SmaTrig 2.1

Simple sound trigger for cameras and flashes

Experiments with the external trigger function in my EOS 400D inspired me to develop a simple do-it-yourself sound trigger. The circuit detects sound pulses that exceed a certain pressure level. The sensitivity of the trigger is limited, but high enough to detect popping champagne bottles, bursting balloons or eggs hitting the floor. It can trigger cameras as well as flashes, as both are activated by the same mechanism of short-circuiting two wires.

The Circuit

sound trigger schematics

The electret microphone is connected in the usual way with R4 and C3. I used the MCE-2000, but any other electret type will do. If you need more sensitivity look for a high V/Pa ratio mic. This value is usually specified in dB. C3 forms a high-pass filter and blocks the DC component in the mic signal. The voltage divider R1-R6 sets the polarisation voltage of the transistor slightly below the conduction threshold which is about 0.7V. Now, if the mic generates a positive signal, is adds up to the polarisation voltage and opens the transistor. The collector voltage is now pulled down from VCC to ground. The falling edge triggers the 555 for one pulse (mono-stable operation). The pulse duration is set by R3 and C2 according to the formula

tpulse = 1.1 R3 C2   [in seconds]

Higher values for C2 or R3 result in longer pulses. The output pin of the 555 can be connected to an opto-coupler or an NPN transistor. The fist option provides some additional safety for the camera or flash unit.

In older flash units the trigger pin is connected directly to high voltage. In such cases an opto-triac or opto-thyristor should be used. Special care must be taken to avoid electrical shock!

The values of most components can vary between about -20% and +50%. The transistor is an NPN small signal type, e.g. BC547-550. If using a modern CMOS version of the 555 timer (e.g. LMC555, TLC555) the supply voltage can be reduced to less than 3V allowing operation from two 1.5V batteries.

The schematics of the circuit for Eagle can be downloaded here.


Before use the polarisation voltage has to be adjusted with R1 to obtain maximum sensitivity. Turn the voltage up slowly until the timer has triggered. Now go back a little to get below the threshold - ready. Turning the voltage further down reduces the trigger sensitivity. This might be useful in some situations.

Now you have to adjust the trigger pulse duration with R3 according to your needs. For high-speed photography it's a good idea to block the trigger for a few seconds after detection by selecting a long pulse duration. This helps to avoid secondary triggering due to ricocheting champagne corks, falling glass fragments and so on. Longer trigger pulses also allow to use the serial image mode of the camera to shoot multiple images after detection.

Example images

sound trigger sound trigger

The pics above were shot by triggering the camera directly. The shutter lag is clearly visible in the right picture. If you need more information on shutter lag issues, have a look here or here. For real high-speed shots the flash unit must be triggered as in the pics below.

bursting balloon bursting balloon

It is important to limit the flash duration by reducing the flash power. Values of 1/32 and 1/64 gave good results on my Canon Speedlite 430EX. At higher flash outputs the fast moving parts are blurred. By moving the mic away from the balloon the captured phase of the burst can varied. I used a small tube as a blowgun to puncture the balloons with a screw.

popping champagne bottle 1 popping champagne bottle 2

In the image above left the sensitivity of the sound trigger was too high, so a "parasitic" sound pulse was captured.

Shutter lag measurements for the Canon EOS 400D
Comparison of Shutter Lag & Startup Delay
Remote trigger pinout of the Canon EOS 400D
Supply for popping cork experiments

Comments (24)

Components replacement
Great project and very useful. I am not electronic guy, I do photographing as a hobby.
Some parts of the circuit are not found in my country. Can I replace (SFH-610) with (TLP621, TLP721, PC817)?
#24 - Mohamed - 12/26/2020 - 18:52
Sound trigger camera
I recommend to test the circuit without the camera first. Just use an LED instead of the opto-coupler and see if it flashes when you make a sound.
Note, that the potentiometer R1 needs to adjusted first. Turn it up until the circuit triggers without any sound and then turn it back a bit.
#23 - Luk - 11/11/2019 - 19:59
Sound trigger camera
Help me to do this project I am not able to get output. How we connect camera to this circuit.
#22 - Manoj - 11/11/2019 - 13:38
Love this project
You should really sell this! I'd get one in a heartbeat
#21 - John - 12/03/2016 - 23:42
Hey, that's a really handy sound trigger, I could use that.
#20 - Steve - 07/11/2016 - 22:36
Power supply
The power supply or battery is connected between the 2 points in the circuit labelled GND and +5V. Of course, all the GND connections are wired together.

