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the problem is, that you cannot draw much current out of an rs-232 port.
You have to take an unused handshake-line like DTR, and set it to
digital '0'-pegel. This pegel will be converted to +9V at the output of the
rs-232 (inverted logic!). This line can be loaded with max. 10mA
(depending on type of PC, a notebook can't drive as much).
Now you can regulate this voltage with a low drop linear regulator, which
should have a low quiescent current (e.g. LM2931).