The following circuit is an example how to get power from RS-232 serial port. It gives regulated +5V power for logic circuits and also unregulated positive and negative power supplies for RS-232 transmitting circuit. The circuit gives only few milliampres power output because the power available from serial port is limited (and resistors R1, R2 and R3 limit current more).
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Modern PCs have typically 9 pin port instead of this older 25 pin port. If you want to use this circuit with such PC you have two options: use a 9 to 25 pin adapter or modify the circuit to 9 pin port. You can get a little bit more current from the circuit if you leave out the resistors (R1, Rs, R3) and replace them with short piece of wire. 78L05 regulator takes 3-4 mA current all the time and needs at least 2V voltage drop, so if you can find a similar regulator which takes less current and has lower voltage drop, you get more current for your circuit. Some small circuits signal input circuit I have seen have taken positive and negative supplies for simple operational amplifier circuit just by using DTR and RTS lines. Just by driving one of them to 1 and other to 0 using suitable software routine, there are available positive and negative voltages from those pins.
How much you can really draw from a serial port depends on the circuit technology used in the serial port. B&B Electronics Connection Newsletter number 2 has a good article "Tips For Using Port Powered Converters" about how much you can really get power from different serial ports. The following data is extracted from the article text:
Type of State of Voltage Voltage/current Power available from
Driver Line at no load per driver (max) 3 driver outputs
1488 positive +9..11V +5V @ 6.5 mA 98 mW
1488...
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