Everybody that has a small digital camera  knows the problem of using slaves because of the pre-flash that the camera uses to set the exposure: the preflash triggers the slave flash that thus fires before the shutter opens.

Plenty of solutions have been proposed and a number of circuits were published in the electronics monthly 'Elektor'  but frankly, generally these proved not to work or at least cause problems with a lot of people trying to build them.

I managed to build a working unit with a minimum of parts that I have build several times and they all work, so it is certainly 'reproducable'. The picture shows two finished units: one with a fototransistor and one with a fotodiode

One thing though. The circuit I am proposing is for a camera with 1 preflash. Also, it is not an original circuit. I first found something  like it on foto website Flickr that had a working version and a non working version and after some digging i came on a russian website (!)  that gave me a lot of useful information as well.

The circuit is pretty sensitive: I got it to react inside with a distance of 5 meters and an indirect flash

Let's start with the Bill of materials:

Phototransistor (e.g. BPW 40)  or photodiode (e.g. BP104)
CD4001 quadruple NOR-gate
R 220k  3x
R 120 K
R 620 k
R 27 k
R 10 k
Diode 1n4148
C 1nF
C100 nF
C 1uF (elco)
C 10uF (elco)
perfboard (18x12 holes) (I did build one on a 9x10 holes but you dont want to torture yourself)
Thyristor TIC106D
9V battery clip
14 pin IC foot

small bridge rectifier
switch 1 x make
switch 1x switch

The circuit is pretty straightforward: The photo transistor/-diode picks up a pulse that gets filtered and amplified in the circuit around the BC547.
I will not go into detail in the exact signals on the various nandgates but what happens is that the first pulse of the flash opens the nandgates for about 100ms, waiting for a second puls if that comes the final nand gate becomes high, opening the Thyristor T3.
T3 can be used to directly trigger a flash, if you are sure about the polarity (most likely the center pin is the positive and should go to the anode)  but one could add 4 diodes or a small bridge rectifier (make that a couple of 100 Volt unless you are sure about the voltage on the flash shoe)  to make the polarity unimportant.

If you are using a photodiode make sure the kathode is connected to the '+' and the anode is connected to the junction of R1 and C6, as is shown in the second figure.

If you are pretty sure about what the + and the - of your sync jack are, then you can leave out the bridge diodes as in figure 3

If you want to feed the circuit from your flash rather than from a battery, figure 4 is for you. Realise though that not all flashes will work with that so if you are building it, I suggest you wait with soldering the 4M7 in place and test the circuit first with a battery, so if things dont work you will know it is the circuit and not your flash having a bad reaction.


Step 1: Slaveflash for automatic camera's with pre flash: construction

My way of constructing it was to solder a 14 pin IC holder on the perfboard and first start connecting the pins of the various nand gates: so i put a blob of solder between pin 1 and 2 and between 8 and 9. Then I took a bit of wire and connected 5 with one and 2. Then I took the components and found them a place as close as possible to the connections that needed to be made and used wires at the underside of the perfboard to connect everything. That seems daunting, but in fact it proved quite easy, just make sure that you cross out the connections you made with a felt pen on a print of the circuit and you'll be fine.

You could add the switch to switch off the supply voltage, but I did not bother, i just unhook the battery. You could also use a switch as indicated to switch between preflash ignore and no preflash ignore but I did not bother with that either.

It is possible to omit the 9Volt battery and to feed the circuit from the sync jack (via a resistor a diode and a zener) but I would not bother. I tried it and it works for some flashes but not for others: apparently the load on the sync jack can influence the recycling time.

PCB: it is of course very well possible to build this on a PCB and you might be able to make it pretty small if you use smd components.
I have added a fritzing file to print the PCB. Realize though that I have not build it on this print, I uesed the perfboard as seen above. I have added the PCB design for your convenience and I double checked it, but I have not tested it (yet)

I put my finished circuit in a small translucent plastic box, the kind that is used to pack bolts, nuts and screws (I use a lot of these boxes in my projects). They are quite handy, they are basically free as I already bought the bolts and it is a kind of recycling by keeping them out of the waste circuit. The box is translucent enough to still let the phototransistor/-diode pick up the flash 
<p>There is a problem with the theoretical diagram: it does not connect the CD4001 to the power supply. It is shown in the schematic for the circuit board, however. </p><p>Connecting pin 7 to the ground and pin 14 to the battery +ve makes all the difference!</p>
Are the Gates NOR or NAND? 4001 or 4011
my Mistake brushless54 <br>I am using a HEF 4001 (a CD4001 equivalent) and that of course is a NOR gate. The circuit mentions NOR-gate as well, but yes my BOM causes confusion. I will correct that. Thanks for pointing it out
For anyone wondering, the &quot;MUA&quot; girl in the first photo is the Make Up Artist. :]
She posed for me on other occasions finton. This time she brought a friend to pose and she did the make-up. I ended up shooting both :-)
Whew! Tough job - but somebody's got to do it... :] <br>I was just wondering what a MUA was, so looked it up. Great Instructable BTW. If I still did photography, I'd definitely make one of these.
indeed. the sacrifices I make :-) <br>Thanks <br>Not sure if i'd make one again as there are so many flashes now that cater for the pre-flash, but for existing outfit it will do great
nifty. Seems easy to build. I saw the other one: http://www.instructables.com/id/Slaveflash-trigger-for-digital-with-Attiny24/ but yours does not need any programming
Thanks Robin. The one you link to has a version 2 somewhere and as far as i understand it is suitable for more than 1 preflash as well, but yes, you need to program a microcontroller for that one. My simple version is built within an hour, without programming
cool! so how could this be expanded to cameras with more than one preflash?
Tnx for your interest in my flash slave. One could extend it to ignore more flashes by adding some nand gates. However getting the timing right is going to be difficult then and one needs more peripheral components (resistors capacitors) too and the failure rate would go up. <br> <br> It would be simpler then to use a microcontroller (Attiny or PIC) that can just count flashes and has a 'learning' mode to make it suitable for each and every flash. There are some of those circuits available on the net (with a PIC) and it seemed foolish to just reproduce that :-). <br> <br>Maybe if i feel like it, I may develop such a ting with an attiny 13 but currently i have no need for it :-)

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