PASSING FL180 IN CLIMB; B737 FLT CREW EXPERIENCES NUMEROUS ELECTRICAL MALFUNCTIONS INCLUDING CABIN PRESSURE LOSS. EMERGENCY DECLARED WITH RETURN TO FIELD.

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ON CLBOUT; AS PNF; AT APPROX FL180 WE RECEIVED A TCAS FAIL. AFTER CHKING THE CIRCUIT BREAKERS AND RECYCLING; THE TCAS WAS STILL INOP. THEN CTR RPTED OUR MODE C HAD STOPPED RPTING. AFTER CYCLING AND SWITCHING BOTH XPONDERS; THEY WERE NOW INOP. NOW; BOTH XPONDERS AND TCAS ARE NOT WORKING. WE RECEIVED A CLRNC TO CLB TO FL330 AND RPT VACATING FL250. AT THIS TIME THE CAPT WAS HAND FLYING AND INFORMED ME THE ACFT WASN'T RESPONDING TO THE COMMANDS ON THE AUTOPLT AND/OR THE AUTOTHROTTLES WEREN'T ENGAGING AS WELL. AS I WAS THE PNF; HE COULD BETTER RPT AS TO THE FLYING QUALITIES AND WHAT THE PLANE WAS OR WASN'T DOING BETTER THAN ME. AT ABOUT THIS TIME; WE RECEIVED THE CABIN ALT WARNING HORN. I CANCELED IT AND CHKED THE CABIN ALT; IT WAS NEAR 10000 FT AND APPEARED TO BE CLBING; WE THEN REQUESTED LOWER; DECLARED AN EMER; PUT ON OXYGEN MASKS; ESTABLISHED COMS AND RAN THE EMER DSCNT CHKLIST. DURING THE DSCNT; THE CAPT NOTICED THE WX AND MADE THE DECISION TO RETURN TO ZZZ. ONCE THE PLANE WAS STABILIZED AT 10000 FT AND 250 KTS HEADED TO ZZZ; WE TOOK OFF OUR OXYGEN MASKS; THE CAPT GAVE ME CTL AT WHICH TIME I ENGAGED THE AUTOPLT; GOT THE WX FOR ZZZ; SET UP FOR THE APCH AND GOT THE LNDG DATA; WHILE HE TRIED DISPATCH AND TALKED TO THE FLT ATTENDANTS. CTR ASKED US IF WE WERE SQUAWKING 7700; WHICH WE WERE; BUT THEY WEREN'T PICKING IT UP; BUT ZZZ APCH WAS? I BELIEVE THE OUTCOME WAS SUCCESSFUL AND SHOULD/COULD HAVE GONE EVEN SMOOTHER IF: 1) THE PA WAS LOUDER -- THE PAX SAID THEY COULDN'T CLEARLY HEAR THE CAPT'S ANNOUNCEMENTS; AFTER WE WERE DEPLANING. 2) THE INTERPHONE COULD HAVE BEEN LOUDER AND OR CLEARER SO THE INFO AND SITUATION COULD HAVE BEEN MORE CLEARLY UNDERSTOOD AND DEALT WITH. 3) WE COULD HAVE COMMUNICATED WITH DISPATCH THROUGH VOICE -- MAYBE WE WERE TOO LOW? 4) WE WERE INTERRUPTED COUNTLESS TIMES BY ATC; BUT LATER THE CAPT AND I UNDERSTOOD THAT THEY COULDN'T SEE OUR XPONDER; SO WE HAD TO CONTINUALLY ANSWER QUESTIONS; RPT POS; ETC. 5) COMMUNICATING THROUGH THE OXYGEN MASK AND SPEAKER IN THE PLANE IS MORE DIFFICULT THAN THE SIM; IN REGARDS TO CLARITY AND LOUDNESS. I DON'T BELIEVE THERE WERE ANY HUMAN FACTORS INVOLVED AT THIS TIME IN THIS EVENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER CONFIRMS THAT A FAULTY GROUND/AIR SENSING CIRCUIT WAS RESPONSIBLE FOR THE ANOMALIES REPORTED AND WAS REPAIRED BY MAINTENANCE.