pcmhacking.net

I don't know if this has been tried before but I bought the mic plug adapter for the android phone , and hooked it up to a vn knock sensor downloaded an app called Spectroid , and I now have have a knock box on my android phone .
anyone into writing apps could probably make a specific app for it.
but the Spectroid app seems to work fine but would be nice to filter out some specific frequencies

Nice I use the same setup into a spyear amplifier might have to try that app

Photo not clear but you need the 3 segment 3.5 mm plug for the mic