus in a manner that said first and second signals are substantially equal, said window comparator means being configured in a manner that said output terminal has a predetermined output signal state in response to the absolute value of the difference between said first and second signals being greater than a predetermined constant, and means responsive to the output of said window comparator means for coupling said sonic alarm across said battery in response to said predetermined state.