Add privacy mode to white-LED camera flash

Under Italian law, a flash camera must display a privacy light immediately before taking a picture using the flash. Because of cost and because of obvious board-area limitations within the camera, the LED used for flash should also serve as a source of reduced-intensity illumination for the required privacy warning.

The privacy-warning light can be implemented by adding a simple pulse-width modulation (PWM) circuit to the Mode-Select inputs (EN1, EN2) of the boost-converter IC (MAX8607) that drives the LED (Figure 1 ). Pulsing the Flash mode of that device with an appropriate duty cycle yields the desired warning light.

Figure 1: This dual SPDT analog switch (MAX4685) provides a controlled-brightness privacy warning by applying PWM to the LED current.

(Click to enlarge image)

The Privacy Mode Select signal controls a dual-SPDT analog switch (MAX4685), which in turn controls the LED current by applying PWM signals to the boost converter's Mode Select inputs: EN1 = EN2 = 1 gives Flash mode; EN1 = EN2 = 0 gives Shutdown mode. (These inputs also provide “Movie” mode and “Disco” mode, for other purposes.)

Switching between Flash and Shutdown modes makes the LED brightness proportional to the PWM duty cycle. To ensure a stable output for driving the white LED, you should set the PWM frequency much lower than the boost converter's 1-MHz operating frequency, i.e., no higher than 10 kHz.

The PWM signal can be obtained from a simple comparator circuit (Figure 2 ),

Figure 2: A simple comparator circuit generates the PWM signal in Figure 1.

(Click to enlarge image)

in which the input rise/fall times are controlled by capacitor C to produce (ideally) a triangle wave at the comparator's noninverting input. R1 and potentiometer R2 provide an adjustable reference voltage at the inverting input. The pot then controls the duty cycle of the comparator's PWM output. Increasing the duty cycle brightens the LED.

About the authors
John Robinson is with Maxim Integrated Products Inc., Sunnyvale, CA. Robinson, a Field Applications Engineer, graduated from Oxford Brookes University in 1990 with a first class honors in Electrical and Electronic Engineering, and also earned a Masters degree in Communications from Cranfield University in 1992. John is currently pursuing a PhD at Oxford Brookes University. He has worked as a mixed-signal engineer for 15 years.

Derek Valleroy was a Member of the Technical Staff when this article was written; he is no longer a Maxim.

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