Road vehicle rear window heater circuits

Claim 1

A road vehicle rear window heater circuit, which comprises: a rear window of a road vehicle having rear window heating means associated therewith; a battery connected to said rear window heating means for supplying electrical energy thereto; a first switch connected in series between said battery and said rear window heating means; and timing circuit means connected between said battery and said rear window heating means for selectively actuating said rear window heating means for a predetermined time interval after the closure of said first switch, said timing circuit means comprising: a second switch which is resiliently urged to an open position; a capacitor connected to said battery through said second switch and said first switch, said capacitor being charged upon momentary closure of said second switch; a bistable trigger circuit comprising first and second transistors connected in a Schmitt trigger configuration, the base of said first transistor connected to receive the charge from said capacitor to initially be rendered conductive while said second transistor is rendered non-conductive; a third transistor connected across said second transistor, said third transistor being initially driven into conduction by a connection through said first switch to said battery; a relay having a coil, and a pair of contacts connected between said first switch and said heating means, said coil and contacts being actuated by the output from said third transistor for energizing said rear window heating means; said first transistor being rendered non-conductive after the discharge of said capacitor to thereby render said second transistor conductive and said third transistor non-conductive to thereby deactivate said relay and disconnect said rear window heating means from said battery.