The SCR has a second function. It serves to isolate the amplifier from the voltage across the terminals of the flash unit. This can be as high as 330 V for older flash units. Newer units, however, may have low-voltage trigger circuits. In any case, an SCR rated for 400 V is sufficient.
Sound triggers are convenient to use in many situations.
If the event to be photographed produces a loud, sharp sound, piezoelectric triggers work fine. The pressure of the sound pulse on a piece of piezoelectric film distorts the film, which produces a voltage pulse in response. A transistor switch serves to gate a sensitive SCR, that is, one with a low gate current threshold. While one can spend more than a hundred dollars for a professional model sound trigger, there are inexpensive alternatives that work just as well in many situations. The SCR must be sensitive to 0.2 ma of gate current or less.