The current flow produces a potential difference across the first diode to turn on {HEAD ~ another transistor} to cause {TAIL ~ an alarm} to be given 
to cause
The current flow produces a potential difference across the first diode to turn on { HEAD ~ another transistor } to cause { TAIL ~ an alarm } to be given
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH