A simple and low cost TTL to RS232 level converter using only two transistors as the active component can be seen on the following schematic diagram :
Any small signal transistor is suitable for this RS232 converter. The collector of Q1 transistor is connected to TxD to borrow a negative voltage for Q1. This will make the converter works half duplex in order to make the negative voltage always present when transmitting data from TTL to RS232 level. The 4.7k resistor can be modified by connecting it to the ground, so it will not borrow the negative voltage from RS232 TxD, then the converter will work full duplex. This will work for most RS232 port.
[Source: Wichit Sirichote’s project]