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