One button circuit with thyristor, controling power suply
I have been looking for something like this recently. I came across this setup and worked it out.
The schematic in the picture has been confirmed on a breadboard, only not with an actual microprocessor (Arduino) but with myself doing the GPIO-pin CTRL-things (connecting to 5V HIGH or GND/LOW).
I've been adding some pinout descriptions for clarification to myself.