Electrical – Signal invertor for power switch using 3.5mm audio jack switch

Neat idea. I just finished building a Bluetooth controlled Canon SLR remote, but it's controlled from the phone, not by the phone.

I think you can use this idea to drive the input to a MOSFET (a 2N7000 should do). When the signal from your phone drives the opamp high, the MOSFET will act like a switch, shorting your shutter release pin to the camera/circuit ground. If it turns out that the signal is too short in duration for the shutter to fire, just use a 555 in monostable operation in between your phone and the MOSFET, I'll bet the audio signal will work as the trigger input. If it doesn't, use both ideas together, have the square wave from the opamp trigger the 555 timer.

This sort of pop is a common problem with audio connections, particularly guitar leads as they can get swapped live. It's caused by capacitance in the systems being connected holding different dc levels, and results in a step pulse in the power output stage, which isn't good for your speakers and probably is what overloads your battery.

One means to neutralise is a reed switch in the jack, though I don't know of a mini stereo version.

The other means are to reduce capacitance in the cables and input stage of the amp. It may also be possible to use a transformer to isolate the input, or possibly a band pass filter to remove both the suddenness of the step change and the dc offset. But a filter may well have to be in the audio range to be effective, and that is the range you want to maintain.

If there was an automatic circuit which did this well then every electric guitarist would buy one, and there doesn't seem to be any out there (normally you have manual mute switches or the fore-mentioned silent jacks), so if you do get it to work there's quite a market.

An answer in another question pointed to this circuit as a possibility.