Which be output voltage and voltage this increase continually poverty voltage that equal to 12 the volt. By have R7 be limit current already change come in the way pin B of Q4 make Q4 work be make Q3 stop bias. Then make output voltage be 0 volt, but when positive pulse signal that made come to then change come to at R6 then make Q3 bias again. And work character as before but there are many speeds then can make output that appear can come out 12 volt all the time by have C3 perform in something current filter smoothly before increasingly lead output voltage go to be usable