Diagram SOlution

This is one of the porters of the series "battery chargers that will never do." It uses the serial port of a PC to power a battery charger. A serial interface port can not supply enough current to charge batteries more powerful, but low capacity battery Nickel Cadmium (NiCd), this circuit is more than enough.You could, for example, use the batteries in a radio and charge them while using the PC. 

The three serial port connections TxD, DTR and RTS, when not in use, are -10 V and can provide a current of about 10 to 20 mA. The circuit shown supplies a charging current of about 30 mA. If you need to change the polarity of the charging circuit, then it is a simple job, just reverse the diodes and use of software, change door signs to 10 V. Those interested can also write a software routine that automatically recharges the batteries.