Philco Engineers used tiny floating jewel in record groove to rotate a paper thin mirror ( shown as vertical switch) covered with vaporized aluminum to send audio light signals to photocell ( shown as NPN transistor) to increase sound fidelity and reduce sharp needle wear on records.
Design considerations: Lamp is a special gas-filled lamp to eliminate flicker which would register as hum at high frequency (1.8 MHz) to get rid of hum. Note: bulb filament supports are extra heavy to prevent "howl" from filament shaking.
Circuit shown in simulator is inside head of pick up arm.
