I believe that after Shabbos one must say “Boruch hamavdil bein kodesh lechol” before doing a melacha. Please can someone supply a source for this?
"The Tur, in Orach Chaim 299, brings the opinion of Rabbeinu Asher and the Ba'al Halachot Gedolot that before doing melacha one should say "Hamavdil" with "Shem v'Malchut", that is, in a normative bracha format. Rashi, however, says that only the words "Baruch HaMavdil bein kodesh l'chol" must be said and this is the position adopted by the Shulchan Aruch. This is the common practice." http://www.yeshiva.co/ask/?id=3513
AMAZING......took a while...and I THINK I found something: http://shulchanaruchharav.com/halacha/baruch-hamavdil-or-havdala-before-doing-melacha/?hilite=hamavdil#ftn8
I was looking only about a case...or flipping on a switch...or gatting candles...NOT CLEANING THE HOUSE OR INSTALLING A LIGHT FIXTURE.... so:
Other Opinions brought in Admur: There are those opinions [Rabbeinu Yerucham] which rule that only those Melachas which are time consuming and take effort, such as weaving, writing and chopping wood, were forbidden by the Sages before saying Havdala or Baruch Hamavdil . However those Melachas which are performed without any effort, such as lighting a candle or carrying 4 Amos in a public domain, are permitted even if they are of Biblical nature, if the proper time of night has arrived, even if one has not yet said Hamavdil .
OK...in the Maariv..it is CUSTOMARY (not obligatory) to draw out the VEHU RACHUM...and BARECHU, in order to "add from the mundane to the sacred"..an interesting BACK ENGINEERING of extending Shabbat....bringing MOTZEI into...
That is what was not answered in the other link. We understand by DAYBREAK, Sunday, saying HAMAVDIL is not necessary. I guess the question stands: After "enough" time after Nightfall...3 stars...and an additional as per requirement...if one turns on a light (A NON LABOR INTENSIVE ACT), is there and ISSUR DaRabbanan?