Single touch flash charger circuit with timer control

Abstract

A single touch flash capacitor charging circuit particularly useful in low cost cameras incorporating an oscillation timeout circuit to arrest self-sustaining oscillations for conserving battery consumption. The timeout circuit includes a reference voltage integration circuit coupled to the secondary of the oscillation transformer to develop a reference voltage that is independent of battery voltage. A timer charging circuit is coupled to the voltage reference circuit and determines the timeout period. Closing of a start charge switch resets the timer charging circuit to a fixed voltage differential from the integration circuit reference voltage. When the start charge switch is opened, the timer circuit begins charging to the voltage reference. Once initiated, the flash capacitor charger circuit continues charging the flash capacitor for the duration of the timeout period set by the timer circuit. At the end of the timeout period, the timer charging circuit turns on a pair of switching transistors which operate to shunt off the oscillation transistor in the flash capacitor charger circuit. The start switch resets the timer circuit either through the emitter-base coupled transistors or by auxiliary switch contacts which couple the timer circuit directly to the battery terminal. A low voltage LED charge indicator circuit is also coupled to the transformer secondary to indicate full charge of the flash capacitor.