555 was just one option... any oscillator will do that is easily keyed by the
hardware morse instruments - to allow the oscillator to travel from the OSC output - into the LINE/MIC input port of the sound card...
it's the change in pitch... from 2, 7 , 11, and 19 Khz(prime numbers) that makes the big deal...
since the new Trigger Midi MONO(TMM) app now has a good audio bandpass filter on its input port...
so you can tune out everything but the pitch that will determine the MIDI CW NOTE output timing for that TMM
be it a DIT/dah or st key or cw keyboard keying it... etc...
the 555's in this example represent any tone oscillator CPO etc...
the 555 does not do any of the critical cw timing work though...just sends a steady Khz tone as per 'keyed by'
dit handle,dah handle, st key or cw keyboard's RIG KEYING jack... sending a solid steady tone for the duration,
of the mechanical PRESS to close, sending the tone into the sound card's INPUT JACK...
the RECRI CW KEYER app does all the timing math from there provided by the CW KEYING data
from the TRIGGER MIDI MONO app(s) as per which one is being targeted...