The 5V connection is connected to the battery positive terminal and the ground (GND) is the negative. This is almost always the case EXCEPT with split rail supplies, which this isn't.

As you cannot get a 5V battery easily, here are a few thoughts:
- I am confident 6V will not damage the circuit. The NE555 chip will operate at over 15V (from memory) so although things like timings may vary very slightly, the adjustments in the circuit will compensate. Note that I have not tried this but commenting based on experience.
- A battery pack of 4 x 1.5V cells will deliver 6V which will be completely satisfactory.
- That same holder can be used with 4 x rechargeable Nicads or nickel-metal-hydride batteries which at 1.2V each will give you 4.8V - again, completely satisfactory. Using these battery chemistries reduces the long term cost of powering the circuit.
- The circuit will almost certainly run from 4.5V, meaning a 3 cell battery holder with 3 x standard AA cells will operate adequately.
- Larger cells, such as C's will work and live longer.
- If you have a supply and really want to "lose" some voltage, simply wire a diode in series with the positive connection and you will get rid of about .6-.7V per diode. Use silicon diodes for this, a 1N4001 is a cheap and commmon device costing only a few cents. The more diodes you wire in series, the bigger the drop. The 1N4004 is another common diode/rectifiier which will work.

Hope this helps. My email is should there be other simple queries...
#19 - Brett - 07/11/2012 - 08:37
Really great stuff
#18 - Best Heart Rate Monitor - 12/25/2011 - 13:24
Looking for someone with good knowledge on building sound triggers for project. Will pay for your time.
I have a project that involves a sound trigger. Due to my lack of electronic knowledge and a baby due any day now I realise that I won't be able to complete it without help. I am happy to pay someone for their time on this. Please contact me at if you have the know how and time for this. It should be interesting. Thank you .. Jon
#17 - Jon - 10/05/2011 - 10:26
Battery connection...
Connect the positive lead to the 5V(top center of schematic) and the negative lead to the GND(bottom center of schematic).
#16 - Solomon - 04/04/2011 - 15:23
Sound Trigger
A commercial unit for those who are not familiar with electronics:
#15 - edgar - 02/17/2011 - 19:18
large rugs
Renske gaat toch maar proberen te twitteren, misschien helpt het bij die keelonsteking... kakkeel. Thank Yous! massjo
#14 - massjo - 01/21/2011 - 06:38
Extra Sensitivity
Awesome circuit. Found it much easier to follow and versatile than the one by lullabye. One addition you can make is to swap R5 (20k ohms) with a 250k POT. This allows you to tune the trigger into the 555 timer. First set it at a low value ~50k-100k, and tune R1 for the transistor sensitivity like you say. Then slowly increase R5 until a small signal will saturate the circuit. After saturation u need to reset the circuit either by dropping the resistance on R1 or simply shorting R1. This allows you to trigger on a pin drop from across the room. Don't know if this is useful for anyone, or how technically sound it is, but it worked for me.
#13 - Will - 10/22/2010 - 03:50
As you can see from the other article the shutter has a delay of 60 to 120 mS because of the mechanical parts that has to be moved, so I don`t know if it`ll be fast enough to catch the lightning :) Lightnings are usually catched with the BULB mode, you release it after the flash.
#12 - Kiril Nedev - 10/18/2010 - 13:26
Hi Bernard,

you are absolutely right, RESET should be connected to VCC for better stability. 10nF at pin 5 is also recommended, but I achieved good results without the cap. It depends on the application.

