In a previous post to this thread :
Chuck Norcutt proposed redesigning the OM TTL flash circuitry
by adding an external adapter placed between the flash and
the camera. This proposed adapter would memorise the flash
duration by monitoring the signals starting and stoping the flash
while in TTL mode. Then, when switched from monitor mode to
remember time mode it would always time the stop signal to give the
same memorised flash duration independent of the camera's
stop flash signal.
He went on to remark and asked for comment:
<<
Now you can use your 18 0ray card for TTL flash!!! Remember, you saw
it "exposed" here first! OK, who can turn the logic into hardware?
Tim, are you there? >>
Warning : Long Engineering Content follows!
Chuck,
this is conceptually very simple to design although there are a
few issues that would need addressing and some difficulties that may
make it impractical at very short duration flashes
(eg macro use without diffuser).
Your suggestion of using a toggle or slide switch to both select modes
and thereby show which mode is selected is good as it saves power over
an LED. The unit could then be designed with such a low power
consumption' as to not need an on/off switch and run off two or three
357 cells (small).
Why is the very short duration potentially more difficult? The shortest
flash duration for the Olympus flashes listed on Hans's OM-sif is 1/50000
second or 20micro seconds (T32). (The camera and flash combination probably
runs a bit longer given the low power electronics in the camera.)
This 20uS means if we allow the light level error to be as much as 50% (!!)
then
the allowable timing **uncertainty** from start of flash output to stop
flash must be less than 10uS. This may be hard given that
triggering the flash tube to fire may easily have more than 10uS uncertainty
associated with it. (e.g.As the flash tube warms up from use the
trigger voltage changes significantly.)
I designed and built high power stroboscopes for human gait analysis
photography many years ago, as a vacation job. I soon discovered that
triggering flash tubes of the type used in amateur flashes is not as
predictable as one would like. Very high speed stroboscopes use a
different kind of flash tube to give more predictable firing.
(They use an additional pair of internal electrodes in the flash tube,
unlike the amateur flashes which initiate firing with an external electrode
capacitively coupled through the tube wall).
Thus the timing uncertainty and the relatively long time from trigger
signal to actual start of light output may be too variable for
short duration flash timing. OM2S and most later bodies trigger
the flash with an electronic switch so at least the mechanical
switch bounce which would make timing impossible is eliminated
in these cameras. It would be fairly simple to test the trigger to
light time reproducibility with a timer and photo diode to see
if this really is the problem I suspect. This would need careful
testing to verify light start times are reproduceable enough.
A way around this potential problem would be to add a photo
sensor to detect ** the start of light output** and time from there rather
than
from the camera's trigger signal.
Another possible problem is that the voltage on the flash capacitor
may vary enough from flash to flash (impatient user!) to cause significant
variation in exposure with a fixed time (This is eliminated by normal TTL
flash.)
There are now CMOS programmable logic devices with essentially
zero power consumption running at low clock frequencies
as would be required here.) that are easy to customise (program off PC)
but some old cheap metal gate CMOS counters and gates could be used
just as easily. Additionally some discrete transistors with
resistors and protection diodes would be needed to level shift and clamp
the input signals. Regular packaged crystal oscillators are power hogs
and start very slowly ( 100mS) so a fast start
RC oscillator started and stopped by the flash trigger circuits
would be more than accurate enough here. (1 op amp).
very rough estimated parts count :
2-4 transistors , 8 disctrete parts r's ,c's etc
10 bit counter (1uS clock ,1 mS max flash duration) ( to time and store
number)
10 bit presettable counter
(times subsequent flash durations, preset from 1st timer o/p)
a few gates and/or a flip flop or two
mechanical switch.
op amp.
357 batteries +holder
housed in OM flash extender
Incidently, I once met Mr Flash himself ( Prof Edgerton) from MIT : he
was an amazing character. He held many basic patents on electronic flash
and built very high power flashes operated from aircraft for photographing
enemy
positions during WW2. These flashes were powerful enough to ignite paper
at some significant distance! He also held the original patent on the
autoflash (with
dump tube) . He did get one thing wrong though he was a N*k*n user :(
He delighted in estimating flash energy (Joules) by holding his hand in front
of the flash (at
about 3 inches) and feeling how warm his hand got when he set it off.
Regards,
Tim Hughes
Hi100@xxxxxxx
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