a dual light source voltage - modulated reciprocal control circuit for a scanner . the circuit includes a voltage modulation circuit , a first lamp driving circuit , a second lamp driving circuit and a reciprocal control circuit . the voltage - modulating circuit generates a modulated voltage whose magnitude can be adjusted through pulse width modulation of a square wave . the first lamp driving circuit receives the modulated voltage to drive a first lamp . similarly , the second lamp driving circuit receives the modulated voltage to drive a second lamp . the reciprocal control circuit redirects the modulated voltage to the first lamp driving circuit or the second lamp driving circuit according to the dictate of a reciprocal logic signal .