i had difficulty getting my head round the comparator output-- when they are "on" they are in effect off, if followed by an npn transistor, which maybe acts as an invertor.
even a sim was treating them as op amp output.
so you were right to go with pnp, when the output of the comparator transistor is "on",it provides the base current to switch on the transistor.

i found the link to the buzzer to be critical as it seemed to be loading the 555 preventing oscillation. pic

as usual, comments welcome,
dougal

 

Tibbles, are you working on the same assignment?

 

sorry bill ,am i so far off track?- marx was using pnp transistors in his drawings,

dougal

 

Nope, but you occasionally seem to drop references that you're doing something similar. I was just curious if you were doing it as an assignment too.

 

bill,i was merely responding to marx query on the output of the camparators, but you have gone off topic a bit. see chat.