R74 = 0 ohms burn out buzzer and possibly transistor, if you haven't already.
Buzzer coil is min 13 ohms. Limit of 100 mA.
R74 ~= (3.3 - Vcesattransistor) /.1 amps = ~ 3.1 / .1 ~= 30 ohms. Power of R = I**2 x R ~=
30 x .01 or .3 W. Use 1/2W Resistor.
Base drive wants to be ~ Icollector /10, or 10 mA. This insures transistor is in saturation when on.
R72 ~= ( Vohpwm - Vbe) / .01 amps so look up Voh of processor and calc
that value.
If I use Voh ~= 2.5V, Vbe ~= .7V then R72 ~= ( 2.5 - .7 ) / .01 ~= 180 ohms.
Buzzer helps if mechanical cone placed over it. Small end on buzzer. Use a piece
of cardboard to try out. Normally in finished products this is a plastic piece. It wants
to be audio reflective, a hard surface like metal or plastic.
Regards, Dana.