Privacy has become a major issue with the near ubiquitous availability
of cell-phone cameras. Some people have called for a preliminary “privacy flash” that goes off just before the full flash in order to warn people that their picture is being taken. In fact, such a flash is required in Italy.
You can implement a combination privacy- and full-flash light using simple logic gates along with an LED driver (see the figure). The circuit uses an IA2505 high-efficiency, low-cost, four-channel LED driver current source, designed for flash and backlighting
LEDs. The circuit’s LED current can be set from 5 mA to 80 mA on each channel, and LED pulse current can be as high as 200 mA per channel for a duration of 20 ms or less.
The privacy- and full-flash LED current, IOUT, on each output pin of the IA2505 is programmed by R1 and R2, respectively:
IOUT (privacy flash) = 1200/R1
IOUT (full flash) = 1200/R2
With R1 = 100 kΩ, IOUT (privacy flash) = 12 mA and with R2 = 5.6 kΩ, IOUT (full flash) = 214 mA. Since the four output pins are connected in parallel, the current to the LED is 48 mA for the privacy flash and 856 mA for the full flash.
The driver is kept in shutdown mode by R3 and can only be enabled through the LM555 when pushbutton switch SW1 is activated.
The LM555 generates a 500-ms pulse with a width set by R5 and C4. The LM555 operates in a monostable mode, with the timer functioning as a one-shot.
Because we want to separate the flashes by a delay of 400 to 500 ms, we need to generate two pulses of 20 ms (or less) triggered by only one shot. This is accomplished by sending the 500-ms pulse to an XOR gate (U5A), but to delay one of its pins with the R4-C1 combination. Once the XOR gate generates the two 10-ms pulses, the two NAND gates send the first one to Q1 and the second one to Q2. Q1 triggers the privacy flash and Q2 triggers the full flash.
