serial TX bit                12vDC power supply
__________                     ^
          \                    |
           |                   \__
           |                      @
           |                      @ buzzer coil or relay
           |                      @
           |                    --
           |                   /
           |                   |
           |                   /
opto LED   V  /\/\light/\/\/\ | opto photodiode
          ---                  \
           |                    v
           /                    |
sio gnd ---                     |
                                |
                               ---
                                =  gnd