Optical flash trigger with second pulse detection

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Update 27.1.2007 Mobile version
Update 27.1.2007 Another sample from Zdenek Maly


I was not satisfied with results of on-camera flash of my EOS 300. The lighting was flat and it often caused red eye. As there were too big differences between object and background, I also often got nasty shadows. I tried to solve the problem by system flash (Metz 54-MZ3) with tilt/swivel head. The results were much better due to possibility to bounce the flash off a wall or ceiling. However, I was not completely satisfied with the solution - the flash was heavy (heavier than EOS 300 + 50/1.8 lens) and the light was too diffused when I bounced the light off ceiling. I got the best results by bouncing off a wall behind me but this was not always possible.

I had a possibility to buy several old AC powered flashes for $2 each. Their power was 16-20 GN (in meters) and triggering voltage 40-80 V. I permanently installed two of them to my living room and fire them by the trigger described below. They give me very diffused light at F/4 @ ISO 100. I use them now together with on-camera flash (for fill-in and to trigger the permanent flashes).

To make the trigger compatible with digital cameras and with Canon E-TTL flash mode a second pulse detector had to be added. As digital cameras don't use film, they cannot use normal TTL flash. Instead, two flash pulses are used. The first one is used with closed shutter just for measuring. Then, shutter is opened and usually 50-100ms later a second pulse is fired according to pre-measured values. To make the trigger compatible with digital cameras, it must ignore the first pulse and fire after the second one.

Please note that not all digital cameras use the previously described technique of pre-flash followed by main flash. Some less expensive models (like my Fuji A310) have external flash sensor for measuring flash intensity. As the flash is not measured through the lens, there is no need for two pulses. In this case, you don't have to include the second-pulse detector. If you are not sure whether your camera uses one or two flash pulses then take a photodiode and connect it into microphone input of soundcard in your PC. Then use program like WaveFlow to analyze what happens when you take a photo using flash.

Complete scheme

Below is the complete scheme of the trigger (the scheme was created in CircuitMaker). Here is bigger image for printing. It should be possible to build the trigger for less than $10 depending on the number of thyristors.

Count Value Mark               
Count Value Mark

3 33k R5,R11,R13

2 1M

1 10M R8

330    R9

1k R10,R14,R17




78L05 U1

74HC74 U2



Individual components

The complete scheme can be divided into several main sub-blocks. The blocks are marked with different colors and they are described below. The exact values of individual components may vary - small changes of values (470nF or 330nF, 1k ohm or 2k2) usually won't have any influence. Only working point of Q4 should be set exactly (0.55V in my case by combination of R15 R16 and R13).

Stabilized power source. Working point of Q4 must be set with relatively high accuracy. Therefore, stabilized power source (7805 in my case) is needed. The total power consumption of the trigger is relatively low and 7805 does not need any cooling. Even the cheaper TO220 package can be used. To achieve higher sensitivity, optical sensor is driven by full voltage (9V in my case). If your power source is not stable enough then the optical sensor can be also connected to stabilized +5V. Sensitivity will be slightly smaller but it should not be real problem.
Optical sensor. Phototransistor BPW42 is used here to detect light pulse. After filtering by R11, C2 and R13 only rapid changes of lighting are sent to amplifier. The optical sensor must be located so that it can be reached by light from your master flash. Fortunately, due to very good sensitivity, the sensor does not have to "see" your master flash directly. Light reflected from walls is sufficient. In my case I just made a hole into plastic box with the trigger and put the sensor to this hole - this was enough.
Phototransistor BPW42 is sensitive also to infrared light. Therefore, IR filter can be used over the on-camera master flash. This way, the on-camera flash can be completely filtered out from your images. The cheapest IR filter is an unexposed but developed slide film (e.g. black leading part).
Amplifier. The pulse from phototransistor is amplified by Q4. The base voltage (working point) is set so that even very small pulse (just 50mV) will open the transistor. Transistor Q5 just inverts pulse from negative back to positive. Sensitivity of this combination sensor/amplifier is so high that the trigger can be located even in a different room and it will still fire.
Second pulse detector. This circuit will detect second pulse for Canon ETTL and for digital cameras. IC 7474 is used with self-reset after approximately 1 second (R4 R6 C3). Therefore, you don't have to worry about synchronization. The auto-reset is needed because the trigger is very sensitive and it can sometimes incorrectly react also to other events like switching light on (especially fluorescent lights).
Also, some minimum time between the pulses is needed (R5 C4). It set to approximately 10ms - this should avoid problems with "shape" of the first measuring pulse (it can sometimes consist of many very small pulses). Just for information, the delay between the first measuring and the second main pulse for Canon EOS300/30 is 65 and 75ms. Olympus C4000Z has 100ms between pulses.
If you don't have neither Canon EOS camera with SpeedLite flash nor digital camera, the second-pulse detector can be completely removed.
Switches. S1 is used for firing your flashes manually. This way, you can check if all flashes are firing correctly. ETTL switch can be used to switch between first/second pulse detection. When the switch is in upper level then flashes are fired immediately after the first pulse (usually used with built-in flash of your analog camera). When the switch is in lower level then the trigger waits for the second pulse (when you use system flash with E-TTL or if you have a digital camera). You can use a dual switch here and add a LED diode for indication if you are in ETTL mode.
Thyristor array. Ground of all flashes was connected together. The positive pin was connected to a thyristor. The 400V/3A thyristors used here should survive just any flash. In my particular case, 8 thyristors were used. This way I could simulate variable flash power (I used two soft boxes with two flashes each and this allowed me to use 0/50/100% of power). The remaining thyristors can be used for fill, background, hair light....
The scheme here uses just one resistor R2 common to all thyristors. If the thyristors are not exactly the same then some of them may not fire. In this case it would be better to have separate resistor for each thyristor. 4k7 should be a good value here (not tested).
Flash confirmation. After I used the device for a while I found that I want to know if the flashes fired. The problem was that when I looked through viewfinder of my SLR I did not see the flash due to mirror black-out. Usually, there was no problem as the trigger is very sensitive but anyway I wanted to be able to check if the flashes fired. The C1, R7 and R8 combination together with very high Beta of Q2 ensure that LED1 is lighting for 5-15 seconds after flash.

The final build

The trigger was built into a plastic case. Mono 3.5mm jacks were used for flash connection. I had no simple way how to make a real printed circuit and therefore "paper circuit" was used :-). I just drew the components on hard paper, made holes and connected the pins on the opposite side.

The box on these pictures has many connectors because it was used also for lighting objects with RGB LED (mosquito crystal).


Here are some examples with just built-in flash and with several flashes.
direct flash
direct flash - hard shadow, dark background
direct flash + bounce from ceiling
direct flash + bounce from ceiling

flat light and hard shadows from on-camera flash

several flashes, softbox

Another sample

Several flash triggers have been already built based on this description. I have got slightly modified version of the trigger together with PCB from Zdenek Maly. Because he had problem with obtaining phototransistor BPW42, it was replaced by photodiode 1PP75. Optical senzor is in this case external - the advantage is possibility to attach the cable direcly to camera.


LeeSoft - original source of secondary pulse detector
Low-Budget Umbrella Flash for Dummies
Sam's Strobe FAQ
Sound Synchronizers for Hight Speed Photography
DIY for photography links
HiViz DIY flash triggers
forum Digineff - odpalovani blesku (in Czech)
Bob Atkins Canon Flash FAQ
Flash Photography with Canon EOS Cameras

(c) Libor Sindlar, 18 April 2003. Last change 28 January 2007 home