Control circuit for rear view mirrors provided with a liquid crystal cell

Claim 1

Control circuit for controlling the potential difference applied to conductive layers of a liquid crystal cell forming part of a rear view mirror for motor vehicles, said circuit being adapted to be supplied from a direct current low voltage source, and comprising, in combination: resistive potential divider means, connectible to said source and including a resistance and a photoresistor; a logic invertor, connected in parallel with the photoresistor, to invert the voltage across the photoresistor; oscillator means connectible to said source for generating a square wave; a first logic NAND gate having a first input connected to the oscillator, a second input connected to the output of the logic invertor and an output; a second logic NAND gate, having a first input connected to the output of the first NAND gate, a second input connected to the output of the logic invertor and an output; conductor means connecting the output of the second logic NAND gate to one of the conductive layers of the liquid crystal cell; and further conductor means connecting the output of said first logic NAND gate to the other conductive layer of the liquid crystal cell, whereby, when the photoresistor is activated, in use of the circuit, by incident light in excess of a predetermined threshold intensity, the voltage between the conductive layers of the liquid crystal cell has a peak-to-peak amplitude double the amplitude of the square wave generated by the oscillator means.