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.


25 Responses to “Flash Trigger”

  1. Sven on October 14th, 2009 9:36 am

    That truly is More Than Meets The Eye!

  2. Steve on May 28th, 2011 7:27 am

    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.

  3. Jon on May 28th, 2011 10:55 am

    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!

  4. WalidSaladin on November 8th, 2011 4:35 pm

    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.

  5. Jon on November 8th, 2011 6:38 pm

    Locally the box came from Jaycar but I am sure it can be sourced internationally on Farnell or Digikey.

    Something similar on Farnell (UK):

    No exactly the same but it does not matter that much as long as it holds a 9V battery.

  6. erica on November 23rd, 2011 3:43 pm

    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.

  7. Jon on November 24th, 2011 12:55 pm

    It depends on the model and sadly data is often not published in manuals. As noted on this site:

    “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:

    Or Google your flash model with the words “high speed” and someone may have already tested it.

  8. Daniel on January 6th, 2012 10:01 am

    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

  9. Jon on January 6th, 2012 11:51 am

    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.

  10. Mick Favager on February 24th, 2012 8:05 pm

    Thanks for the circuit I made the Amplifier circuit on breadborad and tested sucessfully. I have now started on the Debounce/Delay Circuit and have a question, regarding C6 (0.47uf) and C11 (22uf). I can only source electrolytic and was wondering which way I should attach the negative side of the cap. Electronics is not my strong suit, but I can build from plans. Also is there a particular value for the LED

  11. Jon on February 24th, 2012 10:01 pm

    Hi Mick. Any place you can get electrolytics you should be able to get plastic/ceramic/tantulum caps? Although not really intended for the task if you really want to put electrolytics in you just need to orient them with the negative leg (marked or shorter leg) towards the side of least potential. In the case of this circuit that would be towards the battery ground/negative to the LEFT hand side of the diagram.

  12. Mick on March 2nd, 2012 5:58 am

    Almost completed the project, the amp works just fine. Still struggling in the UK to source 22 microfarad ceramic caps. Found Tantalum but even they are poloarised. I am over in teh states in June I will try FRY’s.


  13. Jon on March 2nd, 2012 11:32 am

    Hi again Mick. Only the C3 in the amp is recommended to be a ceramic. Don’t worry and just put whatever you can get in there – I think mine actually has a polarised cap in it actually. Just make sure the positive leg is on the 9V end (right hand side of the debounce/delay circuit).

    Technically I should update the diagram to show polarisation of the cap (like C1) given caps that big are often polarised. As I said before, except is special cases (like C3), a polarised cap can be substituted making sure the positive leg is towards the greater potential (positive).

  14. Mick Favager on March 3rd, 2012 2:11 am

    Finally got it all working Delay included on breadboard in anycase. Occasional self fireing of the flash when the gain is past its mid point,but delay works great.
    Look forward to popping a few champagne corks soon.

  15. Jon on March 9th, 2012 2:47 pm

    Yeah the high end of my gain is pretty useless too. Much to sensitive. As noted above this pot could be switched to a logarithmic one, like a real volume control. This would make more of the range usable, they just simply did not have one in that mini size the day I bought the parts.

  16. TYHO on July 23rd, 2012 7:59 am

    I already have the opamp part working (and it works great). now my supplier has finaly the LM556 on stock so i can built the second part.

    I really like your post… thank you!

  17. TYHO on July 26th, 2012 7:36 am

    Dear Jon,

    I completed the circuit! And tested succesfully.

    I have one problem: When i put the mic sensetivity higher than a quarter, after triggering it keeps triggering again and a again. Like he is singing around.

    Anyone have this problem too? Any solutions?

    Again thnx jon for your clear explenation!

    Greetz Tijmen

  18. Jon on July 26th, 2012 12:08 pm

    Not quite sure what you mean here. You mean it pops in rapid succession to every little sound, resulting in double exposures? eg. You break a glass and the initial sound is captured but then when the pieces fall that sound results in unwanted flash pops?

    This is a common issue with these circuits, sometimes it is desirable but often not. However it was covered by my original design in the De-bounce/Delay part of the circuit that hooks to the Amplifier section? Looking at the circuit again I have two outputs “Flash Instant” and “Flash Delayed”. Only the “Flash Delayed” output will be de-bounced for approx 2.5s (the LED will indicate the de-bounce period, see notes). Meaning no further pops will be allowed until the de-bounce period expires (and the LED goes off).

    Further to that I have added a switch (SW2) which allows the physical jacks to connect to either one or the other. So you can switch between de-bounce/delay (which you can turn the delay to nothing on if you wish) OR an instant output, giving the best of both worlds?

    Hope this helps.

  19. Jon on July 26th, 2012 12:13 pm

    Further to this be sure you are using an electret type mic on the amp as that is what the component values are set for. Different types of mics will require different pre-amp component values, if they even work at all.

    See these:

    This is the most common type of mic found in cheap PC headsets etc.

  20. TYHO on July 26th, 2012 10:17 pm

    Dear Jon,

    The problem is that once it’s activated it will flash. But after de-bounce period it flash again and again. But only with sensetive podmeter @ a half of more. Below a half it runs very well.

    I use a piezo electric element. I think that causes the problem. I will buy an electred mic and then i report back.

    thanks again 😉

  21. TYHO on July 27th, 2012 3:34 am

    Tryed it with a Electret mic but still the same. I geuss it becomes to sensetive past the half. (like 5K)

    But why, when i turn the sensetive wide open and i keep quiet it will not trigger. Then i make a snap with my fingers and the trigger activates. and after the de-bounce time it comes back continuously?

    I just don’t get it, why it will come back on the background sound and don’t trigger on that sound?

    (I used a dB meter and the background sound stays below 44dB).

    Any solutions?

  22. TYHO on July 27th, 2012 7:02 am

    Ok i got a solution for my problem:

    If anyone has looping problems similar to above try a higher resistor for R8. Like 22k.

    Thanks again Jon for your support.

  23. Jon on July 27th, 2012 10:48 am

    Great stuff! As noted, because of the variances in microphone characteristics simple amplifiers can actually be a little tricky. It is best to get your amp working with a little PC speaker (in place of R3) before trying to hook it to the timer.

    By increasing R8 you have increased the RC time constant for the de-bounce. It is possible your amplifier is oscillating – ie. reverberating for a period following a sound. Again a speaker could check this. But hey, if it works it works! Well done.

    Also worth mentioning is that the amp should really have a LOG pot on it rather than linear – this will give a much smoother ramp up in terms of dB.

    Please post some high speed shots when you take it out for a spin.

  24. TYHO on August 8th, 2012 11:33 pm

    Dear Jon,

    As you asked i made a picture with my trigger and one from my trigger. I have to play more with it for more perfect results.

    You can find these on http://www.tshot.nl

    Thanks again for all support!

  25. Erik on August 12th, 2019 8:52 pm

    Hi Jon
    I am interested in water wigs photography and found your trigger/delay circuit here.

    I am puzzling my head over your design with the first timer.

    You have connected the “left” side of C6 with pin 3, which is the control input. All other designs are connecting the timing capacity to ground. Is this a bug or a feature ?

    Perhaps it is working in combination with C7, but I don’t understand the reason of your design.
    Thanks for sharing this circuit and I hope, you have an answer after all these years.

    Regards Erik

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