Using MAX232 integrated circuit, you can build a TTL-to-RS232 level converter that totally comply with RS232 electrical specification. The IC generate the higher positive and negative voltage (+10 and -10 Volts)  from TTL power supply (5V DC) using switched capacitor. The switching circuitry is provided inside the chip, so you need only few caps for external components. Here is the schematic diagram of the circuit:

ttl rs232 converter circuit schematic

In applications that are sensitive to power-supply noise, VCC should be decoupled to ground with a capacitor (C5) of the same value as C1 and C2 connected as close as possible to the device. [Source: Maxim Integrated Products Application Notes]