Automatic backlight on/off control apparatus for liquid crystal display television

Claim 2

An automatic backlight on/off control apparatus for use in a liquid crystal display television, which comprises: a buffer amplifier means for buffer-amplifying and integrating composite video signals entering a composite video signal input terminal, a synchronizing signal detecting means for detecting and outputting a synchronizing signal according to a pulse output level of said buffer amplifier means, a switching control means for outputting a switching control signal with a differential amplification applied after integrating the output signal of said synchronizing signal detecting means, and a voltage control means for supplying or cutting off a voltage of a backlight voltage terminal to a backlight by switching operation in accordance with the switching control signal of said switching control means.

Claim 3

The automatic backlight on/off control apparatus of claim 1, wherein the synchronizing signal detecting means is constituted by connecting the output of said buffer amplifier means through a first capacitor and a first resistor in the parallel connection to the emitter of a first transistor of which base is applied with a constant bias voltage, connecting the emitter of a second transistor of which base is applied with a constant bias voltage to the base of a third transistor as well as to the collector of said first transistor through said first resistor, and connecting the collector of said third transistor to the base of a fourth transistor as well as to the emitter of said fourth transistor and a second resistor through a third resistor in order that a high potential is outputted only in case the output voltage of said buffer amplifier means is below a determined level.

Claim 4

The automatic backlight on/off control apparatus of claim 1, wherein the switching control means is constituted by connecting the output of said synchronizing signal detecting means to the base of a fifth transistor in the darlington connection with a sixth transistor and a second capacitor, connecting the collector of seventh and eighth transistors in the darlington connection to which a constant bias voltage is applied to the bases of ninth and tenth transistors as a constant current source and the collector of said ninth transistor connecting the emitters of said seventh and eighth transistors to the collector of an eleventh transistor to which a constant bias voltage is applied, and connecting the collector of said tenth transistor to a third resistor in order that a low potential signal continues to be outputted from said synchronizing detecting signal keeps on being outputted.