Using only transistors as the active components, we can build a simple and low cost TTL-to-RS232 level converter. It uses only two transistors as the active components. Here is the schematic diagram of the converter:
TTL-RS232 Level Converter
You can use any small signal transistor for this RS232 converter circuit. You can see the collector of Q1 transistor is connected to TxD, and the purpose of this connection is to borrow a negative voltage for Q1. This condition make the converter works only for half duplex, so the negative voltage is always present when transmitting the data from TTL level to RS232 level. You can modify the 4.7k resisor to be connected to ground, not borrowing the negative voltage from RS232 TxD, and the converter will work for full duplex. This will work for most RS232 port even the conversion from TTL transmission doesn’t swing to negative. [Source: Wichit Sirichote's project]
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