Sea & Sea TTL
(Motormarine II, SX-1000) Flash Interface
" - " indicates ground. |
The principal differences between the Sea & Sea and the Nikonos interface protocols are as follows:
X: The flash ready function is provided by a positive
voltage (+5.6V nominal) appearing on the X terminal (i.e. the
X-terminal is at 0V when the flash is not ready).
S: Integration of light falling on the film is initiated
when the flash unit places +3 to +4V on the S (start integration)
terminal. This +3V serves as the power supply for a 2903 comparator
in the camera (see circuit below). Phototransistor (PT) current
charges a capacitor, and when the capacitor voltage exceeds the
ISO (Film DX coded) set-point, the comparator shorts the Q-line
to ground. The leading edge of the S pulse momentarily turns
on a transistor (Q4) which discharges the integration capacitor
to initialise the system. |
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Q: Two-way communication occurs on the Q line. The pull-up
resistor for the Q input of the strobe is only powered while
the X line is low. When X goes low, Q is pulled up (goes high).
The camera then pulls the Q line low when integration is complete.
The flash unit asserts a high on the Q line (via a resistor)
for a few seconds if quenching is successful, to light the TTL
OK LED in the camera viewfinder. |
TTL Flash adapter.
Sea & Sea Camera to Ikelite Strobe.
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This device is designed to take the TTL flash signal from a Sea
& Sea camera (MM II, MM II-ex, SX1000, etc) and use it to
control an Ikelite TTL strobe. The same circuit can also be used
to control a Nikon strobe (SB102,3,4,5), by the addition of a
resistor of 27K - 47K form SP to RDY at the strobe interface. |
Circuit Diagram (30K
GIF) 1024 x 683 pixels.
Circuit description (display the circuit diagram while you read this):
A pull-down on the camera X line turns on Q1 (the bar above the
symbol indicates that it is 'active low'). This event ignites
the strobe via Q2 and asserts a high on the S (start integration)
line, thereby powering the 2902 comparator in the camera. When
the integrated light level crosses the TTL set point, the camera
asserts a low on its quench line (Q), which is transmitted via
Q3 and Q4 to the strobe quench line. The totem pole configuration
of Q1 and Q3 permits quenching to occur only while X is low or
until C1 discharges. The quench event also triggers the CMOS
555 timer, asserting a high on the camera Q line for a few seconds
to light the TTL-OK LED in the viewfinder. The circuit is powered
from the ready line of the strobe. Capacitors C3 and C4 hold
the circuits up when ready goes low after ignition.
Design issues:
R1 + C1 = debounce network, intended to maintain a clean start
signal (S) when the X-contact closes.
R2 - original choice was 10K, but this gave random triggering
with new batteries (6V) in unregulated strobes, due to the current
drawn by the ready light circuit in the Motormarine II.
C3 - original choice 100mF, but this
made the viewfinder ready LED stay on far too long after the
RDY line from the strobe had gone low.
C4 - increased from 100mF to 220mF, to make the viewfinder TTL-OK LED stay
on for a reasonable time.
R10 + C5 - timing. OK light stays on for ~ 3s, slightly less
if RDY goes low.
R5, C2, C6 - Glitch problem. Original circuit had R5=10K and
C2, C6 not fitted. The 2902 comparator in the camera tends to
output a short glitch when S is asserted. This caused early false
quenching, at random, especially with new batteries in the strobe.
Reducing R5 to 1K, in an attempt to swamp the Q line capacitance,
reduced the probability but did not cure the problem. Adding
C6 made the mono too slow to trigger on the glitch, so that OK
lights in strobe and camera disagreed when premature quenching
occurred. Adding C2 cured the problem. C2 delays the Q signal
by ~ 1ms, which is not significant.
C6 is probably no longer necessary, but is useful diagnostically.
555 - usa a CMOS timer. Prototyped with Texas TLC555P.
Possible improvements:
When the strobe batteries start to droop, triggering will at
some point become unreliable due to the extremely tight voltage
headroom of the circuit. Replacing the silicon PN 1N4148 diodes
(Vf~0.65V) with Schottky signal diodes
such as the IN6263, IN5711 (Vf~0.2V),
or the old-fashioned Ge-Au OA47 (Vf~0.15V),
will give a small but useful improvement to the voltage headroom. |
Artwork and text © D. W. Knight. 1999, 2002, 2004.
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