So-called "one-touch" flash charging circuits are in common use in flash apparatus employed in single use cameras. Such circuits, as exemplified in commonly assigned U.S. Pat. No. 5,870,639, include a self oscillating circuit coupled through a voltage step-up transformer and rectifier diode to a flash capacitor in the flash illumination circuit. In the exemplary charger circuit of the '639 patent, oscillation is initially started by depressing a momentary, "one touch"switch which applies a forward bias to a transistor base terminal in the oscillation circuit. In the above referenced copending application Ser. No. 09/149,688, a feedback path is provided that is directly responsive to the charge voltage on the main flash capacitor to turn off the self oscillating circuit when the flash capacitor reaches full charge of, for example, 320 volts. To automatically restart the oscillator after taking a picture, a normally reverse biased diode is connected between the start terminal of the oscillator circuit and a shutter actuated flash synchronization switch in the flash illumination circuit. During the momentary period that the flash synchronization switch is closed to initiate a flash operation, the diode is forward biased providing a momentary forward bias on the transistor base terminal of the oscillation circuit thereby re-starting the oscillator circuit to recharge the main flash capacitor.
In this copending application Ser. No. 09/149,688, an optical data recording circuit is provided with one or a pair of individually selectable LED illumination devices that allows the camera user to record on the film a selected print aspect ratio (PAR) to be used in reproducing a hard copy print from the exposed image frame on the film. Details of this encodement arrangement are set forth in specifications for the Advanced Photo System. The data recording circuit has one terminal connected to the flash synchronization switch and the other terminal connected to the charged voltage side of the main flash capacitor. When the flash synchronization switch is closed, the selected LED or LEDs in the optical recording circuit are energized by the charge voltage on the flash capacitor to become illuminated for the brief period the synchronization switch is closed, thereby optically recording the PAR data encodement on the camera film.
To ensure that there is sufficient charge voltage on the flash capacitor to energize the LED, even after long periods of non-use of the camera when the flash capacitor might normally self discharge to zero volts, the flash charging circuit is provided with a DC circuit path leading from the battery power source to the flash capacitor so that the flash capacitor never discharges below the level of the battery voltage. In the embodiment disclosed in the application, two battery cells are used as the power source. Since an LED typically requires only about 1.7 volts to initiate illumination, there is always sufficient residual charge voltage on the flash capacitor to cause the LED or LEDs to illuminate, even if the batteries have lost some of their voltage output as a result of exposing a number of the available frames on the film strip. It is known that AA batteries typically have a fresh voltage output of about 1.6 volts per battery (3.2 volts for the pair). Nearing the end of a roll of film, the individual battery output voltage typically declines to about 1.4 volts per cell (2.8 volts for the pair). Thus, in the above mentioned circuit, there is always sufficient voltage to energize the LEDs which only require about 1.7 volts to be illuminated.
For cost competitive reasons, there is a desire to provide combined flash apparatus and optical data recording circuits of the type described that operate effectively and reliably from a single AA battery of nominal 1.5 volt rating. As can be seen from the above discussion, it is generally not feasible to energize an LED illumination device from a single AA battery since even the 1.6 volt of a fresh battery would be insufficient to adequately illuminate the LED thereby losing ability to record the desired
encodements on the film.
In U.S. Pat. No. 5,784,658, a solution to the problem is proposed in which an LED-based optical recording circuit derives is LED operating voltage from flyback pulses derived in the primary or tertiary winding of the step-up transformer of the flash charging circuit. Such pulses are inherently of high enough voltage to ensure illumination of the LED or LEDs irrespective of the normal decline in battery output voltage. An activating circuit driven by closure of the flash synchronization switch ensures that the flash charging circuit is turned on so as to provide pulses required to drive the LED optical recording device. If the flash capacitor is fully charged at the time the flash charging circuit is turned on, the frequency of the flyback pulses is sufficiently high to create the necessary energizing pulses for the LED during the period that the flash synchronization switch is closed, usually a period of about 700 microseconds. On the other hand, if the flash capacitor is in a fully discharged or almost fully discharged state at the time the synchronizing switch is closed to start the charging circuit, the frequency of the flyback pulses is so low during the initial startup phase that no flyback pulse may occur during the brief time that the flash synchronization switch is closed. In such an instance, even though the flash charging circuit is started, no optical data recording occurs. Japanese patent publications 11-84,511 and 11-84512 propose the addition of circuitry intended to overcome this difficulty. In one case, a timer circuit is added to ensure that the flash charging circuit remains on long enough to capture the necessary flyback pulses needed to energize the LED. In another case, a flash capacitor decoupling circuit is added to initially decouple the flash capacitor from the charging circuit to avoid the adverse effect the discharged flash capacitor has on frequency of the flyback pulses. Clearly, the addition of such circuitry is counterproductive to the intended result of reducing cost by eliminating one of battery cells in the power source.
There is therefore a need for a simple, low cost combined flash apparatus and optical data recording circuit that works effectively and reliably in the camera utilizing a single cell battery (1.5 volts) even in the case when the flash capacitor may be fully discharged at the time the flash charging circuit is re-started to provide energy for activating the optical data recording device.