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This instructable explains how to built a slave flash with pre-flash rejection.
When flashing with digital compact cameras, the camera usually uses several small flashes before making the actual picture. This is o.k. if the built-in flash is the only flash you have, but if you want to use an external second flash you have a problem: The second flash, also called slave-flash, fires if the master-flash fires. But most of the flashes are to slow to provide three or more flashes in such a short period of time as the digital camera fires them. The digital cameras use not all the energy on the measuring flashes and expensive modern flashes do it the same, but hey we don't need this over-priced stuff, we built them ourself.
With this device you can chose how many pre-flashes you want to ignore until the slave-flash should fire. So it is suitable for nearly all cameras and all flashes.
With a slave flash you can make really crazy pictures.
But let's start!
When flashing with digital compact cameras, the camera usually uses several small flashes before making the actual picture. This is o.k. if the built-in flash is the only flash you have, but if you want to use an external second flash you have a problem: The second flash, also called slave-flash, fires if the master-flash fires. But most of the flashes are to slow to provide three or more flashes in such a short period of time as the digital camera fires them. The digital cameras use not all the energy on the measuring flashes and expensive modern flashes do it the same, but hey we don't need this over-priced stuff, we built them ourself.
With this device you can chose how many pre-flashes you want to ignore until the slave-flash should fire. So it is suitable for nearly all cameras and all flashes.
With a slave flash you can make really crazy pictures.
But let's start!
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Signing UpStep 1What you need
Well this instructable is something between electronics, photography and software. In just the right portions, oh I like it...
You need experience with the Atmel AVR microcontrollers and a possibility to flash them with a programmer. Getting startet with AVRs is really easy because there are a lot of good tutorials on the web. The Arduinios are just AVRs with a special bootloader to make it even more easy to programm them, but the extra bootloader costs some cents extra. So I decided to go for the real thing.
Then you need an old, but still working flash and some small electronic parts like resistors and so on. A soldering iron would be helpful too.
The complete material list is:
Part Value
R1 100k near the photo-diode
R2 10k pull reset pin high
R4 300 to the opto-coupler
R5 470 to the LED
R6 1k to the reset-button (I didn't use this)
R7 1k to the hex-switch (I didn't use this)
C1 2.2n between the photodiode and the pin PB2
C2 100n Voltage regulator
C3 100n Voltage regulator
C4 10u Voltage regulator
C5 10u Voltage regulator
IC1 ATTINY24/44/84-PU The microcontroller
IC2 7805TV the voltage regulator
LED1 green the green LED
OK1 MOC3021M an opto-coupler
PHOTODIODE BP104 the sensor
Resetbutton
S1 4bit rotary selector
an female flash-shoe with a tripod connector, no special function needed, take the cheapest you can get.
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Just a few remarks:
The optocoupler is not really necessary. Yes there can be a high voltage on an old flash, but instead of an optocoupler, a Thyristor would also work, so if you have that, don't buy an optocoupler.
Using a microcontroller is quite handy, but if you have no access to a programmer, it is a big obstacle. If your camera has only 1 preflash (like in most compacts), there is a simple schedule that works with a CD4001 and just a few other compounds that can be built pretty small. (See picture)
If you have a PIC programmer and not an Atmel tiny programmer, you may consider a PIC based schedule, such as developed by Pavel Janko (Google). The advantage of that one is that it has a 'learning mode' so there is no need to even know how many preflashes the camera has.
Not trying to put down yr effort, not at all, I could never have come up with it, just trying to point out some more possibilities to those looking for preflash ignore
But the idea of the learning mode is good. I'm working on a second version of it and didn't have any more of the hex-switches, so I tried to implement the learning version and it worked quite well!
Result: Smaller board, less parts and saved 2$ for the hex-switch!
I will soon publish the second version as an own instructable, because it really has a lot of improvements!
I just published the new version of the slave-flash-trigger here:
http://www.instructables.com/id/Slaveflash-with-Attiny24-ver-20/
It's really much better than this version, it is cheaper, more flexible, light-weight and just awesome. Check it out!
Andreas