Build a High-Power LED Strobe
demand much more juice than what
four AA cells can realistically provide.
The evaluation board is preset to
750 mA, which dictates that 700 mA
grade stars be used. I would have liked
to use 1,500 mA stars, but that means
substantial EV board modifications,
which are beyond the scope of this
article.
Control Circuit
Description
I had some goals for the control
circuit: It should be capable of driving
the HV9930 evaluation board from
three different trigger sources:
component video to synchronize with
a camera; an external open collector
or TTL level signal; and an internal
variable frequency oscillator. It must
also have calibrated power levels
achieved by precisely controlling the
light pulse’s output width. The
schematic is shown in Figure 4.
A + 12 volt DC external source is
fed via J1 to both the evaluation board
EV1 and a five volt regulator U5 that
feeds the control circuit. Capacitor C9
should be located very close to EV1,
to smooth out the demand for high
current pulses.
An external NTSC/PAL video
component signal is applied to J3,
with resistor R1 providing the proper
termination impedance, and is fed via
C1 to sync separator U4. This IC has
many functions, but we are using only
two: The composite sync output is
employed to turn on MOSFET Q5,
which discharges capacitor C4. This
low voltage applied to NAND gate
U3a disables it from oscillating when a
valid video signal has been connected. Another of U4’s functions — the
Odd/Even field output — is employed.
A little explanation is required
here. Individual TV images — or
frames — are comprised by two interlaced fields, which each include the
odd and even scan lines. Modern
video sensors capture the entire frame
simultaneously, but to comply with
this legacy TV standard, the signal is
processed and interlaced before
sending it to the video output. For the
purposes of this project, this would
create double light pulsing while the
frame is acquired, causing a double
exposure. The Odd/Even output
effectively chooses a single field,
synchronized to the video capture.
Since the CCD’s actual frame capture
timing (with respect to this signal) is
unknown, we must delay its phase
such as both coincide. This is achieved
by monostable U2b, which is
triggered on O/E’s falling edge.
The phase delay is set via C10, R6,
and potentiometer R7. By adjusting
the latter, the light pulse can be made
to coincide with the actual video capture. This can easily be accomplished
empirically: One turns R7 fully to one
end, and then start backing up slowly
until the image on the camera’s
viewfinder becomes the brightest.
Both the composite sync and O/E
outputs become a logic low when a
video signal is absent. This has two
effects; first, U2b is no longer
triggered and no further pulses occur.
Second, Q5 is no longer turned on,
and C4 charges through R3. When a
logic high is reached, U3a is allowed
to oscillate via positive feedback via
R4 and R5, and in conjunction with
C5, sets the frequency. This is coupled
to U3c which is no longer receiving
pulses from U2b, which serves to
select which pulse (video or free run)
passes through.
In essence, all this circuitry selects
a free running pulse when no video is
available, and a field synchronized
pulse when a valid video signal is
present. U3d only buffers the
signal to drive a small red LED,
which indicates oscillator activity.
The selected signal goes to
the normally closed contact of
“external trigger” jack J2. When
no plug is connected, the signal
will continue to U3b and then
to the next monostable U2a.
The reason for employing U3b
is to provide a Schmitt-Trigger
action for the external signal,
which provides noise immunity
and the fast trigger edge
required by U2a. This monostable is
wired in a non-retriggerable configuration, to prevent pulsing stretching due
to noise.
The external trigger may be either
a TTL-compatible square wave, a
switch, or a transistor closure to
ground. In this last instance, R11
serves as a pull-up.
The pulse width is selected by
switch SW1 and associated components C12, R9, and R10. This will
provide an output pulse duration of
either 1/250 or 1/500 second for high
or low power mode. This pulse is now
applied to LED driver evaluation board
EV1 which, in turn, drives the four
series-connected Luxeon LEDs.
A jumper wire must be installed to
select an operating mode: Connecting
EV1’s pins 1 and 2 together will turn
on the LEDs continuously, regardless
of any other conditions, and is useful
for testing. Connecting EV1’s pin 1 to
U2a pin 6 reverts the circuit to normal
operational mode.
Assembling the Circuit
As shown in Figure 5, the whole
project is assembled in a small
aluminum box. Prior to any electronic
assembly, all the holes must be drilled
and deburred. Afterwards using a
pencil, mark the location of the LED
stars, noting the orientation of the plus
and minus signs to allow them to be
connected in series. Attach them
using the thermal compound and
■ FIGURE 5. Completed project.
January 2008 35
