Be very, very sure the BOB output is truely open collector if Vs = 24v or at best it won't work (I'll leave it to the reader to explain why :) ) and at worst you could fry the BOB!
Alternatively make Vs = 5v
Rb for Vs = 5v is 1k
Rb for Vs = 24v is 4k7
1/4w resistors will be fine