#11 - luk - 03/10/2010 - 08:46
Excellent article. Juste une remarque concernant le 555. La broche 4 (reset) doit être connectée au 5V. La broche 5 à un condensateur de 10nF à la masse.
Ceci pour une meilleure stabilité du composant.

I try in english (but my english is very bad, sorry)
The pin 4 of 555 must be connected to the supply voltage.
The pin 5 to a capacitor 10 nF to the ground
For a better stability of this component

#10 - bernard - 03/09/2010 - 17:01
you should package this and sell it. exactly what i need but i don't have thre first clue on how to build it! oh man!
#9 - free wallpapers - 10/19/2009 - 15:18
Nice work
Just found this interesting circuit and have just ordered the parts to build it. Not too expensive. I've also programmed a PIC microprocessor to connect into the circuit. There will be a 3 position switch, in position 1 (set sensitivity) the PIC microprocessor takes the output from the sound trigger circuit and turns an LED on for 0.25 secs, you will be able to keep making noises and set the sensitivity in this mode. Position 2 is the off position, and position 3 is the 'Start system position.

When in 'Start system', the LED blinks 3 times, the camera shutter is opened (must be in BULB mode), tere is a 0.5 sec delay and then the PIC waits for the sound trigger signal. When it gets this, it waits for the set time and then triggers the flash, waits 0.5 secs again and then closes the shutter.

The delay time will be done with a 16 position hex BCD switch, which represents a time delay (in tens on milli secs). After the sound trigger is activated, the circuit waits for the set time before firing the flash.

I haven't got the components yet, so haven't yet tested it. Also I'm am not sure if having the hex BCD switch setting the delay in 'tens of milli secs' will work, maybe it should be 'milli secs'

#8 - Craig Smith - 10/05/2009 - 15:39
Sound Triiger
I'm no eletronics technophobe (but know a man who is).
I need a sound trigger to set off a flash unit for track and field athletics. Obviously the sound is a pistol report, unfortunately
a .38 calibre so the report is quite loud. How can your project
be adapted for the large inital sound produced. Any help would be gratefully received.
#7 - Mike - 09/19/2009 - 19:10
I admit, I have not been on this webpage in a long time... however it was another joy to see It is such an important topic and ignored by so many, even professionals. Thanks for these expert advices for the professionals as well as for the newbies.
Optical trigger
It should be possible to convert the sound trigger into a flash/lightning trigger if you replace the mic with a photo diode. The power supply resistor R4 (2.2k) should be left out then. I'd also try out a lower value for C3 to suppress triggering by slow changes of light intensity.
#5 - luk - 06/03/2009 - 08:56
Optical Slave Trigger
Great work and explanation, I'm going to try it as soon as possible.
I was wondering if you could create an optical trigger so I can use the flash from my Canon EOS 350D camera to trigger a slave flash.
A friend and I tried to create the "Lullaby DIY Universal Flash" but we can't make it work. (
#4 - Matias - 05/20/2009 - 18:45
The problem is that shutter lag of the camera is much greater than the trigger lag of the flash unit. I measured the shutter lag of my EOS 400D and observed delays between 60 and 120 ms (see You need to trigger the cam fist, wait until the exposure starts and then fire the flash. To take high-speed shots people usually use a dark room, trigger the cam for 15s or so and initiate the trigger event in that time.
Of course you can trigger the cam only and use the cameras sync contact to fire the flash.
#3 - luk - 04/30/2009 - 08:22
Hi Brad,
you can delay the trigger slightly by moving the mic away. One meter distance makes delays about 3 ms (speed of sound / distance). I'll think about a delay stage for the circuit.
#2 - luk - 12/30/2008 - 15:51
Great project
Hi there thanks very much for this project I was looking around for a good circuit that would be adjustable sensitivity wise and have a timer to block multiple flashes. This one did the job perfectly! The parts cost about $20 AU and worked perfectly first time.

Looking forward to playing with this now and getting some great shots. Would love to have the ability to delay the trigger by small amounts (in the milliseconds range) to capture bursting balloons or eggs at various stages.

Thanks and well done!
#1 - Brad - 12/28/2008 - 10:18
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