Commonly assigned U.S. Pat. No. 5,574,337 discloses a "one touch" flash circuit useful in a single use camera. A flash charging circuit is initially started by depression of a "one touch" button which applies a forward bias potential to an input terminal coupled to a base terminal of an oscillation transistor to initiate self-regenerated oscillations used to charge a flash capacitor. The charging circuit continues oscillating even after the "one touch" button is released. When the flash capacitor is fully charged, a feedback circuit responsive to charge potential on the flash capacitor terminates the oscillations. When a picture is taken with flash operation, energy from discharging of the flash capacitor is fed back via the secondary winding of the step-up transformer to the base of the oscillation transistor of the flash charging circuit to drive the oscillation transistor into conduction thus restarting the self-regenerating oscillations.
In commonly assigned U.S. Pat. No. 5,634,153, there is disclosed a similar circuit in which a resistor is coupled between the base input terminal of the oscillator circuit to ground to suppress the possibility of static charge voltage initiating oscillator operation at undesired times. When the camera is in use but left idle for a period of time, such as two hours, the flash capacitor voltage declines to, for example, 200-240 volts. While this is sufficient to cause the flash tube to fire, the base resistor in the oscillator may, in some cases, prevent restart of the oscillator. To ensure restart of the oscillator after flash operation under these conditions, a feedback capacitor of appropriate size is coupled directly from the discharge terminal of the flash charge capacitor to the start input terminal of the charging circuit. While this circuit is fully effective for restarting oscillation when sufficient charge exists on the flash capacitor to cause firing of the flash tube, there are occasions when a picture is taken at a time when insufficient charge on the flash capacitor exists to fire the flash tube. This can occur when the camera is first used after a prolonged period of inactivity, e.g. several months, without first charging the flash capacitor. Without firing of the flash tube, i.e. the flash capacitor does not discharge, there is no energy pulse for feedback to the base of the oscillator transistor and thus no auto-restart of the flash charging cycle.
In the above cross-referenced application Ser. No. 08/992,873, a solution to this problem is described in which a resistor-capacitor restart feedback path is coupled between the flash trigger circuit and the charging circuit start input terminal. With this arrangement, a residual charge voltage on the flash capacitor of as low as eight volts, which also appears in the trigger, is sufficient to be differentiated by the resistor-capacitor feedback path at the time the shutter sync trigger switch is closed to restart oscillations in the charging circuit to thereby assure re-charging of the flash capacitor. It is true that the first picture taken after this prolonged period of non-use will not have the benefit of flash, but at least the next picture will have flash because of the automatic restart of the flash charging circuit.
The proper operation this auto-restart circuit is premised on the assumed fact that even after prolonged non-use, there will be sufficient residual charge left on the flash capacitor to create a feedback pulse to restart the charger. It has been found, however, that the self-discharge of the flash capacitor can completely eliminate any residual charge on the flash capacitor, going all the way down to a zero residual charge. In this event, there is insufficient charge voltage to create a feedback restart pulse when the first picture is taken.
In the above cross-referenced application Ser. No. 08/822,094, a data bit recording circuit is described in which energy to operate the recording is provided from energy stored in the flash capacitor. The recording circuit is capable of being operated at very low residual flash capacitor charge voltage levels. However, should the flash capacitor self-discharge down to a zero residual charge voltage, the recording circuit would not operate until the flash capacitor is recharged after taking the first picture.
There is therefore a need in such cameras for an arrangement that ensures that flash charger oscillation will automatically restart even in situations in which prolonged non-use of the camera results in a zero residual charge on the flash charge storage capacitor. Further, if the flash circuit is used to operate a data bit recording circuit, there is a need to ensure that there will be sufficient residual charge voltage to operate the recording circuit, even after prolonged non-use of the camera.