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