Circuit Schematic Diagram of Simple TTL to RS232 Level Converter using Transistor

April 19th, 2010

We can build a low-cost simple TTL to RS232 level converter by using only two transistors as the active components. Take a look at the following circuit diagram :

simple ttl-rs232 level converter using transistor circuit diagram

1. Data Carrier Detect
2. Receive Data (RxD)
3. Transmit Data (TxD)
4. Data Terminal Ready (DTR)
5. Ground
6. Data set ready
7. Request to send
8. Clear to send
9. Ring Indicator

Any small signal transistor can be used for this RS232 converter. The figure above shows that the collector of Q1 transistor is connected to TxD which purpose is to borrow a negative voltage for Q1. Thus, this makes the converter works only for half duplex so that the negative voltage is always present when transmitting the data from TTL level to RS232 level. The 4.7k resistor can be modified by connecting it to ground, not borrowing the negative voltage from RS323 TxD so the converter will work for full duplex. Even the conversion from TTL transmission doesn’t swing to negative, this will work for most RS232 ports. [Source: Wichit Sirichote's project]

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