Here is an efficient and economical circuit for a wireless remote camera flash trigger, useful for capturing scenes invisible to the naked eye. This high-speed/time-wrap photography accessory is, in fact, a simple sound-triggered flash controller built around a handful of discrete electronics components (I prefer not using a microcontroller for such a simple project — that would be overkill).
The Electronics
The circuit is configured to trigger an external camera flash when sound pulses exceed a certain pressure level. The trigger sensitivity can be adjusted for different situations — for instance, to detect popping champagne bottles, bursting bubbles, or popping water balloons. As you can see, the flash circuit is powered with a 9-V battery. The front end of the circuit is an electret microphone (MIC), a type MCE-2000 by Panasonic with a sensitivity of 6mV/Pa/1 kHz, ±4dB* (though any other type of electret will suffice). Signals captured by this microphone are amplified by the LM386 (IC1), and the pre-processed signal is compared with a constant threshold by the LM393 comparator (IC2). Finally, the comparator’s falling edge triggers the LM555 timer (IC3) wired in monostable mode and the pulse output from the monostable triggers the external flash by the MPSA44 high-voltage transistor switch (T1).
Here, MIC is connected in the usual way with R1. C1 removes dc components in the signal, while C2 bumps off potential high-frequency rackets. The gain of IC1 is 200 (46 dB) because of C3 connected between Pins 1 and 8 of IC1. The 10K potentiometer (P1) sets the trigger sensitivity, which could be useful in a variety of situations (when the signal level goes above a threshold determined by the potentiometer, falling edge occurs at the output of the comparator, triggering the monostable). The moon shot pulse duration is set by R3 and C5, according to the formula tpulse [in seconds] = 1.1 x R3 (M) x C5 (uF). Higher values for these RC components result in longer pulses. For high-speed photography, it’s good to hinder the trigger for a few seconds after detection by selecting a long pulse duration.
Note that this circuit can trigger flashes as well as cameras because both are activated by the same mechanism of short-circuiting two electric points**. I selected a small ABS enclosure for this project. The enclosure contains a 3.5-mm audio connector (J1) to enable a direct connection with the remote flash by a standard 3.5-mm sync cable (see the connection diagram shown below).
About Off-Camera Flash Trigger
There are a number of different ways that you can get your camera to trigger the flashes in tune with the shutter release or upshots. The popular method used for wired off-camera flash is the use of a PC sync cord, which will simply trigger the flash. The good thing about PC sync cords is that they are quite cheap. Here, you can attach your finished system to your flash through the PC sync cord. When the system receives a valid sound signal, it will trigger the flash to which it is connected. In short, the device works as a flash controller to fire the connected flash by sound impulses. Using your SLR camera, you can start to capture awesome images of sound-generating events!
*International standards have established 1 pascal (Pa) as 94 dBSPL (sound pressure level). This reference point is now accepted for specifying the sensitivity of microphones.
