A C172 SINGLE PLT; WHILE DIVERTING TO 61B DUE TO A TFR THAT PRECLUDED LNDG AT HIS INTENDED DEST; LAS; CHANGED HIS XPONDER WITHOUT L30'S PERMISSION; THEREBY CAUSING A TFR VIOLATION.

Narrative

THE FLT ORIGINATED FROM WINSLOW ARPT; AZ (INW) AND THE PLANNED DEST WAS NORTH LAS VEGAS ARPT; NV (VGT). THE AIRPLANE TOOK OFF; WITH ME AS PIC IN THE MORNING. NO FLT PLAN WAS FILED AND THE WHOLE FLT WAS CONDUCTED UNDER VFR IN VMC CONDITIONS AND UNDER 'FLT FOLLOWING' ASSISTANCE (ZAB THEN ZLA AND FINALLY LAS VEGAS APCH). WHILE APCHING THE LAS VEGAS CLASS B AIRSPACE; ZLA INFORMED ME ABOUT A TFR OVER LAS VEGAS AREA. BECAUSE OF THIS TFR; ALL 3 LAS VEGAS ARPTS WOULD HAVE REMAINED CLOSED UNTIL XC00 LCL TIME. ONCE IN CONTACT WITH LAS VEGAS APCH; AND ONCE OBTAINED CLRNC TO ENTER INTO CLASS B AIRSPACE; I DECIDED TO LAND INTO BOULDER CITY ARPT (61B) WHICH WAS STILL OPEN TO THE PUBLIC. LAS VEGAS ATC GAVE ME VECTORS FOR VISUAL APCH INTO BOULDER CITY ARPT. AT ABOUT 4 NM FROM BOULDER CITY ARPT; I CALLED LAS VEGAS ATC AND DECLARED THAT I HAD THE ARPT IN SIGHT. LAS VEGAS ATC THEN INSTRUCTED ME TO SWITCH TO BOULDER CITY ARPT UNICOM FREQ. WHILE SWITCHING FREQ FROM LAS VEGAS APCH ATC TO BOULDER CITY ARPT UNICOM FREQ; I ALSO CHANGED THE XPONDER CODE WHICH I WAS GIVEN INITIALLY FROM LAS VEGAS APCH ATC TO THE NORMAL 'VFR' XPONDER CODE '1200.' I CHANGED THE XPONDER CODE EVEN THOUGH I HAD NOT RECEIVED INSTRUCTIONS TO DO SO BY LAS VEGAS APCH ATC AND THIS RESULTED IN A TFR VIOLATION. I BELIEVE THIS HAPPENED BECAUSE OF THE FOLLOWING REASONS. NORMALLY; WHILE APCHING TO AN ARPT THE ATC FACILITY USUALLY GIVES INSTRUCTIONS TO SWITCH FREQ TO 'TWR' OR 'UNICOM;' THE ATC FACILITY ALSO USUALLY INFORMS THE PLT THAT 'RADAR SVC IS TERMINATED' AND INSTRUCTS HIM TO SET INTO THE XPONDER THE 'VFR' 1200 CODE. IN MY CASE; THE LAS VEGAS APCH ATC GAVE ME ONLY INSTRUCTIONS TO CHANGE RADIO FREQ. I ASSUMED THAT BECAUSE OF LAS VEGAS APCH FREQ CONGESTION; THE CTLR FORGOT TO TELL ME TO CHANGE THE XPONDER CODE HE GAVE ME INITIALLY INTO THE NORMAL 'VFR' 1200 CODE. FOR THIS REASON I CHANGED THE XPONDER CODE BY MYSELF WITHOUT REAL AND EXPLICIT PERMISSION FROM LAS VEGAS APCH ATC. A CONTRIBUTING FACTOR TO MY MISTAKE WAS ALSO THE CONGESTED BOULDER CITY ARPT UNICOM FREQ AS MANY PVT AND COMMERCIAL FLTS WERE EITHER DEPARTING; ARRIVING; OR DIVERTING TO THAT ARPT. ONCE LANDED AT BOULDER CITY ARPT; AND ONCE I PARKED THE ACFT; AN FAA INSPECTOR PRESENTED HIMSELF TO ME BY SHOWING A PICTURE IDENT; ASKED ME TO SHOW HIM MY PLT'S LICENSE AND MEDICAL CERTIFICATE AND ALL ACFT DOCUMENTS. THE FAA INSPECTOR THEN EXPLAINED TO ME THAT I HAD VIOLATED A TFR AND THAT; FOR THIS REASON; I WILL RECEIVE A LETTER FROM THE FAA; CONTAINING THE DESCRIPTION OF THIS HAPPENING; IN WHICH I WILL BE ALSO INVITED TO ANSWER IN ORDER TO EXPLAIN THIS HAPPENING FROM MY SIDE. ONCE RECEIVED; THE FAA WILL DECIDE EITHER TO SUSPEND MY LICENSE FOR 30 DAYS OR TO 'FORGIVE' ME.