Here’s a cost-effective and streamlined design for a wireless remote camera flash trigger, designed to capture moments that are typically imperceptible to the unaided eye. This device, which enables high-speed and time-warp photography, is essentially a straightforward flash controller triggered by sound, constructed using only a small number of discrete electronic components. I opted not to employ a microcontroller for this relatively uncomplicated project, as it would be excessive for its requirements.
The circuit is designed to activate an external camera flash when sound pulses surpass a specific pressure level. This trigger sensitivity can be customized to suit various scenarios, such as identifying the sound of champagne bottles popping, bubbles bursting, or water balloons bursting. Notably, the flash circuit is powered by a 9-Volt battery.
At the front end of the circuit, you’ll find an electret microphone (MIC), specifically a Panasonic MCE-2000 with a sensitivity of 6mV/Pa/1 kHz, ±4dB* (though any other type of electret microphone will suffice). The signals picked up by this microphone are first amplified by the LM386 (IC1). The pre-processed signal is then compared to a constant threshold using the LM393 comparator (IC2). Subsequently, when the comparator detects a falling edge, it triggers the LM555 timer (IC3) configured in monostable mode. The pulse output from the monostable timer then activates the external flash through the MPSA44 high-voltage transistor switch (T1).
In this setup, the MIC is connected in the conventional manner with the addition of R1. C1 is used to eliminate direct current components from the signal, while C2 filters out potential high-frequency noise. The gain of IC1 is set to 200 (equivalent to 46 dB) due to the presence of C3 connected between Pins 1 and 8 of IC1. The 10K potentiometer (P1) enables adjustment of the trigger sensitivity, which can be handy in different scenarios. When the signal level surpasses a threshold determined by the potentiometer, a falling edge is generated at the comparator’s output, triggering the monostable timer.
The duration of the pulse for capturing the desired moment is determined by R3 and C5, following the formula tpulse [in seconds] = 1.1 x R3 (M) x C5 (uF). Larger values for these RC components result in longer pulses. For high-speed photography, it is advisable to delay the trigger for a few seconds after detection by selecting a longer pulse duration.
It’s important to recognize that this circuit can activate both camera flashes and cameras themselves, as they both operate through the same mechanism of creating a short circuit between two electrical points. For this project, I opted for a compact ABS enclosure, which houses a 3.5-mm audio connector (J1). This connector facilitates a direct link to the remote flash using a standard 3.5-mm sync cable, as illustrated in the connection diagram provided below.
About Off-Camera Flash Trigger
There are various approaches available to synchronize your camera with the firing of flashes, whether it’s during the shutter release or when capturing rapid shots. A commonly used method for wired off-camera flash is the utilization of a PC sync cord, which serves the simple purpose of triggering the flash. One advantageous aspect of PC sync cords is their affordability. In this context, you can link your completed system to your flash using a PC sync cord. Whenever the system receives a valid sound signal, it activates the flash to which it’s connected. In essence, this device functions as a flash controller, initiating the connected flash in response to sound impulses. With your SLR camera, you can embark on capturing striking images of events that generate sound!
*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.