If your serial port voltages are too low, you will need to power the interface from an external power supply. Have a look on my web under interfaces. My Serial PIC Interfaces includes circuit diagrams for both internal and external power supplies. The external power supply can be between 5 and 12V DC, please remember to observe the correct polarity.

Basically, you disconnect the wire on the board going to pin 7 of the serial plug (supply from PC), and ensure it does not make contact anywhere.

Where this wire was disconnected, you can connect the external power supply (+) positive rail.

Alsoon the board, connect the (-) negative rail to the wire going to pin 5 of the serial port plug. Do not remove original wire.