{HEAD ~ This device} mainly comprises a relay , a transistor , an optical coupler , and {TAIL ~ a tapping/rectifying circuit}  
comprises
{ HEAD ~ This device } mainly comprises a relay , a transistor , an optical coupler , and { TAIL ~ a tapping / rectifying circuit }
HEAD HEAD HEAD HEAD HEAD HEAD OTH REL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH