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 ...

Once the circuit is built, you can adjust R7 to get the appropriate output at U2d. Obviously, differing amounts of gain were required for the optical and electret circuits.

Apple’s new Smart Battery Cases have a new shutter button, and now we know a little more about how that button works, thanks to an X-ray of one of the cases done by iFixit and Creative Electron.