BY FERNANDO GARCIA
priate delay can be easily set. This is
important as some events may be timed
for maximum photographic effect.
A pair of photos using this
technique is show in Figures 1 and 2.
Circuit Description
This is quite a simple circuit, as
shown in the schematic of Figure 3.
M1 is a 4. 5 volt electret microphone
capsule, which is biased by R1, R2,
and R12. This bias, along with the
audio signal, is fed to op-amp U2. The
gain of this stage may be varied from
6 to 56 by adjusting potentiometer
P1. To simplify the layout, the pot has
an included on/off switch to power
the circuit from the +9V battery terminals. C2 and C3 decouple noise.
The amplified audio signal is
capacitively coupled via C4, to a window comparator comprised of U3 and
associated circuitry. R13 and R14
biases U3 at mid supply, whereas R5,
R6, R7 provide the window. Whenever
the DC level plus the audio signals
exceed either voltage at pins 3 or 6,
one of the output pins 1 or 7 is pulled
low. Since this is a wired-OR connection, the falling edge signal triggers
the first monostable comprised of
U4a, C5, and R9. This provides a 100
millisecond drive to MOSFET Q1,
which turns on and establishes a path
for the external flash to be triggered.
The second monostable,
comprised of U4b, C6, and R10,
provides a one-second blanking pulse.
This second monostable is triggered
by the negated output of the first one,
which also blinks the LED to provide
visual information of the circuit’s
actions. This blanking pulse is important, because I found — the hard way
— that many events have multiple
sound pulses, from secondary
breakage or even echoes. For instance,
glass breaking produces literally
dozens of sounds as it shatters. These
secondary sounds provide multiple
triggers to the flash unit, which results
in a severely overexposed picture.
The MOSFET is a 60 volt rated
device, which means that you can
safely trigger the newer flashes for
digital cams or the older flashes used
for 35mm film cameras, which have a
far larger open circuit voltage.
Building the Circuit
The project is simple and
straightforward, and can be built by
any of the common breadboarding
methods. There are no critical layout
areas, other than to ensure that the
microphone capsule is close to the
op-amp for minimum noise pickup.
Please observe the normal
handling and soldering procedures for
CMOS devices. I’ve designed a small,
double-sided PCB (shown on the Nuts
& Volts website; www.nutsvolts.com)
for a straightforward assembly. Since I
desired a small footprint — basically
■ FIGURE 3. Project’s schematic.
Only three ICs are required.
February 2006 35
