Vehicle burglar alarm circuit

A vehicle burglar alarm circuit includes a transformer (Tr1) including an input unit having taps (tp1,tp2) and a center tap (tp3), and an output unit magnetically coupled to the input unit; a first and a second power source (1,2) each having one electrode connected to the center tap (tp3) to respectively output a higher voltage (V1) and a lower voltage (V2); a driving unit (4) including switching elements (Q1,Q2) respectively having one end connected to the taps and another end respectively connected to the other electrodes of the power sources; a PWM controller (3a) outputting signals by which the switching elements (Q1,Q2) are alternately turned on and off; and a power source selecting unit for selecting one of the power sources. The PWM controller (3a) generates a first on-duty ratio when the first power source (1) is selected and a second on-duty ratio when the second power source (2) is selected such that the first on-duty ratio is smaller than the second on-duty ratio.