Flash Trigger
There are a few posts on this site that make use of a sound activated flash trigger. The following is a (hopefully clear) description of the final version of the trigger used and a circuit diagram for anyone wishing to construct their own. So bear with me while I fumble out some words…
The initial version of this trigger was a very simple single transistor amplifier, a piezo drum and an SCR. It performed well for quite a while but the need to drive the flashes directly (rather than from eBay radio triggers) and to have a system of delay and debounce led me to constructing a Mark II. I also did a more permanent job of the circuit by vero-boarding and casing it.
A few terms:
Delay: The time between a sufficient sound being received and the flash sync signal being sent.
Debounce: The dead period after the flashes fire during which no further signals are allowed pass. Prevents unwanted double exposures.
Gain: The amount amplification applied to the microphone signal, aka sensitivity.
There are a few circuits like this out there on the net but overall I found the quality of the designs quite poor – some almost like they had never actually been built. So I aggregated a few of the designs, added some of my own stuff and came up with this. Rest assured that if you build this correctly it will work as I have a completed one myself. It is a little over engineered and several components could possibly be discarded but I desiged it with best practices in mind and following all the guidelines in the ICs datasheets.
PDF Circuit Diagram and Bill of Materials (BOM)
In the BOM I have indicated Jaycar part numbers for the various non R/C components. However the parts are quite common and I am sure they all can be sourced from any decent supplier or from Digikey/Farnell.
Pictures of my own implementation follow to give an idea of how you might case the circuit. The case I used is a simple Jiffy type box which has the advantage of a battery compartment for the 9V cell required to power the circuit. Makes for a nice clean unit. The pots are miniature types and linear. Finer tuning in the low end could be achieved if they were logarithmic – I guess it depends on your requirements as to weather that would be useful.
This is the circuit on veroboard inside the case. The C5/R2 Zorbel network are the only components not visible as they are on the back of the board having being added as an afterthought. The wires visible run to the pots, switches and connectors. The red tape insulates the circuit from the switch terminals which could potentially touch the board due to their depth.
For input I just stuck my test mic into an old solder tube as shown below but any electret will work. Most cheap PC microphones are of this type.
And finally all squared away and labelled. For output you can either get some cheap eBay 3.5mm to PC Sync cables (or make your own) or run one of the outputs into a radio trigger transmitter. Though the later presents some problems for high speed photography – see post and comments here. However the flashes are connected they must be used in their LOWEST power setting. This ensures the minimum flash period which is essential for high speed. Both Canon and Nikon flashes are capable of producing approx 1/30000s bursts on their lowest setting.
If you are going to have a shot at building it I would recommend initially doing it on a breadboard. Get the amp working first – you can substitute the R3 resistor with a speaker and see if the mic is being reasonably amplified. Keep in mind this is not a high end audio amplifier! As long a there is no oscillating/reverberating or high levels of distortion you are probably good to go. I used a old PC speaker to test with.
After you get the amp going reliably (which has the most potential to cause trouble being analog) proceed to the delay/debounce which is mostly logic. When building the amp try to keep it as compact as possible – long tracks and cables are never an amps best friend.
Have fun and feel free to ask any questions you might have.
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9 Responses to “Flash Trigger”
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That truly is More Than Meets The Eye!
This looks a great trigger, I’ve ordered the parts is it possible you could send me the veroboard layout. Electronics is not my strongest skill and I’d hate to have problems due to not laying out the components correctly.
I afraid I never actually wrote anything down concerning the board layout and instead just made it up as I soldered. You can tell quite a bit from the picture above as to where you could place the major components. From there just pick the tracks you will use of the major buses (voltages, signals etc). If I get time in the coming week I will take a picture of the reverse of the board so you can see where the track cuts are.
I’m sure the layout I used is far from optimal since it involved little to no planning before soldering
There is software out there to actually formally design vero layouts – eg. http://www.heyrick.co.uk/software/verodes/index.html. Unfortunately time is scarce at present but I would love to see any further work you do on the circuit. Good luck!
Thank you for the post. This is exactly what I was looking for.
Just a quick question: Where did you buy the casing? And did you modify it? I’m new to electronics, so excuse me if it’s a stupid question.
Locally the box came from Jaycar but I am sure it can be sourced internationally on Farnell or Digikey.
Something similar on Farnell (UK):
http://uk.farnell.com/multicomp/bc3/case-handheld-pp3-black/dp/301700
No exactly the same but it does not matter that much as long as it holds a 9V battery.
I have a low end external flash. Does it need to have the capacity of 1/30000s? I have purchased and am currently putting together the flash trigger but a new nikon flash is not really in the budget for me at the moment.
It depends on the model and sadly data is often not published in manuals. As noted on this site:
http://chdk.wikia.com/wiki/Samples:_High-Speed_Shutter_%26_Flash-Sync
“Xenon flash units usually control their flash output by duration rather than output level. So when used on the dimmest setting these cameras are shortening the flash output to 1/60000th of as second.”
So it is essential to use the lowest power available on you flash and then try a simple test like the one here:
http://www.scantips.com/speed.html
Or Google your flash model with the words “high speed” and someone may have already tested it.
Dear Jon, your trigger is very interesting and I would like to ask you, if is possible to use it as s remote for camera, or it is just for a flash? Thank you
The trigger applies a voltage to the flash centre pin in the same way a PC sync socket does. I think most cameras employ a short between 2 pins for their shutter release which might not be compatible. You would need to bring both pins into the circuit and connect them with a gate, preferably an opto isolated one. That gate could be driven by the existing flash trigger circuit from the Flash Instant/Delayed outputs. I’m sure info on the shutter releases would be available online.